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<!-- This is a (PRE) block. Make sure it's left aligned or your toc title will be off. -->
<b>AppendCommand: Contents</b><br>&nbsp;&nbsp;<a href="#APPEND _key_ _value_">APPEND _key_ _value_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Examples">Examples</a>
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<h1 class="wikiname">AppendCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="StringCommandsSidebar.html">StringCommandsSidebar</a><h1><a name="APPEND _key_ _value_">APPEND _key_ _value_</a></h1>
<i>Time complexity: O(1). The amortized time complexity is O(1) assuming the appended value is small and the already present value is of any size, since the dynamic string library used by Redis will double the free space available on every reallocation.</i><blockquote>If the <i>key</i> already exists and is a string, this command appends theprovided value at the end of the string.If the <i>key</i> does not exist it is created and set as an empty string, soAPPEND will be very similar to SET in this special case.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Integer reply</a>, specifically the total length of the string after the append operation.<h2><a name="Examples">Examples</a></h2><pre class="codeblock python" name="code">
redis&gt; exists mykey
(integer) 0
redis&gt; append mykey &quot;Hello &quot;
(integer) 6
redis&gt; append mykey &quot;World&quot;
(integer) 11
redis&gt; get mykey
&quot;Hello World&quot;
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<!-- This is a (PRE) block. Make sure it's left aligned or your toc title will be off. -->
<b>AppendOnlyFileHowto: Contents</b><br>&nbsp;&nbsp;<a href="#Append Only File HOWTO">Append Only File HOWTO</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#General Information">General Information</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Log rewriting">Log rewriting</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Wait... but how does this work?">Wait... but how does this work?</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#How durable is the append only file?">How durable is the append only file?</a>
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<h1 class="wikiname">AppendOnlyFileHowto</h1>
<div class="summary">
</div>
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&iuml;&raquo;&iquest;#sidebar <a href="RedisGuides.html">RedisGuides</a>
<h1><a name="Append Only File HOWTO">Append Only File HOWTO</a></h1><h2><a name="General Information">General Information</a></h2>Append only file is an alternative durability option for Redis. What this mean? Let's start with some fact:<br/><br/><ul><li> For default Redis saves snapshots of the dataset on disk, in a binary file called dump.rdb (by default at least). For instance you can configure Redis to save the dataset every 60 seconds if there are at least 100 changes in the dataset, or every 1000 seconds if there is at least a single change in the dataset. This is known as &quot;Snapshotting&quot;.</li><li> Snapshotting is not very durable. If your computer running Redis stops, your power line fails, or you write killall -9 redis-server for a mistake, the latest data written on Redis will get lost. There are applications where this is not a big deal. There are applications where this is not acceptable and Redis <b>was</b> not an option for this applications.</li></ul>
What is the solution? To use append only file as alternative to snapshotting. How it works?<br/><br/><ul><li> It is an 1.1 only feature.</li><li> You have to turn it on editing the configuration file. Just make sure you have &quot;appendonly yes&quot; somewhere.</li><li> Append only files work this way: every time Redis receive a command that changes the dataset (for instance a SET or LPUSH command) it appends this command in the append only file. When you restart Redis it will first <b>re-play</b> the append only file to rebuild the state.</li></ul>
<h2><a name="Log rewriting">Log rewriting</a></h2>As you can guess... the append log file gets bigger and bigger, every time there is a new operation changing the dataset. Even if you set always the same key &quot;mykey&quot; to the values of &quot;1&quot;, &quot;2&quot;, &quot;3&quot;, ... up to 10000000000 in the end you'll have just a single key in the dataset, just a few bytes! but how big will be the append log file? Very very big.<br/><br/>So Redis supports an interesting feature: it is able to rebuild the append log file, in background, without to stop processing client commands. The key is the command <a href="BGREWRITEAOF.html">BGREWRITEAOF</a>. This command basically is able to use the dataset in memory in order to rewrite the shortest sequence of commands able to rebuild the exact dataset that is currently in memory.<br/><br/>So from time to time when the log gets too big, try this command. It's safe as if it fails you will not lost your old log (but you can make a backup copy given that currently 1.1 is still in beta!).<h2><a name="Wait... but how does this work?">Wait... but how does this work?</a></h2>Basically it uses the same fork() copy-on-write trick that snapshotting already uses. This is how the algorithm works:<br/><br/><ul><li> Redis forks, so now we have a child and a parent.</li><li> The child starts writing the new append log file in a temporary file.</li><li> The parent accumulates all the new changes in an in-memory buffer (but at the same time it writes the new changes in the <b>old</b> append only file, so if the rewriting fails, we are safe).</li><li> When the child finished to rewrite the file, the parent gets a signal, and append the in-memory buffer at the end of the file generated by the child.</li><li> Profit! Now Redis atomically renames the old file into the new one, and starts appending new data into the new file.</li></ul>
<h2><a name="How durable is the append only file?">How durable is the append only file?</a></h2>Check redis.conf, you can configure how many times Redis will fsync() data on disk. There are three options:<br/><br/><ul><li> Fsync() every time a new command is appended to the append log file. Very very slow, very safe.</li><li> Fsync() one time every second. Fast enough, and you can lose 1 second of data if there is a disaster.</li><li> Never fsync(), just put your data in the hands of the Operating System. The faster and unsafer method.</li></ul>
Warning: by default Redis will fsync() after <b>every command</b>! This is because the Redis authors want to ship a default configuration that is the safest pick. But the best compromise for most datasets is to fsync() one time every second.
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<!-- This is a (PRE) block. Make sure it's left aligned or your toc title will be off. -->
<b>AuthCommand: Contents</b><br>&nbsp;&nbsp;<a href="#AUTH _password_">AUTH _password_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1 class="wikiname">AuthCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="ConnectionHandlingSidebar.html">ConnectionHandlingSidebar</a><h1><a name="AUTH _password_">AUTH _password_</a></h1><blockquote>Request for authentication in a password protected Redis server.A Redis server can be instructed to require a password before to allow clientsto issue commands. This is done using the <i>requirepass</i> directive in theRedis configuration file.</blockquote>
<blockquote>If the password given by the client is correct the server replies withan OK status code reply and starts accepting commands from the client.Otherwise an error is returned and the clients needs to try a new password.Note that for the high performance nature of Redis it is possible to trya lot of passwords in parallel in very short time, so make sure to generatea strong and very long password so that this attack is infeasible.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Status code reply</a>
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<b>Benchmarks: Contents</b><br>&nbsp;&nbsp;<a href="#How Fast is Redis?">How Fast is Redis?</a><br>&nbsp;&nbsp;<a href="#Latency percentiles">Latency percentiles</a>
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<h1 class="wikiname">Benchmarks</h1>
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<h1><a name="How Fast is Redis?">How Fast is Redis?</a></h1>Redis includes the <code name="code" class="python">redis-benchmark</code> utility that simulates <a href="SETs.html">SETs</a>/GETs done by N clients at the same time sending M total queries (it is similar to the Apache's <code name="code" class="python">ab</code> utility). Below you'll find the full output of the benchmark executed against a Linux box.<br/><br/><ul><li> The test was done with 50 simultaneous clients performing 100000 requests.</li><li> The value SET and GET is a 256 bytes string.</li><li> The Linux box is running <b>Linux 2.6</b>, it's <b>Xeon X3320 2.5Ghz</b>.</li><li> Text executed using the loopback interface (127.0.0.1).</li></ul>
Results: <b>about 110000 <a href="SETs.html">SETs</a> per second, about 81000 GETs per second.</b><h1><a name="Latency percentiles">Latency percentiles</a></h1><pre class="codeblock python" name="code">
./redis-benchmark -n 100000
====== SET ======
100007 requests completed in 0.88 seconds
50 parallel clients
3 bytes payload
keep alive: 1
58.50% &lt;= 0 milliseconds
99.17% &lt;= 1 milliseconds
99.58% &lt;= 2 milliseconds
99.85% &lt;= 3 milliseconds
99.90% &lt;= 6 milliseconds
100.00% &lt;= 9 milliseconds
114293.71 requests per second
====== GET ======
100000 requests completed in 1.23 seconds
50 parallel clients
3 bytes payload
keep alive: 1
43.12% &lt;= 0 milliseconds
96.82% &lt;= 1 milliseconds
98.62% &lt;= 2 milliseconds
100.00% &lt;= 3 milliseconds
81234.77 requests per second
====== INCR ======
100018 requests completed in 1.46 seconds
50 parallel clients
3 bytes payload
keep alive: 1
32.32% &lt;= 0 milliseconds
96.67% &lt;= 1 milliseconds
99.14% &lt;= 2 milliseconds
99.83% &lt;= 3 milliseconds
99.88% &lt;= 4 milliseconds
99.89% &lt;= 5 milliseconds
99.96% &lt;= 9 milliseconds
100.00% &lt;= 18 milliseconds
68458.59 requests per second
====== LPUSH ======
100004 requests completed in 1.14 seconds
50 parallel clients
3 bytes payload
keep alive: 1
62.27% &lt;= 0 milliseconds
99.74% &lt;= 1 milliseconds
99.85% &lt;= 2 milliseconds
99.86% &lt;= 3 milliseconds
99.89% &lt;= 5 milliseconds
99.93% &lt;= 7 milliseconds
99.96% &lt;= 9 milliseconds
100.00% &lt;= 22 milliseconds
100.00% &lt;= 208 milliseconds
88109.25 requests per second
====== LPOP ======
100001 requests completed in 1.39 seconds
50 parallel clients
3 bytes payload
keep alive: 1
54.83% &lt;= 0 milliseconds
97.34% &lt;= 1 milliseconds
99.95% &lt;= 2 milliseconds
99.96% &lt;= 3 milliseconds
99.96% &lt;= 4 milliseconds
100.00% &lt;= 9 milliseconds
100.00% &lt;= 208 milliseconds
71994.96 requests per second
</pre>Notes: changing the payload from 256 to 1024 or 4096 bytes does not change the numbers significantly (but reply packets are glued together up to 1024 bytes so GETs may be slower with big payloads). The same for the number of clients, from 50 to 256 clients I got the same numbers. With only 10 clients it starts to get a bit slower.<br/><br/>You can expect different results from different boxes. For example a low profile box like <b>Intel core duo T5500 clocked at 1.66Ghz running Linux 2.6</b> will output the following:
<pre class="codeblock python python" name="code">
./redis-benchmark -q -n 100000
SET: 53684.38 requests per second
GET: 45497.73 requests per second
INCR: 39370.47 requests per second
LPUSH: 34803.41 requests per second
LPOP: 37367.20 requests per second
</pre>Another one using a 64 bit box, a Xeon L5420 clocked at 2.5 Ghz:<br/><br/><pre class="codeblock python python python" name="code">
./redis-benchmark -q -n 100000
PING: 111731.84 requests per second
SET: 108114.59 requests per second
GET: 98717.67 requests per second
INCR: 95241.91 requests per second
LPUSH: 104712.05 requests per second
LPOP: 93722.59 requests per second
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<!-- This is a (PRE) block. Make sure it's left aligned or your toc title will be off. -->
<b>BgrewriteaofCommand: Contents</b><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="#BGREWRITEAOF">BGREWRITEAOF</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1 class="wikiname">BgrewriteaofCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="ControlCommandsSidebar.html">ControlCommandsSidebar</a><h3><a name="BGREWRITEAOF">BGREWRITEAOF</a></h3>
<blockquote>Please for detailed information about the Redis Append Only File check<a href="AppendOnlyFileHowto.html">the Append Only File Howto</a>.</blockquote>
<blockquote>BGREWRITEAOF rewrites the Append Only File in background when it gets toobig. The Redis Append Only File is a Journal, so every operation modifyingthe dataset is logged in the Append Only File (and replayed at startup).This means that the Append Only File always grows. In order to rebuildits content the BGREWRITEAOF creates a new version of the append only filestarting directly form the dataset in memory in order to guarantee thegeneration of the minimal number of commands needed to rebuild the database.</blockquote>
<blockquote>The <a href="AppendOnlyFileHowto.html">Append Only File Howto</a> contains further details.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Status code reply</a>
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<!-- This is a (PRE) block. Make sure it's left aligned or your toc title will be off. -->
<b>BgsaveCommand: Contents</b><br>&nbsp;&nbsp;<a href="#BGSAVE">BGSAVE</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1 class="wikiname">BgsaveCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="ControlCommandsSidebar.html">ControlCommandsSidebar</a><h1><a name="BGSAVE">BGSAVE</a></h1>
<blockquote>Save the DB in background. The OK code is immediately returned.Redis forks, the parent continues to server the clients, the childsaves the DB on disk then exit. A client my be able to check if theoperation succeeded using the <a href="LastsaveCommand.html">LASTSAVE</a> command.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Status code reply</a>
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<!-- This is a (PRE) block. Make sure it's left aligned or your toc title will be off. -->
<b>BlpopCommand: Contents</b><br>&nbsp;&nbsp;<a href="#BLPOP _key1_ _key2_ ... _keyN_ _timeout_ (Redis &gt;">BLPOP _key1_ _key2_ ... _keyN_ _timeout_ (Redis &gt;</a><br>&nbsp;&nbsp;<a href="#BRPOP _key1_ _key2_ ... _keyN_ _timeout_ (Redis &gt;">BRPOP _key1_ _key2_ ... _keyN_ _timeout_ (Redis &gt;</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Non blocking behavior">Non blocking behavior</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Blocking behavior">Blocking behavior</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Multiple clients blocking for the same keys">Multiple clients blocking for the same keys</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1 class="wikiname">BlpopCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="ListCommandsSidebar.html">ListCommandsSidebar</a><h1><a name="BLPOP _key1_ _key2_ ... _keyN_ _timeout_ (Redis &gt;">BLPOP _key1_ _key2_ ... _keyN_ _timeout_ (Redis &gt;</a></h1> 1.3.1) =
<h1><a name="BRPOP _key1_ _key2_ ... _keyN_ _timeout_ (Redis &gt;">BRPOP _key1_ _key2_ ... _keyN_ _timeout_ (Redis &gt;</a></h1> 1.3.1) =
<i>Time complexity: O(1)</i><blockquote>BLPOP (and BRPOP) is a blocking list pop primitive. You can see this commandsas blocking versions of <a href="LpopCommand.html">LPOP</a> and <a href="LpopCommand.html">RPOP</a> able toblock if the specified keys don't exist or contain empty lists.</blockquote>
<blockquote>The following is a description of the exact semantic. We describe BLPOP butthe two commands are identical, the only difference is that BLPOP pops theelement from the left (head) of the list, and BRPOP pops from the right (tail).</blockquote>
<h2><a name="Non blocking behavior">Non blocking behavior</a></h2><blockquote>When BLPOP is called, if at least one of the specified keys contain a nonempty list, an element is popped from the head of the list and returned tothe caller together with the name of the key (BLPOP returns a two elementsarray, the first element is the key, the second the popped value).</blockquote>
<blockquote>Keys are scanned from left to right, so for instance if youissue <b>BLPOP list1 list2 list3 0</b> against a dataset where <b>list1</b> does notexist but <b>list2</b> and <b>list3</b> contain non empty lists, BLPOP guaranteesto return an element from the list stored at <b>list2</b> (since it is the firstnon empty list starting from the left).</blockquote>
<h2><a name="Blocking behavior">Blocking behavior</a></h2><blockquote>If none of the specified keys exist or contain non empty lists, BLPOPblocks until some other client performs a <a href="RpushCommand.html">LPUSH</a> oran <a href="RpushCommand.html">RPUSH</a> operation against one of the lists.</blockquote>
<blockquote>Once new data is present on one of the lists, the client finally returnswith the name of the key unblocking it and the popped value.</blockquote>
<blockquote>When blocking, if a non-zero timeout is specified, the client will unblockreturning a nil special value if the specified amount of seconds passedwithout a push operation against at least one of the specified keys.</blockquote>
<blockquote>A timeout of zero means instead to block forever.</blockquote>
<h2><a name="Multiple clients blocking for the same keys">Multiple clients blocking for the same keys</a></h2><blockquote>Multiple clients can block for the same key. They are put intoa queue, so the first to be served will be the one that started to waitearlier, in a first-blpopping first-served fashion.</blockquote>
<h2><a name="Return value">Return value</a></h2><blockquote>BLPOP returns a two-elements array via a multi bulk reply in order to returnboth the unblocking key and the popped value.</blockquote>
<blockquote>When a non-zero timeout is specified, and the BLPOP operation timed out,the return value is a nil multi bulk reply. Most client values will returnfalse or nil accordingly to the programming language used.</blockquote>
<a href="ReplyTypes.html">Multi bulk reply</a>
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<!-- This is a (PRE) block. Make sure it's left aligned or your toc title will be off. -->
<b>CommandReference: Contents</b><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Connection handling">Connection handling</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Commands operating on all the kind of values">Commands operating on all the kind of values</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Commands operating on string values">Commands operating on string values</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Commands operating on lists">Commands operating on lists</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Commands operating on sets">Commands operating on sets</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Commands operating on sorted sets (zsets, Redis version &gt;">Commands operating on sorted sets (zsets, Redis version &gt;</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Commands operating on hashes">Commands operating on hashes</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Sorting">Sorting</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Transactions">Transactions</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Publish/Subscribe">Publish/Subscribe</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Persistence control commands">Persistence control commands</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Remote server control commands">Remote server control commands</a>
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<h1 class="wikiname">CommandReference</h1>
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&iuml;&raquo;&iquest;= Redis Command Reference =<br/><br/>Every command name links to a specific wiki page describing the behavior of the command.<h2><a name="Connection handling">Connection handling</a></h2><ul><li> <a href="QuitCommand.html">QUIT</a> <code name="code" class="python">close the connection</code></li><li> <a href="AuthCommand.html">AUTH</a> <code name="code" class="python">simple password authentication if enabled</code></li></ul>
<h2><a name="Commands operating on all the kind of values">Commands operating on all the kind of values</a></h2><ul><li> <a href="ExistsCommand.html">EXISTS</a> <i>key</i> <code name="code" class="python">test if a key exists</code></li><li> <a href="DelCommand.html">DEL</a> <i>key</i> <code name="code" class="python">delete a key</code></li><li> <a href="TypeCommand.html">TYPE</a> <i>key</i> <code name="code" class="python">return the type of the value stored at key</code></li><li> <a href="KeysCommand.html">KEYS</a> <i>pattern</i> <code name="code" class="python">return all the keys matching a given pattern</code></li><li> <a href="RandomkeyCommand.html">RANDOMKEY</a> <code name="code" class="python">return a random key from the key space</code></li><li> <a href="RenameCommand.html">RENAME</a> <i>oldname</i> <i>newname</i> <code name="code" class="python">rename the old key in the new one, destroing the newname key if it already exists</code></li><li> <a href="RenamenxCommand.html">RENAMENX</a> <i>oldname</i> <i>newname</i> <code name="code" class="python">rename the old key in the new one, if the newname key does not already exist</code></li><li> <a href="DbsizeCommand.html">DBSIZE</a> <code name="code" class="python">return the number of keys in the current db</code></li><li> <a href="ExpireCommand.html">EXPIRE</a> <code name="code" class="python">set a time to live in seconds on a key</code></li><li> <a href="TtlCommand.html">TTL</a> <code name="code" class="python">get the time to live in seconds of a key</code></li><li> <a href="SelectCommand.html">SELECT</a> <i>index</i> <code name="code" class="python">Select the DB having the specified index</code></li><li> <a href="MoveCommand.html">MOVE</a> <i>key</i> <i>dbindex</i> <code name="code" class="python">Move the key from the currently selected DB to the DB having as index dbindex</code></li><li> <a href="FlushdbCommand.html">FLUSHDB</a> <code name="code" class="python">Remove all the keys of the currently selected DB</code></li><li> <a href="FlushallCommand.html">FLUSHALL</a> <code name="code" class="python">Remove all the keys from all the databases</code></li></ul>
<h2><a name="Commands operating on string values">Commands operating on string values</a></h2><ul><li> <a href="SetCommand.html">SET</a> <i>key</i> <i>value</i> <code name="code" class="python">set a key to a string value</code></li><li> <a href="GetCommand.html">GET</a> <i>key</i> <code name="code" class="python">return the string value of the key</code></li><li> <a href="GetsetCommand.html">GETSET</a> <i>key</i> <i>value</i> <code name="code" class="python">set a key to a string returning the old value of the key</code></li><li> <a href="MgetCommand.html">MGET</a> <i>key1</i> <i>key2</i> ... <i>keyN</i> <code name="code" class="python">multi-get, return the strings values of the keys</code></li><li> <a href="SetnxCommand.html">SETNX</a> <i>key</i> <i>value</i> <code name="code" class="python">set a key to a string value if the key does not exist</code></li><li> <a href="SetexCommand.html">SETEX</a> <i>key</i> <i>time</i> <i>value</i> <code name="code" class="python">Set+Expire combo command</code></li><li> <a href="MsetCommand.html">MSET</a> <i>key1</i> <i>value1</i> <i>key2</i> <i>value2</i> ... <i>keyN</i> <i>valueN</i> <code name="code" class="python">set a multiple keys to multiple values in a single atomic operation</code></li><li> <a href="MsetCommand.html">MSETNX</a> <i>key1</i> <i>value1</i> <i>key2</i> <i>value2</i> ... <i>keyN</i> <i>valueN</i> <code name="code" class="python">set a multiple keys to multiple values in a single atomic operation if none of the keys already exist</code></li><li> <a href="IncrCommand.html">INCR</a> <i>key</i> <code name="code" class="python">increment the integer value of key</code></li><li> <a href="IncrCommand.html">INCRBY</a> <i>key</i> <i>integer</i><code name="code" class="python"> increment the integer value of key by integer</code></li><li> <a href="IncrCommand.html">DECR</a> <i>key</i> <code name="code" class="python">decrement the integer value of key</code></li><li> <a href="IncrCommand.html">DECRBY</a> <i>key</i> <i>integer</i> <code name="code" class="python">decrement the integer value of key by integer</code></li><li> <a href="AppendCommand.html">APPEND</a> <i>key</i> <i>value</i> <code name="code" class="python">append the specified string to the string stored at key</code></li><li> <a href="SubstrCommand.html">SUBSTR</a> <i>key</i> <i>start</i> <i>end</i> <code name="code" class="python">return a substring out of a larger string</code></li></ul>
<h2><a name="Commands operating on lists">Commands operating on lists</a></h2><ul><li> <a href="RpushCommand.html">RPUSH</a> <i>key</i> <i>value</i> <code name="code" class="python">Append an element to the tail of the List value at key</code></li><li> <a href="RpushCommand.html">LPUSH</a> <i>key</i> <i>value</i> <code name="code" class="python">Append an element to the head of the List value at key</code></li><li> <a href="LlenCommand.html">LLEN</a> <i>key</i> <code name="code" class="python">Return the length of the List value at key</code></li><li> <a href="LrangeCommand.html">LRANGE</a> <i>key</i> <i>start</i> <i>end</i> <code name="code" class="python">Return a range of elements from the List at key</code></li><li> <a href="LtrimCommand.html">LTRIM</a> <i>key</i> <i>start</i> <i>end</i> <code name="code" class="python">Trim the list at key to the specified range of elements</code></li><li> <a href="LindexCommand.html">LINDEX</a> <i>key</i> <i>index</i> <code name="code" class="python">Return the element at index position from the List at key</code></li><li> <a href="LsetCommand.html">LSET</a> <i>key</i> <i>index</i> <i>value</i> <code name="code" class="python">Set a new value as the element at index position of the List at key</code></li><li> <a href="LremCommand.html">LREM</a> <i>key</i> <i>count</i> <i>value</i> <code name="code" class="python">Remove the first-N, last-N, or all the elements matching value from the List at key</code></li><li> <a href="LpopCommand.html">LPOP</a> <i>key</i> <code name="code" class="python">Return and remove (atomically) the first element of the List at key</code></li><li> <a href="LpopCommand.html">RPOP</a> <i>key</i> <code name="code" class="python">Return and remove (atomically) the last element of the List at key</code></li><li> <a href="BlpopCommand.html">BLPOP</a> <i>key1</i> <i>key2</i> ... <i>keyN</i> <i>timeout</i> <code name="code" class="python">Blocking LPOP</code></li><li> <a href="BlpopCommand.html">BRPOP</a> <i>key1</i> <i>key2</i> ... <i>keyN</i> <i>timeout</i> <code name="code" class="python">Blocking RPOP</code></li><li> <a href="RpoplpushCommand.html">RPOPLPUSH</a> <i>srckey</i> <i>dstkey</i> <code name="code" class="python">Return and remove (atomically) the last element of the source List stored at _srckey_ and push the same element to the destination List stored at _dstkey_</code></li></ul>
<h2><a name="Commands operating on sets">Commands operating on sets</a></h2><ul><li> <a href="SaddCommand.html">SADD</a> <i>key</i> <i>member</i> <code name="code" class="python">Add the specified member to the Set value at key</code></li><li> <a href="SremCommand.html">SREM</a> <i>key</i> <i>member</i> <code name="code" class="python">Remove the specified member from the Set value at key</code></li><li> <a href="SpopCommand.html">SPOP</a> <i>key</i> <code name="code" class="python">Remove and return (pop) a random element from the Set value at key</code></li><li> <a href="SmoveCommand.html">SMOVE</a> <i>srckey</i> <i>dstkey</i> <i>member</i> <code name="code" class="python">Move the specified member from one Set to another atomically</code></li><li> <a href="ScardCommand.html">SCARD</a> <i>key</i> <code name="code" class="python">Return the number of elements (the cardinality) of the Set at key</code></li><li> <a href="SismemberCommand.html">SISMEMBER</a> <i>key</i> <i>member</i> <code name="code" class="python">Test if the specified value is a member of the Set at key</code></li><li> <a href="SinterCommand.html">SINTER</a> <i>key1</i> <i>key2</i> ... <i>keyN</i> <code name="code" class="python">Return the intersection between the Sets stored at key1, key2, ..., keyN</code></li><li> <a href="SinterstoreCommand.html">SINTERSTORE</a> <i>dstkey</i> <i>key1</i> <i>key2</i> ... <i>keyN</i> <code name="code" class="python">Compute the intersection between the Sets stored at key1, key2, ..., keyN, and store the resulting Set at dstkey</code></li><li> <a href="SunionCommand.html">SUNION</a> <i>key1</i> <i>key2</i> ... <i>keyN</i> <code name="code" class="python">Return the union between the Sets stored at key1, key2, ..., keyN</code></li><li> <a href="SunionstoreCommand.html">SUNIONSTORE</a> <i>dstkey</i> <i>key1</i> <i>key2</i> ... <i>keyN</i> <code name="code" class="python">Compute the union between the Sets stored at key1, key2, ..., keyN, and store the resulting Set at dstkey</code></li><li> <a href="SdiffCommand.html">SDIFF</a> <i>key1</i> <i>key2</i> ... <i>keyN</i> <code name="code" class="python">Return the difference between the Set stored at key1 and all the Sets key2, ..., keyN</code></li><li> <a href="SdiffstoreCommand.html">SDIFFSTORE</a> <i>dstkey</i> <i>key1</i> <i>key2</i> ... <i>keyN</i> <code name="code" class="python">Compute the difference between the Set key1 and all the Sets key2, ..., keyN, and store the resulting Set at dstkey</code></li><li> <a href="SmembersCommand.html">SMEMBERS</a> <i>key</i> <code name="code" class="python">Return all the members of the Set value at key</code></li><li> <a href="SrandmemberCommand.html">SRANDMEMBER</a> <i>key</i> <code name="code" class="python">Return a random member of the Set value at key</code></li></ul>
<h2><a name="Commands operating on sorted sets (zsets, Redis version &gt;">Commands operating on sorted sets (zsets, Redis version &gt;</a></h2> 1.1) ==<br/><br/><ul><li> <a href="ZaddCommand.html">ZADD</a> <i>key</i> <i>score</i> <i>member</i> <code name="code" class="python">Add the specified member to the Sorted Set value at key or update the score if it already exist</code></li><li> <a href="ZremCommand.html">ZREM</a> <i>key</i> <i>member</i> <code name="code" class="python">Remove the specified member from the Sorted Set value at key</code></li><li> <a href="ZincrbyCommand.html">ZINCRBY</a> <i>key</i> <i>increment</i> <i>member</i> <code name="code" class="python">If the member already exists increment its score by _increment_, otherwise add the member setting _increment_ as score</code></li><li> <a href="ZrankCommand.html">ZRANK</a> <i>key</i> <i>member</i> <code name="code" class="python">Return the rank (or index) or _member_ in the sorted set at _key_, with scores being ordered from low to high</code></li><li> <a href="ZrankCommand.html">ZREVRANK</a> <i>key</i> <i>member</i> <code name="code" class="python">Return the rank (or index) or _member_ in the sorted set at _key_, with scores being ordered from high to low</code></li><li> <a href="ZrangeCommand.html">ZRANGE</a> <i>key</i> <i>start</i> <i>end</i> <code name="code" class="python">Return a range of elements from the sorted set at key</code></li><li> <a href="ZrangeCommand.html">ZREVRANGE</a> <i>key</i> <i>start</i> <i>end</i> <code name="code" class="python">Return a range of elements from the sorted set at key, exactly like ZRANGE, but the sorted set is ordered in traversed in reverse order, from the greatest to the smallest score</code></li><li> <a href="ZrangebyscoreCommand.html">ZRANGEBYSCORE</a> <i>key</i> <i>min</i> <i>max</i> <code name="code" class="python">Return all the elements with score &gt;= min and score &lt;= max (a range query) from the sorted set</code></li><li> <a href="ZcardCommand.html">ZCARD</a> <i>key</i> <code name="code" class="python">Return the cardinality (number of elements) of the sorted set at key</code></li><li> <a href="ZscoreCommand.html">ZSCORE</a> <i>key</i> <i>element</i> <code name="code" class="python">Return the score associated with the specified element of the sorted set at key</code></li><li> <a href="ZremrangebyrankCommand.html">ZREMRANGEBYRANK</a> <i>key</i> <i>min</i> <i>max</i> <code name="code" class="python">Remove all the elements with rank &gt;= min and rank &lt;= max from the sorted set</code></li><li> <a href="ZremrangebyscoreCommand.html">ZREMRANGEBYSCORE</a> <i>key</i> <i>min</i> <i>max</i> <code name="code" class="python">Remove all the elements with score &gt;= min and score &lt;= max from the sorted set</code></li><li> <a href="ZunionstoreCommand.html">ZUNIONSTORE / ZINTERSTORE</a> <i>dstkey</i> <i>N</i> <i>key1</i> ... <i>keyN</i> WEIGHTS <i>w1</i> ... <i>wN</i> AGGREGATE SUM|MIN|MAX <code name="code" class="python">Perform a union or intersection over a number of sorted sets with optional weight and aggregate</code></li></ul>
<h2><a name="Commands operating on hashes">Commands operating on hashes</a></h2><ul><li> <a href="HsetCommand.html">HSET</a> <i>key</i> <i>field</i> <i>value</i> <code name="code" class="python">Set the hash field to the specified value. Creates the hash if needed.</code></li><li> <a href="HgetCommand.html">HGET</a> <i>key</i> <i>field</i> <code name="code" class="python">Retrieve the value of the specified hash field.</code></li><li> <a href="HmsetCommand.html">HMSET</a> <i>key</i> <i>field1</i> <i>value1</i> ... <i>fieldN</i> <i>valueN</i> <code name="code" class="python">Set the hash fields to their respective values.</code></li><li> <a href="HincrbyCommand.html">HINCRBY</a> <i>key</i> <i>field</i> <i>integer</i> <code name="code" class="python">Increment the integer value of the hash at _key_ on _field_ with _integer_.</code></li><li> <a href="HexistsCommand.html">HEXISTS</a> <i>key</i> <i>field</i> <code name="code" class="python">Test for existence of a specified field in a hash</code></li><li> <a href="HdelCommand.html">HDEL</a> <i>key</i> <i>field</i> <code name="code" class="python">Remove the specified field from a hash</code></li><li> <a href="HlenCommand.html">HLEN</a> <i>key</i> <code name="code" class="python">Return the number of items in a hash.</code></li><li> <a href="HgetallCommand.html">HKEYS</a> <i>key</i> <code name="code" class="python">Return all the fields in a hash.</code></li><li> <a href="HgetallCommand.html">HVALS</a> <i>key</i> <code name="code" class="python">Return all the values in a hash.</code></li><li> <a href="HgetallCommand.html">HGETALL</a> <i>key</i> <code name="code" class="python">Return all the fields and associated values in a hash.</code></li></ul>
<h2><a name="Sorting">Sorting</a></h2><ul><li> <a href="SortCommand.html">SORT</a> <i>key</i> BY <i>pattern</i> LIMIT <i>start</i> <i>end</i> GET <i>pattern</i> ASC|DESC ALPHA <code name="code" class="python">Sort a Set or a List accordingly to the specified parameters</code></li></ul>
<h2><a name="Transactions">Transactions</a></h2><ul><li> <a href="MultiExecCommand.html">MULTI/EXEC/DISCARD</a> <code name="code" class="python">Redis atomic transactions</code></li></ul>
<h2><a name="Publish/Subscribe">Publish/Subscribe</a></h2><ul><li> <a href="PublishSubscribe.html">SUBSCRIBE/UNSUBSCRIBE/PUBLISH</a> <code name="code" class="python">Redis Public/Subscribe messaging paradigm implementation</code></li></ul>
<h2><a name="Persistence control commands">Persistence control commands</a></h2><ul><li> <a href="SaveCommand.html">SAVE</a> <code name="code" class="python">Synchronously save the DB on disk</code></li><li> <a href="BgsaveCommand.html">BGSAVE</a> <code name="code" class="python">Asynchronously save the DB on disk</code></li><li> <a href="LastsaveCommand.html">LASTSAVE</a> <code name="code" class="python">Return the UNIX time stamp of the last successfully saving of the dataset on disk</code></li><li> <a href="ShutdownCommand.html">SHUTDOWN</a> <code name="code" class="python">Synchronously save the DB on disk, then shutdown the server</code></li><li> <a href="BgrewriteaofCommand.html">BGREWRITEAOF</a> <code name="code" class="python">Rewrite the append only file in background when it gets too big</code></li></ul>
<h2><a name="Remote server control commands">Remote server control commands</a></h2><ul><li> <a href="InfoCommand.html">INFO</a> <code name="code" class="python">Provide information and statistics about the server</code></li><li> <a href="MonitorCommand.html">MONITOR</a> <code name="code" class="python">Dump all the received requests in real time</code></li><li> <a href="SlaveofCommand.html">SLAVEOF</a> <code name="code" class="python">Change the replication settings</code></li><li> <a href="ConfigCommand.html">CONFIG</a> <code name="code" class="python">Configure a Redis server at runtime</code></li></ul>
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<b>Comparisons: Contents</b><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Memcached">Memcached</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Tokyo Cabinet / Toyo Tyrant">Tokyo Cabinet / Toyo Tyrant</a>
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<h1 class="wikiname">Comparisons</h1>
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&iuml;&raquo;&iquest;if your are asking yourself how is Redis different fom other key-value stores here you will find it compared to some of the most popular contendors (all great software) in this category. <h2><a name="Memcached">Memcached</a></h2><ul><li> Memcached is not persistent, it just holds everything in memory without saving since its main goal is to be used as a cache, while Redis is <a href="Persistence.html">persistent</a>.</li></ul>
<ul><li> Like memcached Redis uses a key-value model, but while keys can just be strings, values in Redis can be <a href="Lists.html">Lists</a>, <a href="Sets.html">Sets</a> or <a href="SortedSets.html">SortedSets</a> and complex operations like intersections, set/get n-th element of lists, pop/push of elements, can be performed against sets and lists.</li></ul>
<h2><a name="Tokyo Cabinet / Toyo Tyrant">Tokyo Cabinet / Toyo Tyrant</a></h2>Redis and Tokyo Cabinet can be used for the same applications, but actually they are <i>very different</i> beasts. If you read Twitter messages of people involved in scalable things both products are reported to work well, but surely there are times where one or the other can be the best choice.<br/><br/><ul><li> Tokyo Cabinet writes synchronously on disk, Redis takes the whole dataset on memory and writes on disk asynchronously. Tokyo Cabinet is safer and probably a better idea if your dataset is going to be bigger than RAM, but Redis is faster (note that Redis supports master-slave replication that is trivial to setup, so you are safe anyway if you want a setup where data can't be lost even after a disaster). </li></ul>
<ul><li> Redis supports higher level operations and data structures. Tokyo Cabinet supports a kind of database that is able to organize data into rows with named fields (in a way very similar to Berkeley DB) but can't do things like server side List and Set operations Redis is able to do: pushing or popping from Lists in an atomic way, in O(1) time complexity, server side Set intersections, Sorting of schema free data in complex ways (By the way TC supports sorting in the table-based database format). Redis on the other hand does not support the abstraction of tables with fields, the idea is that you can build this stuff in software easily if you really need a table-alike approach. </li></ul>
<ul><li> Tokyo Cabinet does not implement a networking layer. You have to use a networking layer called Tokyo Tyrant that interfaces to Tokyo Cabinet so you can talk to Tokyo Cabinet in a client-server fashion. In Redis the networking support is built-in inside the server, and is basically the only interface between the external world and the dataset. </li></ul>
<ul><li> Redis is reported to be much faster, especially if you plan to access Tokyo Cabinet via Tokyo Tyrant. Here I can only say that with Redis you can expect 100,000 operations/seconds with a normal Linux box and 50 concurrent clients. You should test Redis, Tokyo, and the other alternatives with your specific work load to get a feeling about performances for your application. </li></ul>
<ul><li> Redis is not an on-disk DB engine like Tokyo: the latter can be used as a fast DB engine in your C project without the networking overhead just linking to the library. Still in many scalable applications you need multiple servers talking with multiple clients, so the client-server model is almost always needed, this is why in Redis this is built-in. </li></ul>
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<b>ConfigCommand: Contents</b><br>&nbsp;&nbsp;<a href="#CONFIG GET _pattern_ (Redis &gt;">CONFIG GET _pattern_ (Redis &gt;</a><br>&nbsp;&nbsp;<a href="#CONFIG SET _parameter_ _value_ (Redis &gt;">CONFIG SET _parameter_ _value_ (Redis &gt;</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#CONFIG GET _pattern_">CONFIG GET _pattern_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#CONFIG SET _parameter_ _value_">CONFIG SET _parameter_ _value_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Parameters value format">Parameters value format</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#See Also">See Also</a>
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<h1 class="wikiname">ConfigCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="ControlCommandsSidebar.html">ControlCommandsSidebar</a><h1><a name="CONFIG GET _pattern_ (Redis &gt;">CONFIG GET _pattern_ (Redis &gt;</a></h1> 2.0)=
<h1><a name="CONFIG SET _parameter_ _value_ (Redis &gt;">CONFIG SET _parameter_ _value_ (Redis &gt;</a></h1> 2.0)=<br/><br/><blockquote>The CONFIG command is able to retrieve or alter the configuration of a runningRedis server. Not all the configuration parameters are supported.</blockquote>
<blockquote>CONFIG has two sub commands, GET and SET. The GET command is used to readthe configuration, while the SET command is used to alter the configuration.</blockquote>
<h2><a name="CONFIG GET _pattern_">CONFIG GET _pattern_</a></h2><blockquote>CONFIG GET returns the current configuration parameters. This sub commandonly accepts a single argument, that is glob style pattern. All theconfiguration parameters matching this parameter are reported as alist of key-value pairs. Example:</blockquote><pre class="codeblock python" name="code">
$ redis-cli config get '*'
1. &quot;dbfilename&quot;
2. &quot;dump.rdb&quot;
3. &quot;requirepass&quot;
4. (nil)
5. &quot;masterauth&quot;
6. (nil)
7. &quot;maxmemory&quot;
8. &quot;0\n&quot;
9. &quot;appendfsync&quot;
10. &quot;everysec&quot;
11. &quot;save&quot;
12. &quot;3600 1 300 100 60 10000&quot;
$ redis-cli config get 'm*'
1. &quot;masterauth&quot;
2. (nil)
3. &quot;maxmemory&quot;
4. &quot;0\n&quot;
</pre>The return type of the command is a <a href="ReplyTypes.html">bulk reply</a>.<h2><a name="CONFIG SET _parameter_ _value_">CONFIG SET _parameter_ _value_</a></h2><blockquote>CONFIG SET is used in order to reconfigure the server, setting a specificconfiguration parameter to a new value.</blockquote>
<blockquote>The list of configuration parameters supported by CONFIG SET can beobtained issuing a <code name="code" class="python">CONFIG GET *</code> command.</blockquote>
<blockquote>The configuration set using CONFIG SET is immediately loaded by the Redisserver that will start acting as specified starting from the next command.</blockquote>
<blockquote>Example:</blockquote><pre class="codeblock python python" name="code">
$ ./redis-cli
redis&gt; set x 10
OK
redis&gt; config set maxmemory 200
OK
redis&gt; set y 20
(error) ERR command not allowed when used memory &gt; 'maxmemory'
redis&gt; config set maxmemory 0
OK
redis&gt; set y 20
OK
</pre><h2><a name="Parameters value format">Parameters value format</a></h2><blockquote>The value of the configuration parameter is the same as the one of thesame parameter in the Redis configuration file, with the following exceptions:</blockquote>
<ul><li> The <code name="code" class="python">save</code> paramter is a list of space-separated integers. Every pair of integers specify the time and number of changes limit to trigger a save. For instance the command <code name="code" class="python">CONFIG SET save &quot;3600 10 60 10000&quot;</code> will configure the server to issue a background saving of the RDB file every 3600 seconds if there are at least 10 changes in the dataset, and every 60 seconds if there are at least 10000 changes. To completely disable automatic snapshots just set the parameter as an empty string.</li><li> All the integer parameters representing memory are returned and accepted only using bytes as unit.</li></ul>
<h2><a name="See Also">See Also</a></h2>The <a href="InfoCommand.html">INFO</a> command can be used in order to read configuriaton parameters that are not available in the CONFIG command.
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The <code name="code" class="python">redis.conf</code> file included in the source code distribution is a starting point, you should be able to modify it in order do adapt it to your needs without troubles reading the comments inside the file.<br/><br/>In order to start Redis using a configuration file just pass the file name as the sole argument when starting the server:<br/><br/><pre class="codeblock python" name="code">
$ ./redis-server redis.conf
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&iuml;&raquo;&iquest;== Connection handling ==<br/><br/><ul><li> <a href="QuitCommand.html">QUIT</a></li><li> <a href="AuthCommand.html">AUTH</a></li></ul>
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&iuml;&raquo;&iquest;== Control Commands ==<br/><br/><ul><li> <a href="SaveCommand.html">SAVE</a></li><li> <a href="BgsaveCommand.html">BGSAVE</a></li><li> <a href="BgrewriteaofCommand.html">BGREWRITEAOF</a></li><li> <a href="LastsaveCommand.html">LASTSAVE</a></li><li> <a href="ShutdownCommand.html">SHUTDOWN</a></li><li> <a href="InfoCommand.html">INFO</a></li><li> <a href="MonitorCommand.html">MONITOR</a></li><li> <a href="SlaveofCommand.html">SLAVEOF</a></li></ul>
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<h1><a name="Credits">Credits</a></h1><ul><li> The Redis server was designed and written by <a href="http://invece.org" target="_blank">Salvatore Sanfilippo (aka antirez)</a></li><li> <a href="http://brainspl.at/" target="_blank">Ezra Zygmuntowicz (aka ezmobius)</a> - Ruby client lib initial version and hacking</li><li> <a href="http://qix.it" target="_blank">Ludovico Magnocavallo (aka ludo)</a> - Python clinet lib</li><li> <a href="http://www.adroll.com/" target="_blank">Valentino Volonghi of Adroll</a> - Erlang client lib</li><li> <b>brettbender</b> - found and fixed a bug in sds.c that caused the server to crash at least on 64 bit systems, and anyway to be buggy since we used the same vararg thing against vsprintf without to call va_start and va_end every time.</li><li> <a href="http://www.rot13.org/~dpavlin" target="_blank">Dobrica Pavlinusic</a> - Perl client lib</li><li> Brian Hammond - AUTH command implementation, C++ client lib</li><li> <a href="http://www.clorophilla.net/" target="_blank">Daniele Alessandri</a> - Lua client lib</li><li> Corey Stup - C99 cleanups</li><li> Taylor Weibley - Ruby client improvements</li><li> Bob Potter - Rearrange redisObject struct to reduce memory usage in 64bit environments</li><li> Luca Guidi and Brian McKinney - Ruby client improvements</li><li> Aman Gupta - SDIFF / SDIFFSTORE, other Set operations improvements, ability to disable clients timeout.</li><li> Diego Rosario Brogna - Code and ideas about dumping backtrace on sigsegv and similar error conditions.</li></ul>
p.s. sorry to take this file in sync is hard in this early days. Please drop me an email if I forgot to add your name here!
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&iuml;&raquo;&iquest;#sidebar <a href="GenericCommandsSidebar.html">GenericCommandsSidebar</a><h1><a name="DBSIZE">DBSIZE</a></h1><blockquote>Return the number of keys in the currently selected database.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Integer reply</a>
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<b>DelCommand: Contents</b><br>&nbsp;&nbsp;<a href="#DEL _key1_ _key2_ ... _keyN_">DEL _key1_ _key2_ ... _keyN_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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&iuml;&raquo;&iquest;#sidebar <a href="GenericCommandsSidebar.html">GenericCommandsSidebar</a><h1><a name="DEL _key1_ _key2_ ... _keyN_">DEL _key1_ _key2_ ... _keyN_</a></h1>
<i>Time complexity: O(1)</i><blockquote>Remove the specified keys. If a given key does not existno operation is performed for this key. The commnad returns the number ofkeys removed.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Integer reply</a>, specifically:<br/><br/><pre class="codeblock python" name="code">
an integer greater than 0 if one or more keys were removed
0 if none of the specified key existed
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Use random keys instead of incremental keys in order to avoid a single-key that gets incremented by many servers. This can can't be distributed among servers.
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<b>EventLibray: Contents</b><br>&nbsp;&nbsp;<a href="#Event Library">Event Library</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Why is an Event Library needed at all?">Why is an Event Library needed at all?</a>
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<h1 class="wikiname">EventLibray</h1>
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<h1><a name="Event Library">Event Library</a></h1><h2><a name="Why is an Event Library needed at all?">Why is an Event Library needed at all?</a></h2>Let us figure it out through a series of Q&amp;As.<br/><br/>Q: What do you expect a network server to be doing all the time? &lt;br/&gt;
A: Watch for inbound connections on the port its listening and accept them.<br/><br/>Q: Calling <a href="http://man.cx/accept%282%29" target="_blank">accept</a> yields a descriptor. What do I do with it?&lt;br/&gt;
A: Save the descriptor and do a non-blocking read/write operation on it.<br/><br/>Q: Why does the read/write have to be non-blocking?&lt;br/&gt;
A: If the file operation ( even a socket in Unix is a file ) is blocking how could the server for example accept other connection requests when its blocked in a file I/O operation.<br/><br/>Q: I guess I have to do many such non-blocking operations on the socket to see when it's ready. Am I right?&lt;br/&gt;
A: Yes. That is what an event library does for you. Now you get it.<br/><br/>Q: How do Event Libraries do what they do?&lt;br/&gt;
A: They use the operating system's <a href="http://www.devshed.com/c/a/BrainDump/Linux-Files-and-the-Event-Poll-Interface/" target="_blank">polling</a> facility along with timers.<br/><br/>Q: So are there any open source event libraries that do what you just described? &lt;br/&gt;
A: Yes. Libevent and Libev are two such event libraries that I can recall off the top of my head.<br/><br/>Q: Does Redis use such open source event libraries for handling socket I/O?&lt;br/&gt;
A: No. For various <a href="http://groups.google.com/group/redis-db/browse_thread/thread/b52814e9ef15b8d0/" target="_blank">reasons</a> Redis uses its own event library.
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<b>ExistsCommand: Contents</b><br>&nbsp;&nbsp;<a href="#EXISTS _key_">EXISTS _key_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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&iuml;&raquo;&iquest;#sidebar <a href="GenericCommandsSidebar.html">GenericCommandsSidebar</a><h1><a name="EXISTS _key_">EXISTS _key_</a></h1>
<i>Time complexity: O(1)</i><blockquote>Test if the specified key exists. The command returns&quot;0&quot; if the key exists, otherwise &quot;1&quot; is returned.Note that even keys set with an empty string as value willreturn &quot;1&quot;.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Integer reply</a>, specifically:<br/><br/><pre class="codeblock python" name="code">
1 if the key exists.
0 if the key does not exist.
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<b>ExpireCommand: Contents</b><br>&nbsp;&nbsp;<a href="#EXPIRE _key_ _seconds_">EXPIRE _key_ _seconds_</a><br>&nbsp;&nbsp;<a href="#EXPIREAT _key_ _unixtime_ (Redis &gt;">EXPIREAT _key_ _unixtime_ (Redis &gt;</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#How the expire is removed from a key">How the expire is removed from a key</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Restrictions with write operations against volatile keys">Restrictions with write operations against volatile keys</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Setting the timeout again on already volatile keys">Setting the timeout again on already volatile keys</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Enhanced Lazy Expiration algorithm">Enhanced Lazy Expiration algorithm</a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Version 1.0">Version 1.0</a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Version 1.1">Version 1.1</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#FAQ: Can you explain better why Redis deletes keys with an EXPIRE on write operations?">FAQ: Can you explain better why Redis deletes keys with an EXPIRE on write operations?</a>
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&iuml;&raquo;&iquest;#sidebar <a href="GenericCommandsSidebar.html">GenericCommandsSidebar</a><h1><a name="EXPIRE _key_ _seconds_">EXPIRE _key_ _seconds_</a></h1>
<h1><a name="EXPIREAT _key_ _unixtime_ (Redis &gt;">EXPIREAT _key_ _unixtime_ (Redis &gt;</a></h1> 1.1)=
<i>Time complexity: O(1)</i><blockquote>Set a timeout on the specified key. After the timeout the key will beautomatically delete by the server. A key with an associated timeout issaid to be <i>volatile</i> in Redis terminology.</blockquote>
<blockquote>Voltile keys are stored on disk like the other keys, the timeout is persistenttoo like all the other aspects of the dataset. Saving a dataset containingthe dataset and stopping the server does not stop the flow of time as Redisregisters on disk when the key will no longer be available as Unix time, andnot the remaining seconds.</blockquote>
<blockquote>EXPIREAT works exctly like EXPIRE but instead to get the number of secondsrepresenting the Time To Live of the key as a second argument (that is arelative way of specifing the TTL), it takes an absolute one in the form ofa UNIX timestamp (Number of seconds elapsed since 1 Gen 1970).</blockquote>
<blockquote>EXPIREAT was introduced in order to implement [Persistence append only saving mode] so that EXPIRE commands are automatically translated into EXPIREAT commands for the append only file. Of course EXPIREAT can alsoused by programmers that need a way to simply specify that a given key should expire at a given time in the future.</blockquote>
<h2><a name="How the expire is removed from a key">How the expire is removed from a key</a></h2><blockquote>When the key is set to a new value using the SET command, the INCR commandor any other command that modify the value stored at key the timeout isremoved from the key and the key becomes non volatile.</blockquote>
<h2><a name="Restrictions with write operations against volatile keys">Restrictions with write operations against volatile keys</a></h2><blockquote>Write operations like LPUSH, LSET and every other command that has theeffect of modifying the value stored at a volatile key have a special semantic:basically a volatile key is destroyed when it is target of a write operation.See for example the following usage pattern:</blockquote>
<pre class="codeblock python" name="code">
% ./redis-cli lpush mylist foobar /Users/antirez/hack/redis
OK
% ./redis-cli lpush mylist hello /Users/antirez/hack/redis
OK
% ./redis-cli expire mylist 10000 /Users/antirez/hack/redis
1
% ./redis-cli lpush mylist newelement
OK
% ./redis-cli lrange mylist 0 -1 /Users/antirez/hack/redis
1. newelement
</pre><blockquote>What happened here is that lpush against the key with a timeout set deletedthe key before to perform the operation. There is so a simple rule, writeoperations against volatile keys will destroy the key before to perform theoperation. Why Redis uses this behavior? In order to retain an importantproperty: a server that receives a given number of commands in the samesequence will end with the same dataset in memory. Without the delete-on-writesemantic what happens is that the state of the server depends on the timeof the commands to. This is not a desirable property in a distributed databasethat supports replication.</blockquote>
<h2><a name="Setting the timeout again on already volatile keys">Setting the timeout again on already volatile keys</a></h2><blockquote>Trying to call EXPIRE against a key that already has an associated timeoutwill not change the timeout of the key, but will just return 0. If insteadthe key does not have a timeout associated the timeout will be set and EXPIREwill return 1.</blockquote>
<h2><a name="Enhanced Lazy Expiration algorithm">Enhanced Lazy Expiration algorithm</a></h2><blockquote>Redis does not constantly monitor keys that are going to be expired.Keys are expired simply when some client tries to access a key, andthe key is found to be timed out.</blockquote>
<blockquote>Of course this is not enough as there are expired keys that will neverbe accessed again. This keys should be expired anyway, so once everysecond Redis test a few keys at random among keys with an expire set.All the keys that are already expired are deleted from the keyspace. </blockquote>
<h3><a name="Version 1.0">Version 1.0</a></h3><blockquote>Each time a fixed number of keys where tested (100 by default). So ifyou had a client setting keys with a very short expire faster than 100for second the memory continued to grow. When you stopped to insertnew keys the memory started to be freed, 100 keys every second in thebest conditions. Under a peak Redis continues to use more and more RAMeven if most keys are expired in each sweep.</blockquote>
<h3><a name="Version 1.1">Version 1.1</a></h3><blockquote>Each time Redis:</blockquote>
<ol><li> Tests 100 random keys from expired keys set.</li><li> Deletes all the keys found expired.</li><li> If more than 25 keys were expired, it start again from 1.</li></ol>
<blockquote>This is a trivial probabilistic algorithm, basically the assumption isthat our sample is representative of the whole key space,and we continue to expire until the percentage of keys that are likelyto be expired is under 25%</blockquote>
<blockquote>This means that at any given moment the maximum amount of keys alreadyexpired that are using memory is at max equal to max setting operations per second divided by 4.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Integer reply</a>, specifically:<br/><br/><pre class="codeblock python python" name="code">
1: the timeout was set.
0: the timeout was not set since the key already has an associated timeout, or the key does not exist.
</pre><h2><a name="FAQ: Can you explain better why Redis deletes keys with an EXPIRE on write operations?">FAQ: Can you explain better why Redis deletes keys with an EXPIRE on write operations?</a></h2>
Ok let's start with the problem:
<pre class="codeblock python python python" name="code">
redis&gt; set a 100
OK
redis&gt; expire a 360
(integer) 1
redis&gt; incr a
(integer) 1
</pre>
I set a key to the value of 100, then set an expire of 360 seconds, and then incremented the key (before the 360 timeout expired of course). The obvious result would be: 101, instead the key is set to the value of 1. Why?
There is a very important reason involving the Append Only File and Replication. Let's rework a bit hour example adding the notion of time to the mix:
<pre class="codeblock python python python python" name="code">
SET a 100
EXPIRE a 5
... wait 10 seconds ...
INCR a
</pre>
Imagine a Redis version that does not implement the &quot;Delete keys with an expire set on write operation&quot; semantic.
Running the above example with the 10 seconds pause will lead to 'a' being set to the value of 1, as it no longer exists when INCR is called 10 seconds later.<br/><br/>Instead if we drop the 10 seconds pause, the result is that 'a' is set to 101.<br/><br/>And in the practice timing changes! For instance the client may wait 10 seconds before INCR, but the sequence written in the Append Only File (and later replayed-back as fast as possible when Redis is restarted) will not have the pause. Even if we add a timestamp in the AOF, when the time difference is smaller than our timer resolution, we have a race condition.<br/><br/>The same happens with master-slave replication. Again, consider the example above: the client will use the same sequence of commands without the 10 seconds pause, but the replication link will slow down for a few seconds due to a network problem. Result? The master will contain 'a' set to 101, the slave 'a' set to 1.<br/><br/>The only way to avoid this but at the same time have reliable non time dependent timeouts on keys is to destroy volatile keys when a write operation is attempted against it.<br/><br/>After all Redis is one of the rare fully persistent databases that will give you EXPIRE. This comes to a cost :)
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<b>FlushallCommand: Contents</b><br>&nbsp;&nbsp;<a href="#FLUSHALL">FLUSHALL</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1 class="wikiname">FlushallCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="GenericCommandsSidebar.html">GenericCommandsSidebar</a><h1><a name="FLUSHALL">FLUSHALL</a></h1>
<blockquote>Delete all the keys of all the existing databases, not just the currently selected one. This command never fails.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Status code reply</a>
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<b>FlushdbCommand: Contents</b><br>&nbsp;&nbsp;<a href="#FLUSHDB">FLUSHDB</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1 class="wikiname">FlushdbCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="GenericCommandsSidebar.html">GenericCommandsSidebar</a><h1><a name="FLUSHDB">FLUSHDB</a></h1>
<blockquote>Delete all the keys of the currently selected DB. This command never fails.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Status code reply</a>
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<b>FromSqlToDataStructures: Contents</b><br>&nbsp;&nbsp;<a href="#Introduction (IDEA MORE THAN A DRAFT)">Introduction (IDEA MORE THAN A DRAFT)</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Data Structures">Data Structures</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Dude where is my SELECT statement?">Dude where is my SELECT statement?</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#LISTs">LISTs</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#SETs">SETs</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#SORT to the rescue">SORT to the rescue</a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="#SORT BY">SORT BY</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#HASHEs">HASHEs</a>
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<h1 class="wikiname">FromSqlToDataStructures</h1>
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<h1><a name="Introduction (IDEA MORE THAN A DRAFT)">Introduction (IDEA MORE THAN A DRAFT)</a></h1><b>&Acirc;&iquest;Coming from SQLand?</b> <i>&Acirc;&iquest;Who doesn't?</i> Redis is simple, <i>primitive</i> when comapred to the world you are used to in the world of Relational Database Managers (RDBMS) and Structure Query Language (SQL), here you will find insight to build bridges between both worlds to model real life problems.<h2><a name="Data Structures">Data Structures</a></h2>When I was young, happy and single ;) I studied <b>Data Structures</b> at the university, actually I learnt Data Structures and Algorithms <i>before</i> learning anything about Databases, and particularly RDBMS and SQL. This is natural because you need to know about Data Structures and Algorithms to understand a Database.<br/><br/>Redis can be seen as a <b>Data Structures Server</b>, a very simple interface to a extremly fast and efficient <h2><a name="Dude where is my SELECT statement?">Dude where is my SELECT statement?</a></h2><h2><a name="LISTs">LISTs</a></h2>In SQL there is no such thing as a &quot;natural&quot; order, a SELECT statement without a ORDER BY clause will return data in a undefined order. In Redis <a href="LISTs.html">LISTs</a> address the problem of natural ordering, ...<h2><a name="SETs">SETs</a></h2>So you have a bunch of unordered data, <h2><a name="SORT to the rescue">SORT to the rescue</a></h2>But sometimes we <i>need</i> to actually sort a LIST in a order different from its natural or take a SET and have it ordered, there is where the <i>fast</i> SORT commands comes handy... <h3><a name="SORT BY">SORT BY</a></h3>Just SORTing keys would be kind of boring, sometimes useless right? Well, you can SORT...<h2><a name="HASHEs">HASHEs</a></h2>Umm, sorry you will have to wait for a <a href="RoadMap.html">upcoming version of Redis</a> to have Hashes, but here are Idioms you should house to manage Dictionary like data...
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<b>GenericCommandsSidebar: Contents</b>
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<h1 class="wikiname">GenericCommandsSidebar</h1>
<div class="summary">
</div>
<div class="narrow">
&iuml;&raquo;&iquest;== Generic Commands ==<br/><br/><ul><li> <a href="ExistsCommand.html">EXISTS</a></li><li> <a href="DelCommand.html">DEL</a></li><li> <a href="TypeCommand.html">TYPE</a></li><li> <a href="KeysCommand.html">KEYS</a></li><li> <a href="RandomkeyCommand.html">RANDOMKEY</a></li><li> <a href="RenameCommand.html">RENAME</a></li><li> <a href="RenamenxCommand.html">RENAMENX</a></li><li> <a href="DbsizeCommand.html">DBSIZE</a></li><li> <a href="ExpireCommand.html">EXPIRE</a></li><li> <a href="TtlCommand.html">TTL</a></li><li> <a href="SelectCommand.html">SELECT</a></li><li> <a href="MoveCommand.html">MOVE</a></li><li> <a href="FlushdbCommand.html">FLUSHDB</a></li><li> <a href="FlushallCommand.html">FLUSHALL</a></li></ul>
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<b>GetCommand: Contents</b><br>&nbsp;&nbsp;<a href="#GET _key_">GET _key_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1 class="wikiname">GetCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="StringCommandsSidebar.html">StringCommandsSidebar</a><h1><a name="GET _key_">GET _key_</a></h1>
<i>Time complexity: O(1)</i><blockquote>Get the value of the specified key. If the keydoes not exist the special value 'nil' is returned.If the value stored at <i>key</i> is not a string an erroris returned because GET can only handle string values.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Bulk reply</a>
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<b>GetsetCommand: Contents</b><br>&nbsp;&nbsp;<a href="#GETSET _key_ _value_">GETSET _key_ _value_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Design patterns">Design patterns</a>
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<h1 class="wikiname">GetsetCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="StringCommandsSidebar.html">StringCommandsSidebar</a><h1><a name="GETSET _key_ _value_">GETSET _key_ _value_</a></h1>
<i>Time complexity: O(1)</i><blockquote>GETSET is an atomic <i>set this value and return the old value</i> command.Set <i>key</i> to the string <i>value</i> and return the old value stored at <i>key</i>.The string can't be longer than 1073741824 bytes (1 GB).</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Bulk reply</a><h2><a name="Design patterns">Design patterns</a></h2><blockquote>GETSET can be used together with INCR for counting with atomic reset whena given condition arises. For example a process may call INCR against thekey <i>mycounter</i> every time some event occurred, but from time totime we need to get the value of the counter and reset it to zero atomicallyusing <code name="code" class="python">GETSET mycounter 0</code>.</blockquote>
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<b>HackingStrings: Contents</b><br>&nbsp;&nbsp;<a href="#Hacking Strings">Hacking Strings</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Creating Redis Strings">Creating Redis Strings</a>
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<h1 class="wikiname">HackingStrings</h1>
<div class="summary">
</div>
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<h1><a name="Hacking Strings">Hacking Strings</a></h1>The implementation of Redis strings is contained in <b></b>sds.c<b></b> ( sds stands for Simple Dynamic Strings ).<br/><br/>The C structure <i>sdshdr</i> declared in <b>sds.h</b> represents a Redis string:<br/><br/><pre class="codeblock python" name="code">
struct sdshdr {
long len;
long free;
char buf[];
};
</pre>The <i>buf</i> character array stores the actual string.<br/><br/>The <i>len</i> field stores the length of <i>buf</i>. This makes obtaining the length
of a Redis string an O(1) operation.<br/><br/>The <i>free</i> field stores the number of additional bytes available for use.<br/><br/>Together the <i>len</i> and <i>free</i> field can be thought of as holding the metadata of the
<i>buf</i> character array.<h2><a name="Creating Redis Strings">Creating Redis Strings</a></h2>A new data type named <code name="code" class="python">sds</code> is defined in <b>sds.h</b> to be a synonymn for a character pointer:<br/><br/><pre class="codeblock python python" name="code">
typedef char *sds;
</pre><code name="code" class="python">sdsnewlen</code> function defined in <b>sds.c</b> creates a new Redis String: <br/><br/><pre class="codeblock python python python" name="code">
sds sdsnewlen(const void *init, size_t initlen) {
struct sdshdr *sh;
sh = zmalloc(sizeof(struct sdshdr)+initlen+1);
#ifdef SDS_ABORT_ON_OOM
if (sh == NULL) sdsOomAbort();
#else
if (sh == NULL) return NULL;
#endif
sh-&gt;len = initlen;
sh-&gt;free = 0;
if (initlen) {
if (init) memcpy(sh-&gt;buf, init, initlen);
else memset(sh-&gt;buf,0,initlen);
}
sh-&gt;buf[initlen] = '\0';
return (char*)sh-&gt;buf;
}
</pre>Remember a Redis string is a variable of type <code name="code" class="python">struct sdshdr</code>. But <code name="code" class="python">sdsnewlen</code> returns a character pointer!!<br/><br/>That's a trick and needs some explanation.<br/><br/>Suppose I create a Redis string using <code name="code" class="python">sdsnewlen</code> like below:<br/><br/><pre class="codeblock python python python python" name="code">
sdsnewlen(&quot;redis&quot;, 5);
</pre>This creates a new variable of type <code name="code" class="python">struct sdshdr</code> allocating memory for <i>len</i> and <i>free</i>
fields as well as for the <i>buf</i> character array.<br/><br/><pre class="codeblock python python python python python" name="code">
sh = zmalloc(sizeof(struct sdshdr)+initlen+1); // initlen is length of init argument.
</pre>After <code name="code" class="python">sdsnewlen</code> succesfully creates a Redis string the result is something like:<br/><br/><pre class="codeblock python python python python python python" name="code">
-----------
|5|0|redis|
-----------
^ ^
sh sh-&gt;buf
</pre><code name="code" class="python">sdsnewlen</code> returns sh-&gt;buf to the caller.<br/><br/>What do you do if you need to free the Redis string pointed by <code name="code" class="python">sh</code>?<br/><br/>You want the pointer <code name="code" class="python">sh</code> but you only have the pointer <code name="code" class="python">sh-&gt;buf</code>.<br/><br/>Can you get the pointer <code name="code" class="python">sh</code> from <code name="code" class="python">sh-&gt;buf</code>?<br/><br/>Yes. Pointer arithmetic. Notice from the above ASCII art that if you subtract
the size of two longs from <code name="code" class="python">sh-&gt;buf</code> you get the pointer <code name="code" class="python">sh</code>. <br/><br/>The sizeof two longs happens to be the size of <code name="code" class="python">struct sdshdr</code>.<br/><br/>Look at <code name="code" class="python">sdslen</code> function and see this trick at work:<br/><br/><pre class="codeblock python python python python python python python" name="code">
size_t sdslen(const sds s) {
struct sdshdr *sh = (void*) (s-(sizeof(struct sdshdr)));
return sh-&gt;len;
}
</pre>Knowing this trick you could easily go through the rest of the functions in <b>sds.c</b>.<br/><br/>The Redis string implementation is hidden behind an interface that accepts only character pointers. The users of Redis strings need not care about how its implemented and treat Redis strings as a character pointer.
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<h1 class="wikiname">HashCommandsSidebar</h1>
<div class="summary">
</div>
<div class="narrow">
&iuml;&raquo;&iquest;== Hash Commands ==<br/><br/><ul><li> <a href="HsetCommand.html">HSET</a></li><li> <a href="HgetCommand.html">HGET</a></li><li> <a href="HsetnxCommand.html">HSETNX</a></li><li> <a href="HmsetCommand.html">HMSET</a></li><li> <a href="HincrbyCommand.html">HINCRBY</a></li><li> <a href="HexistsCommand.html">HEXISTS</a></li><li> <a href="HdelCommand.html">HDEL</a></li><li> <a href="HlenCommand.html">HLEN</a></li><li> <a href="HgetallCommand.html">HKEYS</a></li><li> <a href="HgetallCommand.html">HVALS</a></li><li> <a href="HgetallCommand.html">HGETALL</a></li></ul>
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<b>Hashes: Contents</b><br>&nbsp;&nbsp;<a href="#Redis Hash Type">Redis Hash Type</a><br>&nbsp;&nbsp;<a href="#Implementation details">Implementation details</a>
</div>
<h1 class="wikiname">Hashes</h1>
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</div>
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&iuml;&raquo;&iquest;#sidebar <a href="HashCommandsSidebar.html">HashCommandsSidebar</a><h1><a name="Redis Hash Type">Redis Hash Type</a></h1>Redis Hashes are unordered maps of <a href="String.html">Redis Strings</a> between fields and values. It is possible to add, remove, test for existence of fields in O(1) amortized time. It is also possible to enumerate all the keys, values, or both, in O(N) (where N is the number of fields inside the hash).<br/><br/>Redis Hashes are interesting because they are very well suited to represent objects. For instance web applications users can be represented by a Redis Hash containing fields such username, encrpypted_password, lastlogin, and so forth.<br/><br/>Another very important property of Redis Hashes is that they use very little memory for hashes composed of a small number of fields (configurable, check redis.conf for details), compared to storing every field as a top level Redis key. This is obtained using a different specialized representation for small hashes. See the implementation details paragraph below for more information.<br/><br/>Commands operating on hashes try to make a good use of the return value in order to signal the application about previous existence of fields. For instance the <a href="HsetCommand.html">HSET</a> command will return 1 if the field set was not already present in the hash, otherwise will return 0 (and the user knows this was just an update operation).<br/><br/>The max number of fields in a set is 232-1 (4294967295, more than 4 billion of members per hash).<h1><a name="Implementation details">Implementation details</a></h1>The obvious internal representation of hashes is indeed an hash table, as the name of the data structure itself suggests. Still the drawback of this representation is that there is a lot of space overhead for hash table metadata.<br/><br/>Because one of the most interesting uses of Hashes is object encoding, and objects are often composed of a few fields each, Redis uses a different internal representation for small hashes (for Redis to consider a hash small, this must be composed a limited number of fields, and each field and value can't exceed a given number of bytes. All this is user-configurable).<br/><br/>Small hashes are thus encoded using a data structure called zipmap (is not something you can find in a CS book, the name is a Redis invention), that is a very memory efficient data structure to represent string to string maps, at the cost of being O(N) instead of O(1) for most operations. Since the constant times of this data structure are very small, and the zipmaps are converted into real hash tables once they are big enough, the amortized time of Redis hashes is still O(1), and in the practice small zipmaps are not slower than small hash tables because they are designed for good cache locality and fast access.<br/><br/>The result is that small hashes are both memory efficient and fast, while bigger hashes are fast but not as memory efficient than small hashes.
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<b>HdelCommand: Contents</b><br>&nbsp;&nbsp;<a href="#HDEL _key_ _field_ (Redis &gt;">HDEL _key_ _field_ (Redis &gt;</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1 class="wikiname">HdelCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="HashCommandsSidebar.html">HashCommandsSidebar</a><h1><a name="HDEL _key_ _field_ (Redis &gt;">HDEL _key_ _field_ (Redis &gt;</a></h1> 1.3.10)=
<i>Time complexity: O(1)</i><blockquote>Remove the specified <i>field</i> from an hash stored at <i>key</i>.</blockquote>
<blockquote>If the <i>field</i> was present in the hash it is returned and 1 is returned, otherwise 0 is returned and no operation is performed.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Integer reply</a>
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<b>HexistsCommand: Contents</b><br>&nbsp;&nbsp;<a href="#HEXISTS _key_ _field_ (Redis &gt;">HEXISTS _key_ _field_ (Redis &gt;</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1 class="wikiname">HexistsCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="HashCommandsSidebar.html">HashCommandsSidebar</a><h1><a name="HEXISTS _key_ _field_ (Redis &gt;">HEXISTS _key_ _field_ (Redis &gt;</a></h1> 1.3.10)=
<i>Time complexity: O(1)</i><blockquote>Return 1 if the hash stored at <i>key</i> contains the specified <i>field</i>.</blockquote>
<blockquote>Return 0 if the <i>key</i> is not found or the <i>field</i> is not present.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Integer reply</a>
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<b>HgetCommand: Contents</b><br>&nbsp;&nbsp;<a href="#HGET _key_ _field_ (Redis &gt;">HGET _key_ _field_ (Redis &gt;</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<i>Time complexity: O(1)</i><blockquote>If <i>key</i> holds a hash, retrieve the value associated to the specified <i>field</i>.</blockquote>
<blockquote>If the <i>field</i> is not found or the <i>key</i> does not exist, a special 'nil' value is returned.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Bulk reply</a>
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&iuml;&raquo;&iquest;#sidebar <a href="HashCommandsSidebar.html">HashCommandsSidebar</a><h1><a name="HKEYS _key_ (Redis &gt;">HKEYS _key_ (Redis &gt;</a></h1> 1.3.10)=
<h1><a name="HVALS _key_ (Redis &gt;">HVALS _key_ (Redis &gt;</a></h1> 1.3.10)=
<h1><a name="HGETALL _key_ (Redis &gt;">HGETALL _key_ (Redis &gt;</a></h1> 1.3.10)=
<i>Time complexity: O(N), where N is the total number of entries</i><blockquote>HKEYS returns all the fields names contained into a hash, HVALS all the associated values, while HGETALL returns both the fields and values in the form of <i>field1</i>, <i>value1</i>, <i>field2</i>, <i>value2</i>, ..., <i>fieldN</i>, <i>valueN</i>.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Multi Bulk Reply</a>
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<b>HincrbyCommand: Contents</b><br>&nbsp;&nbsp;<a href="#HINCRBY _key_ _field_ _value_ (Redis &gt;">HINCRBY _key_ _field_ _value_ (Redis &gt;</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Examples">Examples</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<i>Time complexity: O(1)</i><blockquote>Increment the number stored at <i>field</i> in the hash at <i>key</i> by <i>value</i>. If <i>key</i> does not exist, a new key holding a hash is created. If <i>field</i> does not exist or holds a string, the value is set to 0 before applying the operation.</blockquote>
<blockquote>The range of values supported by HINCRBY is limited to 64 bit signed integers.</blockquote><h2><a name="Examples">Examples</a></h2>
Since the <i>value</i> argument is signed you can use this command to perform both increments and decrements:<br/><br/><pre class="codeblock python" name="code">
HINCRBY key field 1 (increment by one)
HINCRBY key field -1 (decrement by one, just like the DECR command)
HINCRBY key field -10 (decrement by 10)
</pre>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Integer reply</a> The new value at <i>field</i> after the increment operation.
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<b>HlenCommand: Contents</b><br>&nbsp;&nbsp;<a href="#HLEN _key_ (Redis &gt;">HLEN _key_ (Redis &gt;</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<i>Time complexity: O(1)</i><blockquote>Return the number of entries (fields) contained in the hash stored at <i>key</i>. If the specified <i>key</i> does not exist, 0 is returned assuming an empty hash. </blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Integer reply</a>
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<i>Time complexity: O(N) (with N being the number of fields)</i><blockquote>Set the respective fields to the respective values. HMSET replaces old values with new values.</blockquote>
<blockquote>If <i>key</i> does not exist, a new key holding a hash is created.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Status code reply</a> Always +OK because HMSET can't fail
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<i>Time complexity: O(1)</i><blockquote>Set the specified hash <i>field</i> to the specified <i>value</i>.</blockquote>
<blockquote>If <i>key</i> does not exist, a new key holding a hash is created.</blockquote>
<blockquote>If the field already exists, and the HSET just produced an update of thevalue, 0 is returned, otherwise if a new field is created 1 is returned.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Integer reply</a>
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<i>Time complexity: O(1)</i><blockquote>Set the specified hash <i>field</i> to the specified <i>value</i>, if <i>field</i> does not exist yet.</blockquote>
<blockquote>If <i>key</i> does not exist, a new key holding a hash is created.</blockquote>
<blockquote>If the field already exists, this operation has no effect and returns 0.Otherwise, the field is set to <i>value</i> and the operation returns 1.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Integer reply</a>
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<b>IncrCommand: Contents</b><br>&nbsp;&nbsp;<a href="#INCR _key_">INCR _key_</a><br>&nbsp;&nbsp;<a href="#INCRBY _key_ _integer_">INCRBY _key_ _integer_</a><br>&nbsp;&nbsp;<a href="#DECR _key_ _integer_">DECR _key_ _integer_</a><br>&nbsp;&nbsp;<a href="#DECRBY _key_ _integer_">DECRBY _key_ _integer_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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&iuml;&raquo;&iquest;#sidebar <a href="StringCommandsSidebar.html">StringCommandsSidebar</a><h1><a name="INCR _key_">INCR _key_</a></h1>
<h1><a name="INCRBY _key_ _integer_">INCRBY _key_ _integer_</a></h1>
<h1><a name="DECR _key_ _integer_">DECR _key_ _integer_</a></h1>
<h1><a name="DECRBY _key_ _integer_">DECRBY _key_ _integer_</a></h1>
<i>Time complexity: O(1)</i><blockquote>Increment or decrement the number stored at <i>key</i> by one. If the key doesnot exist or contains a value of a wrong type, set the key to thevalue of &quot;0&quot; before to perform the increment or decrement operation.</blockquote>
<blockquote>INCRBY and DECRBY work just like INCR and DECR but instead toincrement/decrement by 1 the increment/decrement is <i>integer</i>.</blockquote>
<blockquote>INCR commands are limited to 64 bit signed integers.</blockquote>
Note: this is actually a string operation, that is, in Redis there are not &quot;integer&quot; types. Simply the string stored at the key is parsed as a base 10 64 bit signed integer, incremented, and then converted back as a string.<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Integer reply</a>, this commands will reply with the new value of <i>key</i> after the increment or decrement.
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<b>InfoCommand: Contents</b><br>&nbsp;&nbsp;<a href="#INFO">INFO</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Notes">Notes</a>
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&iuml;&raquo;&iquest;#sidebar <a href="ControlCommandsSidebar.html">ControlCommandsSidebar</a><h1><a name="INFO">INFO</a></h1><blockquote>The info command returns different information and statistics about the server in an format that's simple to parse by computers and easy to red by huamns.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Bulk reply</a>, specifically in the following format:<br/><br/><pre class="codeblock python" name="code">
edis_version:0.07
connected_clients:1
connected_slaves:0
used_memory:3187
changes_since_last_save:0
last_save_time:1237655729
total_connections_received:1
total_commands_processed:1
uptime_in_seconds:25
uptime_in_days:0
</pre>All the fields are in the form <code name="code" class="python">field:value</code><h2><a name="Notes">Notes</a></h2><ul><li> <code name="code" class="python">used_memory</code> is returned in bytes, and is the total number of bytes allocated by the program using <code name="code" class="python">malloc</code>.</li><li> <code name="code" class="python">uptime_in_days</code> is redundant since the uptime in seconds contains already the full uptime information, this field is only mainly present for humans.</li><li> <code name="code" class="python">changes_since_last_save</code> does not refer to the number of key changes, but to the number of operations that produced some kind of change in the dataset.</li></ul>
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<b>IntroductionToRedisDataTypes: Contents</b><br>&nbsp;&nbsp;<a href="#A fifteen minutes introduction to Redis data types">A fifteen minutes introduction to Redis data types</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Redis keys">Redis keys</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#The string type">The string type</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#The List type">The List type</a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="#First steps with Redis lists">First steps with Redis lists</a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Pushing IDs instead of the actual data in Redis lists">Pushing IDs instead of the actual data in Redis lists</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Redis Sets">Redis Sets</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#A digression. How to get unique identifiers for strings">A digression. How to get unique identifiers for strings</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Sorted sets">Sorted sets</a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Operating on ranges">Operating on ranges</a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Back to the reddit example">Back to the reddit example</a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Updating the scores of a sorted set">Updating the scores of a sorted set</a>
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&iuml;&raquo;&iquest;#sidebar <a href="RedisGuides.html">RedisGuides</a>
<h1><a name="A fifteen minutes introduction to Redis data types">A fifteen minutes introduction to Redis data types</a></h1>As you already probably know Redis is not a plain key-value store, actually it is a <b>data structures server</b>, supporting different kind of values. That is, you can't just set strings as values of keys. All the following data types are supported as values:<br/><br/><ul><li> Binary-safe strings.</li><li> Lists of binary-safe strings.</li><li> Sets of binary-safe strings, that are collection of unique unsorted elements. You can think at this as a Ruby hash where all the keys are set to the 'true' value.</li><li> Sorted sets, similar to Sets but where every element is associated to a floating number score. The elements are taken sorted by score. You can think at this as Ruby hashes where the key is the element and the value is the score, but where elements are always taken in order without requiring a sorting operation.</li></ul>
It's not always trivial to grasp how this data types work and what to use in order to solve a given problem from the <a href="CommandReference.html">Redis command reference</a>, so this document is a crash course to Redis data types and their most used patterns.<br/><br/>For all the examples we'll use the <b>redis-cli</b> utility, that's a simple but handy command line utility to issue commands against the Redis server.<h2><a name="Redis keys">Redis keys</a></h2>Before to start talking about the different kind of values supported by Redis it is better to start saying that keys are not binary safe strings in Redis, but just strings not containing a space or a newline character. For instance &quot;foo&quot; or &quot;123456789&quot; or &quot;foo_bar&quot; are valid keys, while &quot;hello world&quot; or &quot;hello\n&quot; are not.<br/><br/>Actually there is nothing inside the Redis internals preventing the use of binary keys, it's just a matter of protocol, and actually the new protocol introduced with Redis 1.2 (1.2 betas are 1.1.x) in order to implement commands like MSET, is totally binary safe. Still for now consider this as an hard limit as the database is only tested with &quot;normal&quot; keys.<br/><br/>A few other rules about keys:<br/><br/><ul><li> Too long keys are not a good idea, for instance a key of 1024 bytes is not a good idea not only memory-wise, but also because the lookup of the key in the dataset may require several costly key-comparisons.</li><li> Too short keys are not a good idea. There is no point in writing &quot;u:1000:pwd&quot; as key if you can write instead &quot;user:1000:password&quot;, the latter is more readable and the added space is very little compared to the space used by the key object itself.</li><li> Try to stick with a schema. For instance &quot;object-type:id:field&quot; can be a nice idea, like in &quot;user:1000:password&quot;. I like to use dots for multi-words fields, like in &quot;comment:1234:reply.to&quot;.</li></ul>
<h2><a name="The string type">The string type</a></h2>This is the simplest Redis type. If you use only this type, Redis will be something like a memcached server with persistence.<br/><br/>Let's play a bit with the string type:<br/><br/><pre class="codeblock python" name="code">
$ ./redis-cli set mykey &quot;my binary safe value&quot;
OK
$ ./redis-cli get mykey
my binary safe value
</pre>As you can see using the <a href="SetCommand.html">Set command</a> and the <a href="GetCommand.html">Get command</a> is trivial to set values to strings and have this strings returned back.<br/><br/>Values can be strings (including binary data) of every kind, for instance you can store a jpeg image inside a key. A value can't be bigger than 1 Gigabyte.<br/><br/>Even if strings are the basic values of Redis, there are interesting operations you can perform against them. For instance one is atomic increment:<br/><br/><pre class="codeblock python python" name="code">
$ ./redis-cli set counter 100
OK
$ ./redis-cli incr counter
(integer) 101
$ ./redis-cli incr counter
(integer) 102
$ ./redis-cli incrby counter 10
(integer) 112
</pre>The <a href="IncrCommand.html">INCR</a> command parses the string value as an integer, increments it by one, and finally sets the obtained value as the new string value. There are other similar commands like <a href="IncrCommand.html">INCRBY</a>, <a href="IncrCommand.html">DECR</a> and <a href="IncrCommand.html">DECRBY</a>. Actually internally it's always the same command, acting in a slightly different way.<br/><br/>What means that INCR is atomic? That even multiple clients issuing INCR against the same key will never incur into a race condition. For instance it can't never happen that client 1 read &quot;10&quot;, client 2 read &quot;10&quot; at the same time, both increment to 11, and set the new value of 11. The final value will always be of 12 ad the read-increment-set operation is performed while all the other clients are not executing a command at the same time.<br/><br/>Another interesting operation on string is the <a href="GetsetCommand.html">GETSET</a> command, that does just what its name suggests: Set a key to a new value, returning the old value, as result. Why this is useful? Example: you have a system that increments a Redis key using the <a href="IncrCommand.html">INCR</a> command every time your web site receives a new visit. You want to collect this information one time every hour, without loosing a single key. You can GETSET the key assigning it the new value of &quot;0&quot; and reading the old value back.<h2><a name="The List type">The List type</a></h2>To explain the List data type it's better to start with a little of theory, as the term <b>List</b> is often used in an improper way by information technology folks. For instance &quot;Python Lists&quot; are not what the name may suggest (Linked Lists), but them are actually Arrays (the same data type is called Array in Ruby actually).<br/><br/>From a very general point of view a List is just a sequence of ordered elements: 10,20,1,2,3 is a list, but when a list of items is implemented using an Array and when instead a <b>Linked List</b> is used for the implementation, the properties change a lot.<br/><br/>Redis lists are implemented via Linked Lists, this means that even if you have million of elements inside a list, the operation of adding a new element in the head or in the tail of the list is performed <b>in constant time</b>. Adding a new element with the <a href="LpushCommand.html">LPUSH</a> command to the head of a ten elements list is the same speed as adding an element to the head of a 10 million elements list.<br/><br/>What's the downside? That accessing an element <b>by index</b> is very fast in lists implemented with an Array and not so fast in lists implemented by linked lists.<br/><br/>Redis Lists are implemented with linked lists because for a database system is crucial to be able to add elements to a very long list in a very fast way. Another strong advantage is, as you'll see in a moment, that Redis Lists can be taken at constant length in constant time.<h3><a name="First steps with Redis lists">First steps with Redis lists</a></h3>The <a href="RpushCommand.html">LPUSH</a> command add a new element into a list, on the left (on head), while the <a href="RpushCommand.html">RPUSH</a> command add a new element into alist, ot the right (on tail). Finally the <a href="LrangeCommand.html">LRANGE</a> command extract ranges of elements from lists:<br/><br/><pre class="codeblock python python python" name="code">
$ ./redis-cli rpush messages &quot;Hello how are you?&quot;
OK
$ ./redis-cli rpush messages &quot;Fine thanks. I'm having fun with Redis&quot;
OK
$ ./redis-cli rpush messages &quot;I should look into this NOSQL thing ASAP&quot;
OK
$ ./redis-cli lrange messages 0 2
1. Hello how are you?
2. Fine thanks. I'm having fun with Redis
3. I should look into this NOSQL thing ASAP
</pre>Note that <a href="LrangeCommand.html">LRANGE</a> takes two indexes, the first and the last element of the range to return. Both the indexes can be negative to tell Redis to start to count for the end, so -1 is the last element, -2 is the penultimate element of the list, and so forth.<br/><br/>As you can guess from the example above, lists can be used, for instance, in order to implement a chat system. Another use is as queues in order to route messages between different processes. But the key point is that <b>you can use Redis lists every time you require to access data in the same order they are added</b>. This will not require any SQL ORDER BY operation, will be very fast, and will scale to millions of elements even with a toy Linux box.<br/><br/>For instance in ranking systems like the social news reddit.com you can add every new submitted link into a List, and with <a href="LrangeCommand.html">LRANGE</a> it's possible to paginate results in a trivial way.<br/><br/>In a blog engine implementation you can have a list for every post, where to push blog comments, and so forth.<h3><a name="Pushing IDs instead of the actual data in Redis lists">Pushing IDs instead of the actual data in Redis lists</a></h3>In the above example we pushed our &quot;objects&quot; (simply messages in the example) directly inside the Redis list, but this is often not the way to go, as objects can be referenced in multiple times: in a list to preserve their chronological order, in a Set to remember they are about a specific category, in another list but only if this object matches some kind of requisite, and so forth.<br/><br/>Let's return back to the reddit.com example. A more credible pattern for adding submitted links (news) to the list is the following:<br/><br/><pre class="codeblock python python python python" name="code">
$ ./redis-cli incr next.news.id
(integer) 1
$ ./redis-cli set news:1:title &quot;Redis is simple&quot;
OK
$ ./redis-cli set news:1:url &quot;http://code.google.com/p/redis&quot;
OK
$ ./redis-cli lpush submitted.news 1
OK
</pre>We obtained an unique incremental ID for our news object just incrementing a key, then used this ID to create the object setting a key for every field in the object. Finally the ID of the new object was pushed on the <b>submitted.news</b> list.<br/><br/>This is just the start. Check the <a href="CommandReference.html">Command Reference</a> and read about all the other list related commands. You can remove elements, rotate lists, get and set elements by index, and of course retrieve the length of the list with <a href="LLenCommand.html">LLEN</a>.<h2><a name="Redis Sets">Redis Sets</a></h2>Redis Sets are unordered collection of binary-safe strings. The <a href="SaddCommand.html">SADD</a> command adds a new element to a set. It's also possible to do a number of other operations against sets like testing if a given element already exists, performing the intersection, union or difference between multiple sets and so forth. An example is worth 1000 words:<br/><br/><pre class="codeblock python python python python python" name="code">
$ ./redis-cli sadd myset 1
(integer) 1
$ ./redis-cli sadd myset 2
(integer) 1
$ ./redis-cli sadd myset 3
(integer) 1
$ ./redis-cli smembers myset
1. 3
2. 1
3. 2
</pre>I added three elements to my set and told Redis to return back all the elements. As you can see they are not sorted.<br/><br/>Now let's check if a given element exists:<br/><br/><pre class="codeblock python python python python python python" name="code">
$ ./redis-cli sismember myset 3
(integer) 1
$ ./redis-cli sismember myset 30
(integer) 0
</pre>&quot;3&quot; is a member of the set, while &quot;30&quot; is not. Sets are very good in order to express relations between objects. For instance we can easily Redis Sets in order to implement tags.<br/><br/>A simple way to model this is to have, for every object you want to tag, a Set with all the IDs of the tags associated with the object, and for every tag that exists, a Set of of all the objects tagged with this tag.<br/><br/>For instance if our news ID 1000 is tagged with tag 1,2,5 and 77, we can specify the following two Sets:<br/><br/><pre class="codeblock python python python python python python python" name="code">
$ ./redis-cli sadd news:1000:tags 1
(integer) 1
$ ./redis-cli sadd news:1000:tags 2
(integer) 1
$ ./redis-cli sadd news:1000:tags 5
(integer) 1
$ ./redis-cli sadd news:1000:tags 77
(integer) 1
$ ./redis-cli sadd tag:1:objects 1000
(integer) 1
$ ./redis-cli sadd tag:2:objects 1000
(integer) 1
$ ./redis-cli sadd tag:5:objects 1000
(integer) 1
$ ./redis-cli sadd tag:77:objects 1000
(integer) 1
</pre>To get all the tags for a given object is trivial:<br/><br/>$ ./redis-cli smembers <a href="news:1000:tags" target="_blank">news:1000:tags</a>
1. 5
2. 1
3. 77
4. 2<br/><br/>But there are other non trivial operations that are still easy to implement using the right Redis commands. For instance we may want the list of all the objects having as tags 1, 2, 10, and 27 at the same time. We can do this using the <a href="SinterCommand.html">SinterCommand</a> that performs the intersection between different sets. So in order to reach our goal we can just use:<br/><br/><pre class="codeblock python python python python python python python python" name="code">
$ ./redis-cli sinter tag:1:objects tag:2:objects tag:10:objects tag:27:objects
... no result in our dataset composed of just one object ;) ...
</pre>Look at the <a href="CommandReference.html">Command Reference</a> to discover other Set related commands, there are a bunch of interesting one. Also make sure to check the <a href="SortCommand.html">SORT</a> command as both Redis Sets and Lists are sortable.<h2><a name="A digression. How to get unique identifiers for strings">A digression. How to get unique identifiers for strings</a></h2>In our tags example we showed tag IDs without to mention how this IDs can be obtained. Basically for every tag added to the system, you need an unique identifier. You also want to be sure that there are no race conditions if multiple clients are trying to add the same tag at the same time. Also, if a tag already exists, you want its ID returned, otherwise a new unique ID should be created and associated to the tag.<br/><br/>Redis 1.4 will add the Hash type. With it it will be trivial to associate strings with unique IDs, but how to do this today with the current commands exported by Redis in a reliable way?<br/><br/>Our first attempt (that is broken) can be the following. Let's suppose we want to get an unique ID for the tag &quot;redis&quot;:<br/><br/><ul><li> In order to make this algorithm binary safe (they are just tags but think to utf8, spaces and so forth) we start performing the SHA1 sum of the tag. SHA1(redis) = b840fc02d524045429941cc15f59e41cb7be6c52.</li><li> Let's check if this tag is already associated with an unique ID with the command <b>GET tag:b840fc02d524045429941cc15f59e41cb7be6c52:id</b>.</li><li> If the above GET returns an ID, return it back to the user. We already have the unique ID.</li><li> Otherwise... create a new unique ID with <b>INCR next.tag.id</b> (assume it returned 123456).</li><li> Finally associate this new ID to our tag with <b>SET tag:b840fc02d524045429941cc15f59e41cb7be6c52:id 123456</b> and return the new ID to the caller.</li></ul>
Nice. Or better.. broken! What about if two clients perform this commands at the same time trying to get the unique ID for the tag &quot;redis&quot;? If the timing is right they'll both get <b>nil</b> from the GET operation, will both increment the <b>next.tag.id</b> key and will set two times the key. One of the two clients will return the wrong ID to the caller. To fix the algorithm is not hard fortunately, and this is the sane version:<br/><br/><ul><li> In order to make this algorithm binary safe (they are just tags but think to utf8, spaces and so forth) we start performing the SHA1 sum of the tag. SHA1(redis) = b840fc02d524045429941cc15f59e41cb7be6c52.</li><li> Let's check if this tag is already associated with an unique ID with the command <b>GET tag:b840fc02d524045429941cc15f59e41cb7be6c52:id</b>.</li><li> If the above GET returns an ID, return it back to the user. We already have the unique ID.</li><li> Otherwise... create a new unique ID with <b>INCR next.tag.id</b> (assume it returned 123456).</li><li> Finally associate this new ID to our tag with <b>SETNX tag:b840fc02d524045429941cc15f59e41cb7be6c52:id 123456</b>. By using SETNX if a different client was faster than this one the key wil not be setted. Not only, SETNX returns 1 if the key is set, 0 otherwise. So... let's add a final step to our computation.</li><li> If SETNX returned 1 (We set the key) return 123456 to the caller, it's our tag ID, otherwise perform <b>GET tag:b840fc02d524045429941cc15f59e41cb7be6c52:id</b> and return the value to the caller.</li></ul>
<h2><a name="Sorted sets">Sorted sets</a></h2>Sets are a very handy data type, but... they are a bit too unsorted in order to fit well for a number of problems ;) This is why Redis 1.2 introduced Sorted Sets. They are very similar to Sets, collections of binary-safe strings, but this time with an associated score, and an operation similar to the List LRANGE operation to return items in order, but working against Sorted Sets, that is, the <a href="ZrangeCommand.html">ZRANGE</a> command.<br/><br/>Basically Sorted Sets are in some way the Redis equivalent of Indexes in the SQL world. For instance in our reddit.com example above there was no mention about how to generate the actual home page with news raked by user votes and time. We'll see how sorted sets can fix this problem, but it's better to start with something simpler, illustrating the basic working of this advanced data type. Let's add a few selected hackers with their year of birth as &quot;score&quot;.<br/><br/><pre class="codeblock python python python python python python python python python" name="code">
$ ./redis-cli zadd hackers 1940 &quot;Alan Kay&quot;
(integer) 1
$ ./redis-cli zadd hackers 1953 &quot;Richard Stallman&quot;
(integer) 1
$ ./redis-cli zadd hackers 1965 &quot;Yukihiro Matsumoto&quot;
(integer) 1
$ ./redis-cli zadd hackers 1916 &quot;Claude Shannon&quot;
(integer) 1
$ ./redis-cli zadd hackers 1969 &quot;Linus Torvalds&quot;
(integer) 1
$ ./redis-cli zadd hackers 1912 &quot;Alan Turing&quot;
(integer) 1
</pre>For sorted sets it's a joke to return these hackers sorted by their birth year because actually <b>they are already sorted</b>. Sorted sets are implemented via a dual-ported data structure containing both a skip list and an hash table, so every time we add an element Redis performs an O(log(N)) operation, that's good, but when we ask for sorted elements Redis does not have to do any work at all, it's already all sorted:<br/><br/><pre class="codeblock python python python python python python python python python python" name="code">
$ ./redis-cli zrange hackers 0 -1
1. Alan Turing
2. Claude Shannon
3. Alan Kay
4. Richard Stallman
5. Yukihiro Matsumoto
6. Linus Torvalds
</pre>Didn't know that Linus was younger than Yukihiro btw ;)<br/><br/>Anyway I want to order this elements the other way around, using <a href="ZREVRANGE.html">ZrangeCommand</a> instead of <a href="ZRANGE.html">ZrangeCommand</a> this time:<br/><br/><pre class="codeblock python python python python python python python python python python python" name="code">
$ ./redis-cli zrevrange hackers 0 -1
1. Linus Torvalds
2. Yukihiro Matsumoto
3. Richard Stallman
4. Alan Kay
5. Claude Shannon
6. Alan Turing
</pre>A very important note, ZSets have just a &quot;default&quot; ordering but you are still free to call the <a href="SortCommand.html">SORT</a> command against sorted sets to get a different ordering (but this time the server will waste CPU). An alternative for having multiple orders is to add every element in multiple sorted sets at the same time.<h3><a name="Operating on ranges">Operating on ranges</a></h3>Sorted sets are more powerful than this. They can operate on ranges. For instance let's try to get all the individuals that born up to the 1950. We use the <a href="ZrangebyscoreCommand.html">ZRANGEBYSCORE</a> command to do it:<br/><br/><pre class="codeblock python python python python python python python python python python python python" name="code">
$ ./redis-cli zrangebyscore hackers -inf 1950
1. Alan Turing
2. Claude Shannon
3. Alan Kay
</pre>We asked Redis to return all the elements with a score between negative infinite and 1950 (both extremes are included).<br/><br/>It's also possible to remove ranges of elements. For instance let's remove all the hackers born between 1940 and 1960 from the sorted set:<br/><br/><pre class="codeblock python python python python python python python python python python python python python" name="code">
$ ./redis-cli zremrangebyscore hackers 1940 1960
(integer) 2
</pre><a href="ZremrangebyscoreCommand.html">ZREMRANGEBYSCORE</a> is not the best command name, but it can be very useful, and returns the number of removed elements.<h3><a name="Back to the reddit example">Back to the reddit example</a></h3>For the last time, back to the Reddit example. Now we have a decent plan to populate a sorted set in order to generate the home page. A sorted set can contain all the news that are not older than a few days (we remove old entries from time to time using ZREMRANGEBYSCORE). A background job gets all the elements from this sorted set, get the user votes and the time of the news, and compute the score to populate the <b>reddit.home.page</b> sorted set with the news IDs and associated scores. To show the home page we have just to perform a blazingly fast call to ZRANGE.<br/><br/>From time to time we'll remove too old news from the <b>reddit.home.page</b> sorted set as well in order for our system to work always against a limited set of news.<h3><a name="Updating the scores of a sorted set">Updating the scores of a sorted set</a></h3>Just a final note before to finish this tutorial. Sorted sets scores can be updated at any time. Just calling again ZADD against an element already included in the sorted set will update its score (and position) in O(log(N)), so sorted sets are suitable even when there are tons of updates.<br/><br/>This tutorial is in no way complete, this is just the basics to get started with Redis, read the <a href="CommandReference.html">Command Reference</a> to discover a lot more.<br/><br/>Thanks for reading. Salvatore.
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<b>KeysCommand: Contents</b><br>&nbsp;&nbsp;<a href="#KEYS _pattern_">KEYS _pattern_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1 class="wikiname">KeysCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="GenericCommandsSidebar.html">GenericCommandsSidebar</a><h1><a name="KEYS _pattern_">KEYS _pattern_</a></h1>
<i>Time complexity: O(n) (with n being the number of keys in the DB, and assuming keys and pattern of limited length)</i><blockquote>Returns all the keys matching the glob-style <i>pattern</i> asspace separated strings. For example if you have in thedatabase the keys &quot;foo&quot; and &quot;foobar&quot; the command &quot;KEYS foo<code name="code" class="python">*</code>&quot;will return &quot;foo foobar&quot;.</blockquote>
<blockquote>Note that while the time complexity for this operation is O(n)the constant times are pretty low. For example Redis runningon an entry level laptop can scan a 1 million keys databasein 40 milliseconds. <b>Still it's better to consider this one of
<blockquote>the slow commands that may ruin the DB performance if not usedwith care*.</blockquote>
<blockquote>In other words this command is intended only for debugging and *special* operations like creating a script to change the DB schema. Don't use it in your normal code. Use Redis <a href="Sets.html">Sets</a> in order to group together a subset of objects.</blockquote>
Glob style patterns examples:
<blockquote>* h?llo will match hello hallo hhllo* h*llo will match hllo heeeello* h<code name="code" class="python">[</code>ae<code name="code" class="python">]</code>llo will match hello and hallo, but not hillo</blockquote>Use \ to escape special chars if you want to match them verbatim.<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Bulk reply</a>, specifically a string in the form of space separated list of keys. Note that most client libraries will return an Array of keys and not a single string with space separated keys (that is, split by &quot; &quot; is performed in the client library usually).</b></blockquote>
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<b>LastsaveCommand: Contents</b><br>&nbsp;&nbsp;<a href="#LASTSAVE">LASTSAVE</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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&iuml;&raquo;&iquest;#sidebar <a href="ControlCommandsSidebar.html">ControlCommandsSidebar</a><h1><a name="LASTSAVE">LASTSAVE</a></h1>
<blockquote>Return the UNIX TIME of the last DB save executed with success.A client may check if a <a href="BgsaveCommand.html">BGSAVE</a> command succeeded reading the LASTSAVEvalue, then issuing a <a href="BgsaveCommand.html">BGSAVE</a> command and checking at regular intervalsevery N seconds if LASTSAVE changed.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Integer reply</a>, specifically an UNIX time stamp.
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<b>LindexCommand: Contents</b><br>&nbsp;&nbsp;<a href="#LINDEX _key_ _index_">LINDEX _key_ _index_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1 class="wikiname">LindexCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="ListCommandsSidebar.html">ListCommandsSidebar</a><h1><a name="LINDEX _key_ _index_">LINDEX _key_ _index_</a></h1>
<i>Time complexity: O(n) (with n being the length of the list)</i><blockquote>Return the specified element of the list stored at the specifiedkey. 0 is the first element, 1 the second and so on. Negative indexesare supported, for example -1 is the last element, -2 the penultimateand so on.</blockquote>
<blockquote>If the value stored at key is not of list type an error is returned.If the index is out of range an empty string is returned.</blockquote>
<blockquote>Note that even if the average time complexity is O(n) asking forthe first or the last element of the list is O(1).</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Bulk reply</a>, specifically the requested element.
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&iuml;&raquo;&iquest;== List Commands ==<br/><br/><ul><li> <a href="RpushCommand.html">RPUSH</a></li><li> <a href="RpushCommand.html">LPUSH</a></li><li> <a href="LlenCommand.html">LLEN</a></li><li> <a href="LrangeCommand.html">LRANGE</a></li><li> <a href="LtrimCommand.html">LTRIM</a></li><li> <a href="LindexCommand.html">LINDEX</a></li><li> <a href="LsetCommand.html">LSET</a></li><li> <a href="LremCommand.html">LREM</a></li><li> <a href="LpopCommand.html">LPOP</a></li><li> <a href="LpopCommand.html">RPOP</a></li><li> <a href="BlpopCommand.html">BLPOP</a></li><li> <a href="BlpopCommand.html">BRPOP</a></li><li> <a href="RpoplpushCommand.html">RPOPLPUSH</a></li><li> <a href="SortCommand.html">SORT</a></li></ul>
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<b>Lists: Contents</b><br>&nbsp;&nbsp;<a href="#Redis List Type">Redis List Type</a><br>&nbsp;&nbsp;<a href="#Implementation details">Implementation details</a>
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&iuml;&raquo;&iquest;#sidebar <a href="ListCommandsSidebar.html">ListCommandsSidebar</a><h1><a name="Redis List Type">Redis List Type</a></h1>Redis Lists are lists of <a href="Strings.html">Redis Strings</a>, sorted by insertion order. It's possible to add elements to a Redis List pushing new elements on the head (on the left) or on the tail (on the right) of the list.<br/><br/>The <a href="RpushCommand.html">LPUSH</a> command inserts a new elmenet on head, while <a href="RpushCommand.html">RPUSH</a> inserts a new element on tail. A new list is created when one of this operations is performed against an empty key.<br/><br/>For instance if perform the following operations:
<pre class="codeblock python" name="code">
LPUSH mylist a # now the list is &quot;a&quot;
LPUSH mylist b # now the list is &quot;b&quot;,&quot;a&quot;
RPUSH mylist c # now the list is &quot;b&quot;,&quot;a&quot;,&quot;c&quot; (RPUSH was used this time)
</pre>
The resulting list stored at <i>mylist</i> will contain the elements &quot;b&quot;,&quot;a&quot;,&quot;c&quot;.<br/><br/>The max length of a list is 232-1 elements (4294967295, more than 4 billion of elements per list).<h1><a name="Implementation details">Implementation details</a></h1>Redis Lists are implemented as doubly liked lists. A few commands benefit from the fact the lists are doubly linked in order to reach the needed element starting from the nearest extreme (head or tail). <a href="LrangeCommand.html">LRANGE</a> and <a href="LindexCommand.html">LINDEX</a> are examples of such commands.<br/><br/>The use of linked lists also guarantees that regardless of the length of the list pushing and popping are O(1) operations.<br/><br/>Redis Lists cache length information so <a href="LlenCommand.html">LLEN</a> is O(1) as well.
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<b>LlenCommand: Contents</b><br>&nbsp;&nbsp;<a href="#LLEN _key_">LLEN _key_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1 class="wikiname">LlenCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="ListCommandsSidebar.html">ListCommandsSidebar</a><h1><a name="LLEN _key_">LLEN _key_</a></h1>
<i>Time complexity: O(1)</i><blockquote>Return the length of the list stored at the specified key. If thekey does not exist zero is returned (the same behaviour as forempty lists). If the value stored at <i>key</i> is not a list an error is returned.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Integer reply</a>, specifically:<br/><br/><pre class="codeblock python" name="code">
The length of the list.
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<b>LpopCommand: Contents</b><br>&nbsp;&nbsp;<a href="#LPOP _key_">LPOP _key_</a><br>&nbsp;&nbsp;<a href="#RPOP _key_">RPOP _key_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1 class="wikiname">LpopCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="ListCommandsSidebar.html">ListCommandsSidebar</a><h1><a name="LPOP _key_">LPOP _key_</a></h1>
<h1><a name="RPOP _key_">RPOP _key_</a></h1>
<i>Time complexity: O(1)</i><blockquote>Atomically return and remove the first (LPOP) or last (RPOP) elementof the list. For example if the list contains the elements &quot;a&quot;,&quot;b&quot;,&quot;c&quot; LPOPwill return &quot;a&quot; and the list will become &quot;b&quot;,&quot;c&quot;.</blockquote>
<blockquote>If the <i>key</i> does not exist or the list is already empty the specialvalue 'nil' is returned.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Bulk reply</a>
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<b>LrangeCommand: Contents</b><br>&nbsp;&nbsp;<a href="#LRANGE _key_ _start_ _end_">LRANGE _key_ _start_ _end_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Consistency with range functions in various programming languages">Consistency with range functions in various programming languages</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Out-of-range indexes">Out-of-range indexes</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1 class="wikiname">LrangeCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="ListCommandsSidebar.html">ListCommandsSidebar</a><h1><a name="LRANGE _key_ _start_ _end_">LRANGE _key_ _start_ _end_</a></h1>
<i>Time complexity: O(start+n) (with n being the length of the range and start being the start offset)</i>Return the specified elements of the list stored at the specified
key. Start and end are zero-based indexes. 0 is the first element
of the list (the list head), 1 the next element and so on.<br/><br/>For example LRANGE foobar 0 2 will return the first three elements
of the list.<br/><br/><i>start</i> and <i>end</i> can also be negative numbers indicating offsets
from the end of the list. For example -1 is the last element of
the list, -2 the penultimate element and so on.<h2><a name="Consistency with range functions in various programming languages">Consistency with range functions in various programming languages</a></h2>Note that if you have a list of numbers from 0 to 100, LRANGE 0 10 will return
11 elements, that is, rightmost item is included. This <b>may or may not</b> be consistent with
behavior of range-related functions in your programming language of choice (think Ruby's Range.new, Array#slice or Python's range() function).<br/><br/>LRANGE behavior is consistent with one of Tcl.<h2><a name="Out-of-range indexes">Out-of-range indexes</a></h2>Indexes out of range will not produce an error: if start is over
the end of the list, or start <code name="code" class="python">&gt;</code> end, an empty list is returned.
If end is over the end of the list Redis will threat it just like
the last element of the list.<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Multi bulk reply</a>, specifically a list of elements in the specified range.
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<b>LremCommand: Contents</b><br>&nbsp;&nbsp;<a href="#LREM _key_ _count_ _value_">LREM _key_ _count_ _value_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1 class="wikiname">LremCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="ListCommandsSidebar.html">ListCommandsSidebar</a><h1><a name="LREM _key_ _count_ _value_">LREM _key_ _count_ _value_</a></h1>
<i>Time complexity: O(N) (with N being the length of the list)</i><blockquote>Remove the first <i>count</i> occurrences of the <i>value</i> element from the list.If <i>count</i> is zero all the elements are removed. If <i>count</i> is negativeelements are removed from tail to head, instead to go from head to tailthat is the normal behaviour. So for example LREM with count -2 and_hello_ as value to remove against the list (a,b,c,hello,x,hello,hello) willlave the list (a,b,c,hello,x). The number of removed elements is returnedas an integer, see below for more information about the returned value.Note that non existing keys are considered like empty lists by LREM, so LREMagainst non existing keys will always return 0.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Integer Reply</a>, specifically:<br/><br/><pre class="codeblock python" name="code">
The number of removed elements if the operation succeeded
</pre>
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<b>LsetCommand: Contents</b><br>&nbsp;&nbsp;<a href="#LSET _key_ _index_ _value_">LSET _key_ _index_ _value_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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&iuml;&raquo;&iquest;#sidebar <a href="ListCommandsSidebar.html">ListCommandsSidebar</a><h1><a name="LSET _key_ _index_ _value_">LSET _key_ _index_ _value_</a></h1>
<i>Time complexity: O(N) (with N being the length of the list)</i><blockquote>Set the list element at <i>index</i> (see LINDEX for information about the_index_ argument) with the new <i>value</i>. Out of range indexes willgenerate an error. Note that setting the first or last elements ofthe list is O(1).</blockquote>
<blockquote>Similarly to other list commands accepting indexes, the index can be negative to access elements starting from the end of the list. So -1 is the last element, -2 is the penultimate, and so forth.</blockquote><h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Status code reply</a>
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<b>LtrimCommand: Contents</b><br>&nbsp;&nbsp;<a href="#LTRIM _key_ _start_ _end_">LTRIM _key_ _start_ _end_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1 class="wikiname">LtrimCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="ListCommandsSidebar.html">ListCommandsSidebar</a><h1><a name="LTRIM _key_ _start_ _end_">LTRIM _key_ _start_ _end_</a></h1>
<i>Time complexity: O(n) (with n being len of list - len of range)</i><blockquote>Trim an existing list so that it will contain only the specifiedrange of elements specified. Start and end are zero-based indexes.0 is the first element of the list (the list head), 1 the next elementand so on.</blockquote>
<blockquote>For example LTRIM foobar 0 2 will modify the list stored at foobarkey so that only the first three elements of the list will remain.</blockquote>
<blockquote>_start_ and <i>end</i> can also be negative numbers indicating offsetsfrom the end of the list. For example -1 is the last element ofthe list, -2 the penultimate element and so on.</blockquote>
<blockquote>Indexes out of range will not produce an error: if start is overthe end of the list, or start &gt; end, an empty list is left as value.If end over the end of the list Redis will threat it just likethe last element of the list.</blockquote>
<blockquote>Hint: the obvious use of LTRIM is together with LPUSH/RPUSH. For example:</blockquote>
<pre class="codeblock python" name="code">
LPUSH mylist &lt;someelement&gt;
LTRIM mylist 0 99
</pre><blockquote>The above two commands will push elements in the list taking care thatthe list will not grow without limits. This is very useful when usingRedis to store logs for example. It is important to note that when usedin this way LTRIM is an O(1) operation because in the average casejust one element is removed from the tail of the list.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Status code reply</a>
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<b>MgetCommand: Contents</b><br>&nbsp;&nbsp;<a href="#MGET _key1_ _key2_ ... _keyN_">MGET _key1_ _key2_ ... _keyN_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Example">Example</a>
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<h1 class="wikiname">MgetCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="StringCommandsSidebar.html">StringCommandsSidebar</a><h1><a name="MGET _key1_ _key2_ ... _keyN_">MGET _key1_ _key2_ ... _keyN_</a></h1>
<i>Time complexity: O(1) for every key</i><blockquote>Get the values of all the specified keys. If one or more keys dont existor is not of type String, a 'nil' value is returned instead of the valueof the specified key, but the operation never fails.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Multi bulk reply</a><h2><a name="Example">Example</a></h2><pre class="codeblock python" name="code">
$ ./redis-cli set foo 1000
+OK
$ ./redis-cli set bar 2000
+OK
$ ./redis-cli mget foo bar
1. 1000
2. 2000
$ ./redis-cli mget foo bar nokey
1. 1000
2. 2000
3. (nil)
$
</pre>
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<b>MonitorCommand: Contents</b><br>&nbsp;&nbsp;<a href="#MONITOR">MONITOR</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1 class="wikiname">MonitorCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="ControlCommandsSidebar.html">ControlCommandsSidebar</a><h1><a name="MONITOR">MONITOR</a></h1><blockquote>MONITOR is a debugging command that outputs the whole sequence of commandsreceived by the Redis server. is very handy in order to understandwhat is happening into the database. This command is used directlyvia telnet.</blockquote>
<pre class="codeblock python" name="code">
% telnet 127.0.0.1 6379
Trying 127.0.0.1...
Connected to segnalo-local.com.
Escape character is '^]'.
MONITOR
+OK
monitor
keys *
dbsize
set x 6
foobar
get x
del x
get x
set key_x 5
hello
set key_y 5
hello
set key_z 5
hello
set foo_a 5
hello
</pre><blockquote>The ability to see all the requests processed by the server is useful in orderto spot bugs in the application both when using Redis as a database and asa distributed caching system.</blockquote>
<blockquote>In order to end a monitoring session just issue a QUIT command by hand.</blockquote>
<h2><a name="Return value">Return value</a></h2><b>Non standard return value</b>, just dumps the received commands in an infinite flow.
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<b>MoveCommand: Contents</b><br>&nbsp;&nbsp;<a href="#MOVE _key_ _dbindex_">MOVE _key_ _dbindex_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1 class="wikiname">MoveCommand</h1>
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</div>
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&iuml;&raquo;&iquest;#sidebar <a href="GenericCommandsSidebar.html">GenericCommandsSidebar</a><h1><a name="MOVE _key_ _dbindex_">MOVE _key_ _dbindex_</a></h1>
<blockquote>Move the specified key from the currently selected DB to the specifieddestination DB. Note that this command returns 1 only if the key wassuccessfully moved, and 0 if the target key was already there or if thesource key was not found at all, so it is possible to use MOVE as a lockingprimitive.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Integer reply</a>, specifically:<br/><br/><pre class="codeblock python" name="code">
1 if the key was moved
0 if the key was not moved because already present on the target DB or was not found in the current DB.
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<b>MsetCommand: Contents</b><br>&nbsp;&nbsp;<a href="#MSET _key1_ _value1_ _key2_ _value2_ ... _keyN_ _valueN_ (Redis &gt;">MSET _key1_ _value1_ _key2_ _value2_ ... _keyN_ _valueN_ (Redis &gt;</a><br>&nbsp;&nbsp;<a href="#MSETNX _key1_ _value1_ _key2_ _value2_ ... _keyN_ _valueN_ (Redis &gt;">MSETNX _key1_ _value1_ _key2_ _value2_ ... _keyN_ _valueN_ (Redis &gt;</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#MSET Return value">MSET Return value</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#MSETNX Return value">MSETNX Return value</a>
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<h1 class="wikiname">MsetCommand</h1>
<div class="summary">
</div>
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&iuml;&raquo;&iquest;#sidebar <a href="StringCommandsSidebar.html">StringCommandsSidebar</a><h1><a name="MSET _key1_ _value1_ _key2_ _value2_ ... _keyN_ _valueN_ (Redis &gt;">MSET _key1_ _value1_ _key2_ _value2_ ... _keyN_ _valueN_ (Redis &gt;</a></h1> 1.1) =
<h1><a name="MSETNX _key1_ _value1_ _key2_ _value2_ ... _keyN_ _valueN_ (Redis &gt;">MSETNX _key1_ _value1_ _key2_ _value2_ ... _keyN_ _valueN_ (Redis &gt;</a></h1> 1.1) =
<i>Time complexity: O(1) to set every key</i><blockquote>Set the the respective keys to the respective values. MSET will replace oldvalues with new values, while MSETNX will not perform any operation at alleven if just a single key already exists.</blockquote>
<blockquote>Because of this semantic MSETNX can be used in order to set different keysrepresenting different fields of an unique logic object in a way thatensures that either all the fields or none at all are set.</blockquote>
<blockquote>Both MSET and MSETNX are atomic operations. This means that for instanceif the keys A and B are modified, another client talking to Redis can eithersee the changes to both A and B at once, or no modification at all.</blockquote>
<h2><a name="MSET Return value">MSET Return value</a></h2><a href="ReplyTypes.html">Status code reply</a> Basically +OK as MSET can't fail<h2><a name="MSETNX Return value">MSETNX Return value</a></h2><a href="ReplyTypes.html">Integer reply</a>, specifically:<br/><br/><pre class="codeblock python" name="code">
1 if the all the keys were set
0 if no key was set (at least one key already existed)
</pre>
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<b>MultiExecCommand: Contents</b><br>&nbsp;&nbsp;<a href="#MULTI">MULTI</a><br>&nbsp;&nbsp;<a href="#COMMAND_1 ...">COMMAND_1 ...</a><br>&nbsp;&nbsp;<a href="#COMMAND_2 ...">COMMAND_2 ...</a><br>&nbsp;&nbsp;<a href="#COMMAND_N ...">COMMAND_N ...</a><br>&nbsp;&nbsp;<a href="#EXEC or DISCARD">EXEC or DISCARD</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Usage">Usage</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#The DISCARD command">The DISCARD command</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1 class="wikiname">MultiExecCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="GenericCommandsSidebar.html">GenericCommandsSidebar</a><h1><a name="MULTI">MULTI</a></h1>
<h1><a name="COMMAND_1 ...">COMMAND_1 ...</a></h1>
<h1><a name="COMMAND_2 ...">COMMAND_2 ...</a></h1>
<h1><a name="COMMAND_N ...">COMMAND_N ...</a></h1>
<h1><a name="EXEC or DISCARD">EXEC or DISCARD</a></h1><blockquote>MULTI, EXEC and DISCARD commands are the fundation of Redis Transactions.A Redis Transaction allows to execute a group of Redis commands in a singlestep, with two important guarantees:</blockquote>
<ul><li> All the commands in a transaction are serialized and executed sequentially. It can never happen that a request issued by another client is served <b>in the middle</b> of the execution of a Redis transaction. This guarantees that the commands are executed as a single atomic operation.</li><li> Either all of the commands or none are processed. The EXEC command triggers the execution of all the commands in the transaction, so if a client loses the connection to the server in the context of a transaction before calling the MULTI command none of the operations are performed, instead if the EXEC command is called, all the operations are performed. An exception to this rule is when the Append Only File is enabled: every command that is part of a Redis transaction will log in the AOF as long as the operation is completed, so if the Redis server crashes or is killed by the system administrator in some hard way it is possible that only a partial number of operations are registered.</li></ul>
<h2><a name="Usage">Usage</a></h2><blockquote>A Redis transaction is entered using the MULTI command. The command alwaysreplies with OK. At this point the user can issue multiple commands. Insteadto execute this commands Redis will &quot;queue&quot; them. All the commands areexecuted once EXEC is called.</blockquote>
<blockquote>Calling DISCARD instead will flush the transaction queue and will exitthe transaction.</blockquote>
<blockquote>The following is an example using the Ruby client:</blockquote><pre class="codeblock python" name="code">
?&gt; r.multi
=&gt; &quot;OK&quot;
&gt;&gt; r.incr &quot;foo&quot;
=&gt; &quot;QUEUED&quot;
&gt;&gt; r.incr &quot;bar&quot;
=&gt; &quot;QUEUED&quot;
&gt;&gt; r.incr &quot;bar&quot;
=&gt; &quot;QUEUED&quot;
&gt;&gt; r.exec
=&gt; [1, 1, 2]
</pre>
<blockquote>As it is possible to see from the session above, MULTI returns an &quot;array&quot; ofreplies, where every element is the reply of a single command in thetransaction, in the same order the commands were queued.</blockquote>
<blockquote>When a Redis connection is in the context of a MULTI request, all the commandswill reply with a simple string &quot;QUEUED&quot; if they are correct from thepoint of view of the syntax and arity (number of arguments) of the commaand.Some command is still allowed to fail during execution time.</blockquote>
<blockquote>This is more clear if at protocol level: in the following example one commandwill fail when executed even if the syntax is right:</blockquote><pre class="codeblock python python" name="code">
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
MULTI
+OK
SET a 3
abc
+QUEUED
LPOP a
+QUEUED
EXEC
*2
+OK
-ERR Operation against a key holding the wrong kind of value
</pre>
<blockquote>MULTI returned a two elements bulk reply in witch one of this is a +OKcode and one is a -ERR reply. It's up to the client lib to find a sensibleway to provide the error to the user.</blockquote>
<blockquote>IMPORTANT: even when a command will raise an error, all the other commandsin the queue will be processed. Redis will NOT stop the processing ofcommands once an error is found.</blockquote>
<blockquote>Another example, again using the write protocol with telnet, shows howsyntax errors are reported ASAP instead:</blockquote><pre class="codeblock python python python" name="code">
MULTI
+OK
INCR a b c
-ERR wrong number of arguments for 'incr' command
</pre>
<blockquote>This time due to the syntax error the &quot;bad&quot; INCR command is not queuedat all.</blockquote>
<h2><a name="The DISCARD command">The DISCARD command</a></h2><blockquote>DISCARD can be used in order to abort a transaction. No command will beexecuted, and the state of the client is again the normal one, outsideof a transaction. Example using the Ruby client:</blockquote><pre class="codeblock python python python python" name="code">
?&gt; r.set(&quot;foo&quot;,1)
=&gt; true
&gt;&gt; r.multi
=&gt; &quot;OK&quot;
&gt;&gt; r.incr(&quot;foo&quot;)
=&gt; &quot;QUEUED&quot;
&gt;&gt; r.discard
=&gt; &quot;OK&quot;
&gt;&gt; r.get(&quot;foo&quot;)
=&gt; &quot;1&quot;
</pre><h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Multi bulk reply</a>, specifically:<br/><br/><pre class="codeblock python python python python python" name="code">
The result of a MULTI/EXEC command is a multi bulk reply where every element is the return value of every command in the atomic transaction.
</pre>
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<b>ObjectHashMappers: Contents</b><br>&nbsp;&nbsp;<a href="#Object Hash Mappers">Object Hash Mappers</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Ruby">Ruby</a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Ohm">Ohm</a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="#dm-redis-adapter">dm-redis-adapter</a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="#redis-models">redis-models</a>
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</div>
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<h1><a name="Object Hash Mappers">Object Hash Mappers</a></h1>Looking for a higher level if abstraction for your Objects, their Properties and Relationships?<br/><br/>There is not need to stick to the <a href="SupportedLanguages.html">client libraries</a> exposing the raw features of Redis, here you will find a list of <b>Object Hash Mappers</b>, working in the same fashion a ORM does.<h2><a name="Ruby">Ruby</a></h2><h3><a name="Ohm">Ohm</a></h3><ul><li> Object-hash mapping library for Redis. It includes an extensible list of validations and has very good performance.</li><li> Authors: <a href="http://soveran.com/" target="_blank">Michel Martens</a>, <a href="http://twitter.com/soveran" target="_blank">@soveran</a>; and Damian Janowski <a href="http://twitter.com/djanowski" target="_blank">@djanowski</a>.</li><li> Repository: <a href="http://github.com/soveran/ohm" target="_blank">http://github.com/soveran/ohm</a></li><li> Group: <a href="http://groups.google.com/group/ohm-ruby" target="_blank">http://groups.google.com/group/ohm-ruby</a></li></ul>
<h3><a name="dm-redis-adapter">dm-redis-adapter</a></h3><ul><li> This is a DataMapper (ORM that is based on the IdentityMap pattern) adapter for the Redis key-value database.</li><li> Author: <a href="http://whoahbot.com/" target="_blank">Whoahbot</a>, <a href="http://twitter.com/whoahbot" target="_blank">@whoahbot</a>.</li><li> Repository: <a href="http://github.com/whoahbot/dm-redis-adapter/" target="_blank">http://github.com/whoahbot/dm-redis-adapter/</a></li></ul>
<h3><a name="redis-models">redis-models</a></h3><ul><li> Minimal model support for Redis. Directly maps Ruby properties to model_name:id:field_name keys in redis. Scalar, List and Set properties are supported. Values can be marshaled to/from Integer, Float, DateTime, JSON. </li><li> Repository: <a href="http://github.com/voloko/redis-model" target="_blank">http://github.com/voloko/redis-model</a></li></ul>
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<b>Pipelining: Contents</b><br>&nbsp;&nbsp;<a href="#Pipelining (DRAFT)">Pipelining (DRAFT)</a>
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<h1 class="wikiname">Pipelining</h1>
<div class="summary">
</div>
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<h1><a name="Pipelining (DRAFT)">Pipelining (DRAFT)</a></h1>A client library can use the same connection in order to issue multiple commands. But Redis supports <b>pipelining</b>, so multiple commands can be sent to the server with a single write operation by the client, without need to read the server reply in order to issue the next command. All the replies can be read at the end.<br/><br/>Usually Redis server and client will have a very fast link so this is not very important to support this feature in a client implementation, still if an application needs to issue a very large number of commands in s short time, using pipelining can be much faster.<br/><br/>Please read the <a href="ProtocolSpecification.html">ProtocolSpecification</a> if you want to learn more about the way Redis <a href="SupportedLanguages.html">clients</a> and the server communicate.<br/><br/>Pipelining is one of the <a href="Speed.html">Speed</a> <a href="Features.html">Features</a> of Redis, you can also check the support for <a href="MultiBulkCommands.html">send and receive multiple values in a single command</a>.
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<b>ProgrammingExamples: Contents</b><br>&nbsp;&nbsp;<a href="#Programming Examples (DRAFT)">Programming Examples (DRAFT)</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#TODO">TODO</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Java">Java</a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Twayis">Twayis</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#PHP">PHP</a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Retwis">Retwis</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Ruby">Ruby</a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="#twatcher-lite">twatcher-lite</a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Resque">Resque</a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Retwis-rb">Retwis-rb</a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="#scanty-redis">scanty-redis</a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Note Taking">Note Taking</a>
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<h1><a name="Programming Examples (DRAFT)">Programming Examples (DRAFT)</a></h1><h2><a name="TODO">TODO</a></h2><ul><li> Add <a href="http://github.com/jodosha/redis-store" target="_blank">http://github.com/jodosha/redis-store</a></li></ul>
Nothing speaks better than code examples, here you are:<h2><a name="Java">Java</a></h2><h3><a name="Twayis">Twayis</a></h3> <br/><br/>A Java clone of <b>Retwis</b> showcase integration between the <a href="http://www.playframework.org/" target="_blank">Play! framework</a> and Redis <a href="http://code.google.com/p/twayis/" target="_blank">Google Code Project Page</a><h2><a name="PHP">PHP</a></h2><h3><a name="Retwis">Retwis</a></h3>A PHP Twitter clone, the original example of Redis capabilities. With a <a href="http://retwis.antirez.com/" target="_blank">live demo</a>, and an <a href="http://code.google.com/p/redis/wiki/TwitterAlikeExample" target="_blank">article explaining it design</a>. You can find the code in the Downloads tab.<h2><a name="Ruby">Ruby</a></h2><h3><a name="twatcher-lite">twatcher-lite</a></h3>A simplied version of the application running <a href="http://twatcher.com/" target="_blank">http://twatcher.com/</a> from Mirko Froehlich (<a href="http://twitter.com/digitalhobbit" target="_blank">@digitalhobbit</a>) with a full blog post explaining its development at <a href="http://www.digitalhobbit.com/2009/11/08/building-a-twitter-filter-with-sinatra-redis-and-tweetstream/" target="_blank"> Building a Twitter Filter With Sinatra, Redis, and TweetStream</a><h3><a name="Resque">Resque</a></h3>The &quot;simple&quot; Redis-based queue behind Github background jobs, that replaced SQS, Starling, ActiveMessaging, BackgroundJob, DelayedJob, and Beanstalkd. Developed by Chris Wanstrath (<a href="http://twitter.com/defunkt" target="_blank">@defunkt</a>) the code is at <a href="http://github.com/defunkt/resque" target="_blank">http://github.com/defunkt/resque</a>, be sure to read <a href="http://github.com/blog/542-introducing-resque" target="_blank">the introduction</a><h3><a name="Retwis-rb">Retwis-rb</a></h3>A port of <b>Retwis</b> to Ruby and <a href="http://www.sinatrarb.com/" target="_blank">Sinatra</a> written by Daniel Lucraft (<a href="http://twitter.com/DanLucraft" target="_blank">@DanLucraft</a>) Full source code is available at <a href="http://github.com/danlucraft/retwis-rb" target="_blank">http://github.com/danlucraft/retwis-rb</a><h3><a name="scanty-redis">scanty-redis</a></h3>Scanty is <i>minimal</i> blogging software developed by Adam Wiggins (<a href="http://twitter.com/hirodusk" target="_blank">@hirodusk</a>) It is not a blogging engine, but it&acirc;€™s small and easy to modify, so it could be the starting point for your blog. <a href="http://github.com/adamwiggins/scanty-redis" target="_blank">This fork</a> is modified to use Redis, a full featured key-value database, instead of SQL. <h3><a name="Note Taking">Note Taking</a></h3>A <i>very simple</i> note taking example of Ruby and Redis application using <a href="http://www.sinatrarb.com/" target="_blank">Sinatra</a>. Developed by Pieter Noordhuis <a href="http://twitter.com/pnoordhuis" target="_blank">@pnoordhuis</a>, you can check the code at <a href="http://gist.github.com/86714" target="_blank">http://gist.github.com/86714</a>
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<b>ProtocolSpecification: Contents</b><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Networking layer">Networking layer</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Simple INLINE commands">Simple INLINE commands</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Bulk commands">Bulk commands</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Bulk replies">Bulk replies</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Multi-Bulk replies">Multi-Bulk replies</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Nil elements in Multi-Bulk replies">Nil elements in Multi-Bulk replies</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Single line reply">Single line reply</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Integer reply">Integer reply</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Multi bulk commands">Multi bulk commands</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Multiple commands and pipelining">Multiple commands and pipelining</a>
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<h1 class="wikiname">ProtocolSpecification</h1>
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&iuml;&raquo;&iquest;= Protocol Specification =<br/><br/>The Redis protocol is a compromise between being easy to parse by a computer
and being easy to parse by an human. Before reading this section you are
strongly encouraged to read the &quot;REDIS TUTORIAL&quot; section of this README in order
to get a first feeling of the protocol playing with it by TELNET.<h2><a name="Networking layer">Networking layer</a></h2>A client connects to a Redis server creating a TCP connection to the port 6379.
Every redis command or data transmitted by the client and the server is
terminated by &quot;\r\n&quot; (CRLF).<h2><a name="Simple INLINE commands">Simple INLINE commands</a></h2>The simplest commands are the inline commands. This is an example of a
server/client chat (the server chat starts with S:, the client chat with C:)<br/><br/><pre class="codeblock python" name="code">
C: PING
S: +PONG
</pre>An inline command is a CRLF-terminated string sent to the client. The server can reply to commands in different ways:
<ul><li> With an error message (the first byte of the reply will be &quot;-&quot;)</li><li> With a single line reply (the first byte of the reply will be &quot;+)</li><li> With bulk data (the first byte of the reply will be &quot;$&quot;)</li><li> With multi-bulk data, a list of values (the first byte of the reply will be &quot;<code name="code" class="python">*</code>&quot;)</li><li> With an integer number (the first byte of the reply will be &quot;:&quot;)</li></ul>
The following is another example of an INLINE command returning an integer:<br/><br/><pre class="codeblock python python" name="code">
C: EXISTS somekey
S: :0
</pre>Since 'somekey' does not exist the server returned ':0'.<br/><br/>Note that the EXISTS command takes one argument. Arguments are separated
simply by spaces.<h2><a name="Bulk commands">Bulk commands</a></h2>A bulk command is exactly like an inline command, but the last argument
of the command must be a stream of bytes in order to send data to the server.
the &quot;SET&quot; command is a bulk command, see the following example:<br/><br/><pre class="codeblock python python python" name="code">
C: SET mykey 6
C: foobar
S: +OK
</pre>The last argument of the commnad is '6'. This specify the number of DATA
bytes that will follow (note that even this bytes are terminated by two
additional bytes of CRLF).<br/><br/>All the bulk commands are in this exact form: instead of the last argument
the number of bytes that will follow is specified, followed by the bytes,
and CRLF. In order to be more clear for the programmer this is the string
sent by the client in the above sample:<br/><br/><blockquote>&quot;SET mykey 6\r\nfoobar\r\n&quot;</blockquote>
<h2><a name="Bulk replies">Bulk replies</a></h2>The server may reply to an inline or bulk command with a bulk reply. See
the following example:<br/><br/><pre class="codeblock python python python python" name="code">
C: GET mykey
S: $6
S: foobar
</pre>A bulk reply is very similar to the last argument of a bulk command. The
server sends as the first line a &quot;$&quot; byte followed by the number of bytes
of the actual reply followed by CRLF, then the bytes are sent followed by
additional two bytes for the final CRLF. The exact sequence sent by the
server is:<br/><br/><blockquote>&quot;$6\r\nfoobar\r\n&quot;</blockquote>
If the requested value does not exist the bulk reply will use the special
value -1 as data length, example:<br/><br/><pre class="codeblock python python python python python" name="code">
C: GET nonexistingkey
S: $-1
</pre>The client library API should not return an empty string, but a nil object, when the requested object does not exist.
For example a Ruby library should return 'nil' while a C library should return
NULL, and so forth.<h2><a name="Multi-Bulk replies">Multi-Bulk replies</a></h2>Commands similar to LRANGE needs to return multiple values (every element
of the list is a value, and LRANGE needs to return more than a single element). This is accomplished using multiple bulk writes,
prefixed by an initial line indicating how many bulk writes will follow.
The first byte of a multi bulk reply is always <code name="code" class="python">*</code>. Example:<br/><br/><pre class="codeblock python python python python python python" name="code">
C: LRANGE mylist 0 3
S: *4
S: $3
S: foo
S: $3
S: bar
S: $5
S: Hello
S: $5
S: World
</pre>The first line the server sent is &quot;<b>4\r\n&quot; in order to specify that four bulk
write will follow. Then every bulk write is transmitted.<br/><br/>If the specified key does not exist instead of the number of elements in the
list, the special value -1 is sent as count. Example:<br/><br/><pre class="codeblock python python python python python python python" name="code">
C: LRANGE nokey 0 1
S: *-1
</pre>A client library API SHOULD return a nil object and not an empty list when this
happens. This makes possible to distinguish between empty list and non existing ones.<h2><a name="Nil elements in Multi-Bulk replies">Nil elements in Multi-Bulk replies</a></h2>Single elements of a multi bulk reply may have -1 length, in order to signal that this elements are missing and not empty strings. This can happen with the SORT command when used with the GET <i>pattern</i> option when the specified key is missing. Example of a multi bulk reply containing an empty element:<br/><br/><pre class="codeblock python python python python python python python python" name="code">
S: *3
S: $3
S: foo
S: $-1
S: $3
S: bar
</pre>The second element is nul. The client library should return something like this:<br/><br/><pre class="codeblock python python python python python python python python python" name="code">
[&quot;foo&quot;,nil,&quot;bar&quot;]
</pre><h2><a name="Single line reply">Single line reply</a></h2>As already seen a single line reply is in the form of a single line string
starting with &quot;+&quot; terminated by &quot;\r\n&quot;. For example:<br/><br/><pre class="codeblock python python python python python python python python python python" name="code">
+OK
</pre>The client library should return everything after the &quot;+&quot;, that is, the string &quot;OK&quot; in the example.<br/><br/>The following commands reply with a status code reply:
PING, SET, SELECT, SAVE, BGSAVE, SHUTDOWN, RENAME, LPUSH, RPUSH, LSET, LTRIM<h2><a name="Integer reply">Integer reply</a></h2>This type of reply is just a CRLF terminated string representing an integer, prefixed by a &quot;:&quot; byte. For example &quot;:0\r\n&quot;, or &quot;:1000\r\n&quot; are integer replies.<br/><br/>With commands like INCR or LASTSAVE using the integer reply to actually return a value there is no special meaning for the returned integer. It is just an incremental number for INCR, a UNIX time for LASTSAVE and so on.<br/><br/>Some commands like EXISTS will return 1 for true and 0 for false.<br/><br/>Other commands like SADD, SREM and SETNX will return 1 if the operation was actually done, 0 otherwise.<br/><br/>The following commands will reply with an integer reply: SETNX, DEL, EXISTS, INCR, INCRBY, DECR, DECRBY, DBSIZE, LASTSAVE, RENAMENX, MOVE, LLEN, SADD, SREM, SISMEMBER, SCARD<h2><a name="Multi bulk commands">Multi bulk commands</a></h2>As you can see with the protocol described so far there is no way to
send multiple binary-safe arguments to a command. With bulk commands the
last argument is binary safe, but there are commands where multiple binary-safe
commands are needed, like the MSET command that is able to SET multiple keys
in a single operation.<br/><br/>In order to address this problem Redis 1.1 introduced a new way of seding
commands to a Redis server, that uses exactly the same protocol of the
multi bulk replies. For instance the following is a SET command using the
normal bulk protocol:<br/><br/><pre class="codeblock python python python python python python python python python python python" name="code">
SET mykey 8
myvalue
</pre>While the following uses the multi bulk command protocol:<br/><br/><pre class="codeblock python python python python python python python python python python python python" name="code">
*3
$3
SET
$5
mykey
$8
myvalue
</pre>Commands sent in this format are longer, so currently they are used only in
order to transmit commands containing multiple binary-safe arguments, but
actually this protocol can be used to send every kind of command, without to
know if it's an inline, bulk or multi-bulk command.<br/><br/>It is possible that in the future Redis will support only this format.<br/><br/>A good client library may implement unknown commands using this
command format in order to support new commands out of the box without
modifications.<h2><a name="Multiple commands and pipelining">Multiple commands and pipelining</a></h2>A client can use the same connection in order to issue multiple commands.
Pipelining is supported so multiple commands can be sent with a single
write operation by the client, it is not needed to read the server reply
in order to issue the next command. All the replies can be read at the end.<br/><br/>Usually Redis server and client will have a very fast link so this is not
very important to support this feature in a client implementation, still
if an application needs to issue a very large number of commands in short
time to use pipelining can be much faster.
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<b>PublishSubscribe: Contents</b><br>&nbsp;&nbsp;<a href="#UNSUBSCRIBE channel_1 channel_2 ... channel_N">UNSUBSCRIBE channel_1 channel_2 ... channel_N</a><br>&nbsp;&nbsp;<a href="#UNSUBSCRIBE (unsubscribe from all channels)">UNSUBSCRIBE (unsubscribe from all channels)</a><br>&nbsp;&nbsp;<a href="#PSUBSCRIBE pattern_1 pattern_2 ... pattern_N">PSUBSCRIBE pattern_1 pattern_2 ... pattern_N</a><br>&nbsp;&nbsp;<a href="#PUNSUBSCRIBE pattern_1 pattern_2 ... pattern_N">PUNSUBSCRIBE pattern_1 pattern_2 ... pattern_N</a><br>&nbsp;&nbsp;<a href="#PUNSUBSCRIBE (unsubscribe from all patterns)">PUNSUBSCRIBE (unsubscribe from all patterns)</a><br>&nbsp;&nbsp;<a href="#PUBLISH channel message">PUBLISH channel message</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Format of pushed messages">Format of pushed messages</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Unsubscribing from all the channels at once">Unsubscribing from all the channels at once</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Wire protocol example">Wire protocol example</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#PSUBSCRIBE and PUNSUBSCRIBE: pattern matching subscriptions">PSUBSCRIBE and PUNSUBSCRIBE: pattern matching subscriptions</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Messages matching both a pattern and a channel subscription">Messages matching both a pattern and a channel subscription</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#The meaning of the count of subscriptions with pattern matching">The meaning of the count of subscriptions with pattern matching</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#More details on the PUBLISH command">More details on the PUBLISH command</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Programming Example">Programming Example</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Client library implementations hints">Client library implementations hints</a>
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<h1 class="wikiname">PublishSubscribe</h1>
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&iuml;&raquo;&iquest;=SUBSCRIBE channel_1 channel_2 ... channel_N=
<h1><a name="UNSUBSCRIBE channel_1 channel_2 ... channel_N">UNSUBSCRIBE channel_1 channel_2 ... channel_N</a></h1>
<h1><a name="UNSUBSCRIBE (unsubscribe from all channels)">UNSUBSCRIBE (unsubscribe from all channels)</a></h1>
<h1><a name="PSUBSCRIBE pattern_1 pattern_2 ... pattern_N">PSUBSCRIBE pattern_1 pattern_2 ... pattern_N</a></h1>
<h1><a name="PUNSUBSCRIBE pattern_1 pattern_2 ... pattern_N">PUNSUBSCRIBE pattern_1 pattern_2 ... pattern_N</a></h1>
<h1><a name="PUNSUBSCRIBE (unsubscribe from all patterns)">PUNSUBSCRIBE (unsubscribe from all patterns)</a></h1>
<h1><a name="PUBLISH channel message">PUBLISH channel message</a></h1>Time complexity: subscribe is O(1), unsubscribe is O(N) where N is the number of clients already subscribed to a channel, publish is O(N+M) where N is the number of clients subscribed to the receiving channel, and M is the total number of subscribed patterns (by any client). Psubscribe is O(N) where N is the number of patterns the Psubscribing client is already subscribed to. Punsubscribe is O(N+M) where N is the number of patterns the Punsubscribing client is already subscribed and M is the number of total patterns subscribed in the system (by any client).<br/><br/><blockquote>SUBSCRIBE, UNSUBSCRIBE and PUBLISH commands implement the<a href="http://en.wikipedia.org/wiki/Publish/subscribe" target="_blank">Publish/Subscribe messaging paradigm</a> where (citing Wikipedia) senders (publishers) are not programmed to send their messages to specific receivers (subscribers). Rather, published messages are characterized into channels, without knowledge of what (if any) subscribers there may be. Subscribers express interest in one or more channels, and only receive messages that are of interest, without knowledge of what (if any) publishers there are. This decoupling of publishers and subscribers can allow for greater scalability and a more dynamic network topology.</blockquote>
<blockquote>For instance in order to subscribe to the channels foo and bar the clientwill issue the SUBSCRIBE command followed by the names of the channels.</blockquote><pre class="codeblock python" name="code">
SUBSCRIBE foo bar
</pre>
<blockquote>All the messages sent by other clients to this channels will be pushed bythe Redis server to all the subscribed clients, in the form of a threeelements bulk reply, where the first element is the message type, thesecond the originating channel, and the third argument the message payload.</blockquote>
<blockquote>A client subscribed to 1 or more channels should NOT issue other commandsother than SUBSCRIBE and UNSUBSCRIBE, but can subscribe or unsubscribeto other channels dynamically.</blockquote>
<blockquote>The reply of the SUBSCRIBE and UNSUBSCRIBE operations are sent in the formof messages, so that the client can just read a coherent stream of messageswhere the first element indicates the kind of message.</blockquote><h2><a name="Format of pushed messages">Format of pushed messages</a></h2>
<blockquote>Messages are in the form of multi bulk replies with three elements.The first element is the kind of message:</blockquote><ul><li> &quot;subscribe&quot;: means that we successfully subscribed to the channel given as second element of the multi bulk reply. The third argument represents the number of channels we are currently subscribed to.</li><li> &quot;unsubscribe&quot;: means that we successfully unsubscribed from the channel given as second element of the multi bulk reply. The third argument represents the number of channels we are currently subscribed to. If this latest argument is zero, we are no longer subscribed to any channel, and the client can issue any kind of Redis command as we are outside the Pub/sub state.</li><li> &quot;message&quot;: it is a message received as result of a PUBLISH command issued by another client. The second element is the name of the originating channel, and the third the actual message payload.</li></ul><h2><a name="Unsubscribing from all the channels at once">Unsubscribing from all the channels at once</a></h2>
If the UNSUBSCRIBE command is issued without additional arguments, it is equivalent to unsubscribing to all the channels we are currently subscribed. A message for every unsubscribed channel will be received.
<h2><a name="Wire protocol example">Wire protocol example</a></h2>
<pre class="codeblock python python" name="code">
SUBSCRIBE first second
*3
$9
subscribe
$5
first
:1
*3
$9
subscribe
$6
second
:2
</pre>
at this point from another client we issue a PUBLISH operation against the channel named &quot;second&quot;. This is what the first client receives:
<pre class="codeblock python python python" name="code">
*3
$7
message
$6
second
$5
Hello
</pre>
Now the client unsubscribes itself from all the channels using the UNSUBSCRIBE command without additional arguments:
<pre class="codeblock python python python python" name="code">
UNSUBSCRIBE
*3
$11
unsubscribe
$6
second
:1
*3
$11
unsubscribe
$5
first
:0
</pre>
<h2><a name="PSUBSCRIBE and PUNSUBSCRIBE: pattern matching subscriptions">PSUBSCRIBE and PUNSUBSCRIBE: pattern matching subscriptions</a></h2>
Redis Pub/Sub implementation supports pattern matching. Clients may subscribe to glob style patterns in order to receive all the messages sent to channel names matching a given pattern.<br/><br/>For instance the command:
<pre class="codeblock python python python python python" name="code">
PSUBSCRIBE news.*
</pre>
Will receive all the messages sent to the channel news.art.figurative and news.music.jazz and so forth. All the glob style patterns as valid, so multiple wild cards are supported.<br/><br/>Messages received as a result of pattern matching are sent in a different format:
<ul><li> The type of the message is &quot;pmessage&quot;: it is a message received as result of a PUBLISH command issued by another client, matching a pattern matching subscription. The second element is the original pattern matched, the third element is the name of the originating channel, and the last element the actual message payload.</li></ul>
Similarly to SUBSCRIBE and UNSUBSCRIBE, PSUBSCRIBE and PUNSUBSCRIBE commands are acknowledged by the system sending a message of type &quot;psubscribe&quot; and &quot;punsubscribe&quot; using the same format as the &quot;subscribe&quot; and &quot;unsubscribe&quot; message format.
<h2><a name="Messages matching both a pattern and a channel subscription">Messages matching both a pattern and a channel subscription</a></h2>
A client may receive a single message multiple time if it's subscribed to multiple patterns matching a published message, or it is subscribed to both patterns and channels matching the message. Like in the following example:
<pre class="codeblock python python python python python python" name="code">
SUBSCRIBE foo
PSUBSCRIBE f*
</pre>
In the above example, if a message is sent to the <b>foo</b> channel, the client will receive two messages, one of type &quot;message&quot; and one of type &quot;pmessage&quot;.
<h2><a name="The meaning of the count of subscriptions with pattern matching">The meaning of the count of subscriptions with pattern matching</a></h2>
In <b>subscribe</b>, <b>unsubscribe</b>, <b>psubscribe</b> and <b>punsubscribe</b> message types, the last argument is the count of subscriptions still active. This number is actually the total number of channels and patterns the client is still subscribed to. So the client will exit the Pub/Sub state only when this count will drop to zero as a result of unsubscription from all the channels and patterns.
<h2><a name="More details on the PUBLISH command">More details on the PUBLISH command</a></h2>
The Publish command is a bulk command where the first argument is the target class, and the second argument the data to send. It returns an Integer Reply representing the number of clients that received the message (that is, the number of clients that were listening for this class).
<h2><a name="Programming Example">Programming Example</a></h2>
Pieter Noordhuis provided a great example using Event-machine and Redis to create <a href="http://chat.redis-db.com" target="_blank">a multi user high performance web chat</a>, with source code included of course!
<h2><a name="Client library implementations hints">Client library implementations hints</a></h2>
Because all the messages received contain the original subscription causing the message delivery (the channel in the case of &quot;message&quot; type, and the original pattern in the case of &quot;pmessage&quot; type) clinet libraries may bind the original subscription to callbacks (that can be anonymous functions, blocks, function pointers, and so forth), using an hash table.<br/><br/>When a message is received an O(1) lookup can be done in order to deliver the message to the registered callback.
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<b>QuickStart: Contents</b><br>&nbsp;&nbsp;<a href="#Quick Start">Quick Start</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Obtain the latest version">Obtain the latest version</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Compile">Compile</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Run the server">Run the server</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Play with the built in client">Play with the built in client</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Further reading">Further reading</a>
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&iuml;&raquo;&iquest;#sidebar <a href="RedisGuides.html">RedisGuides</a>
<h1><a name="Quick Start">Quick Start</a></h1>This quickstart is a five minutes howto on how to get started with Redis. For more information on Redis check <a href="http://code.google.com/p/redis/wiki/index" target="_blank">Redis Documentation Index</a>.<h2><a name="Obtain the latest version">Obtain the latest version</a></h2>The latest stable source distribution of Redis can be obtained <a href="http://code.google.com/p/redis/downloads/list" target="_blank">at this location as a tarball</a>.<br/><br/><pre class="codeblock python" name="code">
$ wget http://redis.googlecode.com/files/redis-1.02.tar.gz
</pre>The unstable source code, with more features but not ready for production, can be downloaded using git:<br/><br/><pre class="codeblock python python" name="code">
$ git clone git://github.com/antirez/redis.git
</pre><h2><a name="Compile">Compile</a></h2>Redis can be compiled in most <a href="SupportedPlatforms.html">POSIX systems</a>. To compile Redis just untar the tar.gz, enter the directly and type 'make'.<br/><br/><pre class="codeblock python python python" name="code">
$ tar xvzf redis-1.02.tar.gz
$ cd redis-1.02
$ make
</pre>In order to test if the Redis server is working well in your computer make sure to run <code name="code" class="python">make test</code> and check that all the tests are passed.<h2><a name="Run the server">Run the server</a></h2>Redis can run just fine without a configuration file (when executed without a config file a standard configuration is used). To run Redis just type the following command:<br/><br/><pre class="codeblock python python python python" name="code">
$ ./redis-server
</pre>With the <a href="Configuration.html">default configuration</a> Redis will log to the standard output so you can check what happens. Later, you can <a href="Configuration.html">change the default settings</a>.<h2><a name="Play with the built in client">Play with the built in client</a></h2>Redis ships with a command line client that is automatically compiled when you ran <code name="code" class="python">make</code> and it is called <code name="code" class="python">redis-cli</code>For instance to set a key and read back the value use the following:<br/><br/><pre class="codeblock python python python python python" name="code">
$ ./redis-cli set mykey somevalue
OK
$ ./redis-cli get mykey
somevalue
</pre>What about adding elements to a <a href="Lists.html">list</a>:<br/><br/><pre class="codeblock python python python python python python" name="code">
$ ./redis-cli lpush mylist firstvalue
OK
$ ./redis-cli lpush mylist secondvalue
OK
$ ./redis-cli lpush mylist thirdvalue
OK
$ ./redis-cli lrange mylist 0 -1
1. thirdvalue
2. secondvalue
3. firstvalue
$ ./redis-cli rpop mylist
firstvalue
$ ./redis-cli lrange mylist 0 -1
1. thirdvalue
2. secondvalue
</pre><h2><a name="Further reading">Further reading</a></h2><ul><li> What to play more with Redis? Read <a href="IntroductionToRedisDataTypes.html">Fifteen minutes introduction to Redis data types</a>.</li><li> Check all the <a href="Features.html">Features</a></li><li> Read the full list of available commands in the <a href="CommandReference.html">Command Reference</a>.</li><li> Start using Redis from your <a href="SupportedLanguages.html">favorite language</a>.</li><li> Take a look at some <a href="ProgrammingExamples.html">Programming Examples</a>. </li></ul>
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<b>QuitCommand: Contents</b><br>&nbsp;&nbsp;<a href="#Quit">Quit</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1 class="wikiname">QuitCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="ConnectionHandlingSidebar.html">ConnectionHandlingSidebar</a><h1><a name="Quit">Quit</a></h1><blockquote>Ask the server to silently close the connection.</blockquote>
<h2><a name="Return value">Return value</a></h2>None. The connection is closed as soon as the QUIT command is received.
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<b>README: Contents</b><br>&nbsp;&nbsp;<a href="#All data in memory, but saved on disk">All data in memory, but saved on disk</a><br>&nbsp;&nbsp;<a href="#Master-Slave replication made trivial">Master-Slave replication made trivial</a><br>&nbsp;&nbsp;<a href="#It's persistent but supports expires">It's persistent but supports expires</a><br>&nbsp;&nbsp;<a href="#Beyond key-value databases">Beyond key-value databases</a><br>&nbsp;&nbsp;<a href="#Multiple databases support">Multiple databases support</a><br>&nbsp;&nbsp;<a href="#Know more about Redis!">Know more about Redis!</a><br>&nbsp;&nbsp;<a href="#Redis Tutorial">Redis Tutorial</a><br>&nbsp;&nbsp;<a href="#License">License</a><br>&nbsp;&nbsp;<a href="#Credits">Credits</a>
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<h1 class="wikiname">README</h1>
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&iuml;&raquo;&iquest;= Introduction =<br/><br/>Redis is a database. To be specific, Redis is a database implementing a dictionary, where every key is associated with a value. For example I can set the key &quot;surname_1992&quot; to the string &quot;Smith&quot;.
What makes Redis different from many other key-value stores, is that every single value has a type. The following types are supported:<br/><br/><ul><li> <a href="Strings.html">Strings</a></li><li> <a href="Lists.html">Lists</a></li><li> <a href="Sets.html">Sets</a></li><li> <a href="SortedSets.html">Sorted Set</a> (since version 1.1)</li></ul>
The type of a value determines what operations (called commands) are available for the value itself.
For example you can append elements to a list stored at the key &quot;mylist&quot; using the LPUSH or RPUSH command in O(1). Later you'll be able to get a range of elements with LRANGE or trim the list with LTRIM. Sets are very flexible too, it is possible to add and remove elements from Sets (unsorted collections of strings), and then ask for server-side intersection, union, difference of Sets. Each command is performed through server-side atomic operations.
Please refer to the <a href="CommandReference.html">Command Reference</a> to see the full list of operations associated to these data types.<br/><br/>In other words, you can look at Redis as a data structures server. A Redis user is virtually provided with an interface to <a href="http://en.wikipedia.org/wiki/Abstract_data_type" target="_blank">Abstract Data Types</a>, saving her from the responsibility to implement concrete data structures and algorithms. Indeed both algorithms and data structures in Redis are properly choosed in order to obtain the best performance.<h1><a name="All data in memory, but saved on disk">All data in memory, but saved on disk</a></h1>Redis loads and mantains the whole dataset into memory, but the dataset is persistent, since at the same time it is saved on disk, so that when the server is restarted data can be loaded back in memory.<br/><br/>There are two kind of persistence supported: the first one is called snapshotting. In this mode Redis, from time to time, writes a dump on disk asynchronously. The dataset is loaded from the dump every time the server is (re)started.<br/><br/>Redis can be configured to save the dataset when a certain number of changes is reached and after a given number of seconds elapses. For example, you can configure Redis to save after 1000 changes and at most 60 seconds since the last save. You can specify any combination for these numbers.<br/><br/>Because data is written asynchronously, when a system crash occurs, the last few queries can get lost (that is acceptable in many applications but not in all). In order to make this a non issue Redis supports another, safer persistence mode, called <a href="AppendOnlyFileHowto.html">Append Only File</a>, where every command received altering the dataset (so not a read-only command, but a write command) is written on an append only file ASAP. This commands are <i>replayed</i> when the server is restarted in order to rebuild the dataset in memory.<br/><br/>Redis Append Only File supports a very handy feature: the server is able to safely rebuild the append only file in background in a non-blocking fashion when it gets too long. You can find <a href="AppendOnlyFileHowto.html">more details in the Append Only File HOWTO</a>.<h1><a name="Master-Slave replication made trivial">Master-Slave replication made trivial</a></h1>Whatever will be the persistence mode you'll use Redis supports master-slave replications if you want to stay really safe or if you need to scale to huge amounts of reads.<br/><br/><b>Redis Replication is trivial to setup</b>. So trivial that all you need to do in order to configure a Redis server to be a slave of another one, with automatic synchronization if the link will go down and so forth, is the following config line: <code name="code" class="python">slaveof 192.168.1.100 6379</code>. <a href="ReplicationHowto.html">We provide a Replication Howto</a> if you want to know more about this feature.<h1><a name="It's persistent but supports expires">It's persistent but supports expires</a></h1>Redis can be used as a <b>memcached on steroids</b> because is as fast as memcached but with a number of features more. Like memcached, Redis also supports setting timeouts to keys so that this key will be automatically removed when a given amount of time passes.<h1><a name="Beyond key-value databases">Beyond key-value databases</a></h1>All these features allow to use Redis as the sole DB for your scalable application without the need of any relational database. <a href="TwitterAlikeExample.html">We wrote a simple Twitter clone in PHP + Redis</a> to show a real world example, the link points to an article explaining the design and internals in very simple words.<h1><a name="Multiple databases support">Multiple databases support</a></h1>Redis supports multiple databases with commands to atomically move keys from one database to the other. By default DB 0 is selected for every new connection, but using the SELECT command it is possible to select a different database. The MOVE operation can move an item from one DB to another atomically. This can be used as a base for locking free algorithms together with the 'RANDOMKEY' commands.<h1><a name="Know more about Redis!">Know more about Redis!</a></h1>To really get a feeling about what Redis is and how it works please try reading <a href="IntroductionToRedisDataTypes.html">A fifteen minutes introduction to Redis data types</a>.<br/><br/>To know a bit more about how Redis works <i>internally</i> continue reading.<h1><a name="Redis Tutorial">Redis Tutorial</a></h1>(note, you can skip this section if you are only interested in &quot;formal&quot; doc.)<br/><br/>Later in this document you can find detailed information about Redis commands,
the protocol specification, and so on. This kind of documentation is useful
but... if you are new to Redis it is also BORING! The Redis protocol is designed
so that is both pretty efficient to be parsed by computers, but simple enough
to be used by humans just poking around with the 'telnet' command, so this
section will show to the reader how to play a bit with Redis to get an initial
feeling about it, and how it works.<br/><br/>To start just compile redis with 'make' and start it with './redis-server'.
The server will start and log stuff on the standard output, if you want
it to log more edit redis.conf, set the loglevel to debug, and restart it.<br/><br/>You can specify a configuration file as unique parameter:<br/><br/><blockquote>./redis-server /etc/redis.conf</blockquote>
This is NOT required. The server will start even without a configuration file
using a default built-in configuration.<br/><br/>Now let's try to set a key to a given value:<br/><br/><pre class="codeblock python" name="code">
$ telnet localhost 6379
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
SET foo 3
bar
+OK
</pre>The first line we sent to the server is &quot;set foo 3&quot;. This means &quot;set the key
foo with the following three bytes I'll send you&quot;. The following line is
the &quot;bar&quot; string, that is, the three bytes. So the effect is to set the
key &quot;foo&quot; to the value &quot;bar&quot;. Very simple!<br/><br/>(note that you can send commands in lowercase and it will work anyway,
commands are not case sensitive)<br/><br/>Note that after the first and the second line we sent to the server there
is a newline at the end. The server expects commands terminated by &quot;\r\n&quot;
and sequence of bytes terminated by &quot;\r\n&quot;. This is a minimal overhead from
the point of view of both the server and client but allows us to play with
Redis with the telnet command easily.<br/><br/>The last line of the chat between server and client is &quot;+OK&quot;. This means
our key was added without problems. Actually SET can never fail but
the &quot;+OK&quot; sent lets us know that the server received everything and
the command was actually executed.<br/><br/>Let's try to get the key content now:<br/><br/><pre class="codeblock python python" name="code">
GET foo
$3
bar
</pre>Ok that's very similar to 'set', just the other way around. We sent &quot;get foo&quot;,
the server replied with a first line that is just the $ character follwed by
the number of bytes the value stored at key contained, followed by the actual
bytes. Again &quot;\r\n&quot; are appended both to the bytes count and the actual data. In Redis slang this is called a bulk reply.<br/><br/>What about requesting a non existing key?<br/><br/><pre class="codeblock python python python" name="code">
GET blabla
$-1
</pre>When the key does not exist instead of the length, just the &quot;$-1&quot; string is sent. Since a -1 length of a bulk reply has no meaning it is used in order to specifiy a 'nil' value and distinguish it from a zero length value. Another way to check if a given key exists or not is indeed the EXISTS command:<br/><br/><pre class="codeblock python python python python" name="code">
EXISTS nokey
:0
EXISTS foo
:1
</pre>As you can see the server replied ':0' the first time since 'nokey' does not
exist, and ':1' for 'foo', a key that actually exists. Replies starting with the colon character are integer reply.<br/><br/>Ok... now you know the basics, read the <a href="CommandReference.html">REDIS COMMAND REFERENCE</a> section to
learn all the commands supported by Redis and the <a href="ProtocolSpecification.html">PROTOCOL SPECIFICATION</a>
section for more details about the protocol used if you plan to implement one
for a language missing a decent client implementation.<h1><a name="License">License</a></h1>Redis is released under the BSD license. See the COPYING file for more information.<h1><a name="Credits">Credits</a></h1>Redis is written and maintained by Salvatore Sanfilippo, Aka 'antirez'.
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<b>RandomkeyCommand: Contents</b><br>&nbsp;&nbsp;<a href="#RANDOMKEY">RANDOMKEY</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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&iuml;&raquo;&iquest;#sidebar <a href="GenericCommandsSidebar.html">GenericCommandsSidebar</a><h1><a name="RANDOMKEY">RANDOMKEY</a></h1>
<i>Time complexity: O(1)</i><blockquote>Return a randomly selected key from the currently selected DB.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Singe line reply</a>, specifically the randomly selected key or an empty string is the database is empty.
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<h1 class="wikiname">Redis0100ChangeLog</h1>
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<h1><a name="Redis 0.100 Changelog">Redis 0.100 Changelog</a></h1>
<pre class="codeblock python" name="code">
- SUNION, SDIFF, SUNIONSTORE, SDIFFSTORE commands implemented. (Aman Gupta, antirez)
- Non blocking replication. Now while N slaves are synchronizing, the master will continue to ask to client queries. (antirez)
- PHP client ported to PHP5 (antirez)
- FLUSHALL/FLUSHDB no longer sync on disk. Just increment the dirty counter by the number of elements removed, that will probably trigger a background saving operation (antirez)
- INCRBY/DECRBY now support 64bit increments, with tests (antirez)
- New fields in INFO command, bgsave_in_progress and replication related (antirez)
- Ability to specify a different file name for the DB (... can't remember ...)
- GETSET command, atomic GET + SET (antirez)
- SMOVE command implemented, atomic move-element across sets operation (antirez)
- Ability to work with huge data sets, tested up to 350 million keys (antirez)
- Warns if /proc/sys/vm/overcommit_memory is set to 0 on Linux. Also make sure to don't resize the hash tables while the child process is saving in order to avoid copy-on-write of memory pages (antirez)
- Infinite number of arguments for MGET and all the other commands (antirez)
- CPP client (Brian Hammond)
- DEL is now a vararg, IMPORTANT: memory leak fixed in loading DB code (antirez)
- Benchmark utility now supports random keys (antirez)
- Timestamp in log lines (antirez)
- Fix SINTER/UNIONSTORE to allow for &amp;=/|= style operations (i.e. SINTERSTORE set1 set1 set2) (Aman Gupta)
- Partial qsort implemented in SORT command, only when both BY and LIMIT is used (antirez)
- Allow timeout=0 config to disable client timeouts (Aman Gupta)
- Alternative (faster/simpler) ruby client API compatible with Redis-rb (antirez)
- S*STORE now return the cardinality of the resulting set (antirez)
- TTL command implemented (antirez)
- Critical bug about glueoutputbuffers=yes fixed. Under load and with pipelining and clients disconnecting on the middle of the chat with the server, Redis could block. (antirez)
- Different replication fixes (antirez)
- SLAVEOF command implemented for remote replication management (antirez)
- Issue with redis-client used in scripts solved, now to check if the latest argument must come from standard input we do not check that stdin is or not a tty but the command arity (antirez)
- Warns if using the default config (antirez)
- maxclients implemented, see redis.conf for details (antirez)
- max bytes of a received command enlarged from 1k to 32k (antirez)
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<b>Redis0900ChangeLog: Contents</b><br>&nbsp;&nbsp;<a href="#CHANGELOG for Redis 0.900">CHANGELOG for Redis 0.900</a>
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<h1><a name="CHANGELOG for Redis 0.900">CHANGELOG for Redis 0.900</a></h1><pre class="codeblock python" name="code">
2009-06-16 client libraries updated (antirez)
2009-06-16 Better handling of background saving process killed or crashed (antirez)
2009-06-14 number of keys info in INFO command (Diego Rosario Brogna)
2009-06-14 SPOP documented (antirez)
2009-06-14 Clojure library (Ragnar Dahl&Atilde;&copy;n)
2009-06-10 It is now possible to specify - as config file name to read it from stdin (antirez)
2009-06-10 max bytes in an inline command raised to 1024*1024 bytes, in order to allow for very large MGETs and still protect from client crashes (antirez)
2009-06-08 SPOP implemented. Hash table resizing for Sets and Expires too. Changed the resize policy to play better with RANDOMKEY and SPOP. (antirez)
2009-06-07 some minor changes to the backtrace code (antirez)
2009-06-07 enable backtrace capabilities only for Linux and MacOSX (antirez)
2009-06-07 Dump a backtrace on sigsegv/sigbus, original coded (Diego Rosario Brogna)
2009-06-05 Avoid a busy loop while sending very large replies against very fast links, this allows to be more responsive with other clients even under a KEY * against the loopback interface (antirez)
2009-06-05 Kill the background saving process before performing SHUTDOWN to avoid races (antirez)
2009-06-05 LREM now returns :0 for non existing keys (antirez)
2009-06-05 added config.h for #ifdef business isolation, added fstat64 for Mac OS X (antirez)
2009-06-04 macosx specific zmalloc.c, uses malloc_size function in order to avoid to waste memory and time to put an additional header (antirez)
2009-06-04 DEBUG OBJECT implemented (antirez)
2009-06-03 shareobjectspoolsize implemented in reds.conf, in order to control the pool size when object sharing is on (antirez)
2009-05-27 maxmemory implemented (antirez)
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<b>RedisBigData: Contents</b><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#BGSAVE and BGREWRITEAOF blocking fork() call">BGSAVE and BGREWRITEAOF blocking fork() call</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Using multiple cores">Using multiple cores</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Splitting data into multiple instances">Splitting data into multiple instances</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#BGSAVE / AOFSAVE memory usage, and copy on write">BGSAVE / AOFSAVE memory usage, and copy on write</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#BGSAVE / AOFSAVE time for big datasets">BGSAVE / AOFSAVE time for big datasets</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Non blocking hash table">Non blocking hash table</a>
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&iuml;&raquo;&iquest;=Redis Big Data: facts and guidelines=<h2><a name="BGSAVE and BGREWRITEAOF blocking fork() call">BGSAVE and BGREWRITEAOF blocking fork() call</a></h2>
<pre class="codeblock python" name="code">
fork.c &amp;&amp; ./a.out
allocated: 1 MB, fork() took 0.000
allocated: 10 MB, fork() took 0.001
allocated: 100 MB, fork() took 0.007
allocated: 1000 MB, fork() took 0.059
allocated: 10000 MB, fork() took 0.460
allocated: 20000 MB, fork() took 0.895
allocated: 30000 MB, fork() took 1.327
allocated: 40000 MB, fork() took 1.759
allocated: 50000 MB, fork() took 2.190
allocated: 60000 MB, fork() took 2.621
allocated: 70000 MB, fork() took 3.051
allocated: 80000 MB, fork() took 3.483
allocated: 90000 MB, fork() took 3.911
allocated: 100000 MB, fork() took 4.340
allocated: 110000 MB, fork() took 4.770
allocated: 120000 MB, fork() took 5.202
</pre>
<h2><a name="Using multiple cores">Using multiple cores</a></h2>
<h2><a name="Splitting data into multiple instances">Splitting data into multiple instances</a></h2>
<h2><a name="BGSAVE / AOFSAVE memory usage, and copy on write">BGSAVE / AOFSAVE memory usage, and copy on write</a></h2>
<h2><a name="BGSAVE / AOFSAVE time for big datasets">BGSAVE / AOFSAVE time for big datasets</a></h2>
<h2><a name="Non blocking hash table">Non blocking hash table</a></h2>
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<b>RedisEventLibrary: Contents</b><br>&nbsp;&nbsp;<a href="#Redis Event Library">Redis Event Library</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Event Loop Initialization">Event Loop Initialization</a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="#aeCreateEventLoop">aeCreateEventLoop</a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="#aeCreateTimeEvent">aeCreateTimeEvent</a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="#aeCreateFileEvent">aeCreateFileEvent</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Event Loop Processing">Event Loop Processing</a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="#aeProcessEvents">aeProcessEvents</a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="#processTimeEvents">processTimeEvents</a>
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&iuml;&raquo;&iquest;#sidebar <a href="RedisInternals.html">RedisInternals</a><h1><a name="Redis Event Library">Redis Event Library</a></h1>Redis implements its own event library. The event library is implemented in <b>ae.c</b>.<br/><br/>The best way to understand how the Redis event library works is to understand how Redis uses it.<h2><a name="Event Loop Initialization">Event Loop Initialization</a></h2>
<code name="code" class="python">initServer</code> function defined in <b>redis.c</b> initializes the numerous fields of the <code name="code" class="python">redisServer</code> structure variable. One such field is the Redis event loop <code name="code" class="python">el</code>:<br/><br/><pre class="codeblock python" name="code">
aeEventLoop *el
</pre><code name="code" class="python">initServer</code> initializes <code name="code" class="python">server.el</code> field by calling <code name="code" class="python">aeCreateEventLoop</code> defined in <b>ae.c</b>. The definition of <code name="code" class="python">aeEventLoop</code> is below:
<pre class="codeblock python python" name="code">
typedef struct aeEventLoop
{
int maxfd;
long long timeEventNextId;
aeFileEvent events[AE_SETSIZE]; /* Registered events */
aeFiredEvent fired[AE_SETSIZE]; /* Fired events */
aeTimeEvent *timeEventHead;
int stop;
void *apidata; /* This is used for polling API specific data */
aeBeforeSleepProc *beforesleep;
} aeEventLoop;
</pre><h3><a name="aeCreateEventLoop">aeCreateEventLoop</a></h3><code name="code" class="python">aeCreateEventLoop</code> first mallocs aeEventLoop structure then calls ae_epoll.c:aeApiCreate<code name="code" class="python">.
</code>aeApiCreate<code name="code" class="python"> mallocs </code>aeApiState<code name="code" class="python"> that has two fields - </code>epfd<code name="code" class="python"> that holds the epoll file descriptor returned by a call from [http://man.cx/epoll_create%282%29 epoll_create] and </code>events<code name="code" class="python"> that is of type </code>struct epoll_event<code name="code" class="python"> define by the Linux epoll library. The use of the </code>events<code name="code" class="python"> field will be described later.
Next is 'ae.c:aeCreateTimeEvent</code>. But before that <code name="code" class="python">initServer</code> call <code name="code" class="python">anet.c:anetTcpServer</code> that creates and returns a <i>listening descriptor</i>. The descriptor is listens to <b>port 6379</b> by default. The returned <i>listening descriptor</i> is stored in <code name="code" class="python">server.fd</code> field.<h3><a name="aeCreateTimeEvent">aeCreateTimeEvent</a></h3><code name="code" class="python">aeCreateTimeEvent</code> accepts the following as parameters:<br/><br/><ul><li> eventLoop: This is <code name="code" class="python">server.el</code> in <b>redis.c</b></li><li> milliseconds: The number of milliseconds from the curent time after which the timer expires.</li><li> proc: Function pointer. Stores the address of the function that has to be called after the timer expires.</li><li> clientData: Mostly NULL.</li><li> finalizerProc: Pointer to the function that has to be called before the timed event is removed from the list of timed events.</li></ul>
<code name="code" class="python">initServer</code> calls <code name="code" class="python">aeCreateTimeEvent</code> to add a timed event to <code name="code" class="python">timeEventHead</code> field of <code name="code" class="python">server.el</code>. <code name="code" class="python">timeEventHead</code> is a pointer to a list of such timed events. The call to <code name="code" class="python">aeCreateTimeEvent</code> from <code name="code" class="python">redis.c:initServer</code> function is given below:<br/><br/><pre class="codeblock python python python" name="code">
aeCreateTimeEvent(server.el /*eventLoop*/, 1 /*milliseconds*/, serverCron /*proc*/, NULL /*clientData*/, NULL /*finalizerProc*/);
</pre><code name="code" class="python">redis.c:serverCron</code> performs many operations that helps keep Redis running properly.<h3><a name="aeCreateFileEvent">aeCreateFileEvent</a></h3>The essence of <code name="code" class="python">aeCreateFileEvent</code> function is to execute <a href="http://man.cx/epoll_ctl" target="_blank">epoll_ctl</a> system call which adds a watch for <code name="code" class="python">EPOLLIN</code> event on the <i>listening descriptor</i> create by <code name="code" class="python">anetTcpServer</code> and associate it with the epoll descriptor created by a call to <code name="code" class="python">aeCreateEventLoop</code>. <br/><br/>Following is an explanation of what precisely <code name="code" class="python">aeCreateFileEvent</code> does when called from <code name="code" class="python">redis.c:initServer</code>.<br/><br/><code name="code" class="python">initServer</code> passes the following arguments to <code name="code" class="python">aeCreateFileEvent</code>:
<ul><li> server.el: The event loop created by <code name="code" class="python">aeCreateEventLoop</code>. The epoll descriptor is got from server.el. </li><li> server.fd: The <i>listening descriptor</i> that also serves as an index to access the relevant file event structure from the <code name="code" class="python">eventLoop-&gt;events</code> table and store extra information like the callback function.</li><li> AE_READABLE: Signifies that server.fd has to be watched for EPOLLIN event.</li><li> acceptHandler: The function that has to be executed when the event being watched for is ready. This function pointer is stored in <code name="code" class="python">eventLoop-&gt;events[server.fd]-&gt;rfileProc</code>. </li></ul>
This completes the initialization of Redis event loop.<h2><a name="Event Loop Processing">Event Loop Processing</a></h2><code name="code" class="python">ae.c:aeMain</code> called from <code name="code" class="python">redis.c:main</code> does the job of processing the event loop that is initialized in the previous phase.<br/><br/><code name="code" class="python">ae.c:aeMain</code> calls <code name="code" class="python">ae.c:aeProcessEvents</code> in a while loop that processes pending time and file events.<h3><a name="aeProcessEvents">aeProcessEvents</a></h3><code name="code" class="python">ae.c:aeProcessEvents</code> looks for the time event that will be pending in the smallest amount of time by calling <code name="code" class="python">ae.c:aeSearchNearestTimer</code> on the event loop. In our case there is only one timer event in the event loop that was created by <code name="code" class="python">ae.c:aeCreateTimeEvent</code>. <br/><br/>Remember, that timer event created by <code name="code" class="python">aeCreateTimeEvent</code> has by now probably elapsed because it had a expiry time of one millisecond. Since, the timer has already expired the seconds and microseconds fields of the <code name="code" class="python">tvp</code> timeval structure variable is initialized to zero. <br/><br/>The <code name="code" class="python">tvp</code> structure variable along with the event loop variable is passed to <code name="code" class="python">ae_epoll.c:aeApiPoll</code>.<br/><br/><code name="code" class="python">aeApiPoll</code> functions does a <a href="http://man.cx/epoll_wait" target="_blank">epoll_wait</a> on the epoll descriptor and populates the <code name="code" class="python">eventLoop-&gt;fired</code> table with the details:
<ul><li> fd: The descriptor that is now ready to do a read/write operation depending on the mask value. The </li><li> mask: The read/write event that can now be performed on the corresponding descriptor.</li></ul>
<code name="code" class="python">aeApiPoll</code> returns the number of such file events ready for operation. Now to put things in context, if any client has requested for a connection then aeApiPoll would have noticed it and populated the <code name="code" class="python">eventLoop-&gt;fired</code> table with an entry of the descriptor being the <i>listening descriptor</i> and mask being <code name="code" class="python">AE_READABLE</code>.<br/><br/>Now, <code name="code" class="python">aeProcessEvents</code> calls the <code name="code" class="python">redis.c:acceptHandler</code> registered as the callback. <code name="code" class="python">acceptHandler</code> executes [<a href="http://man.cx/accept(2" target="_blank">http://man.cx/accept(2</a>) accept] on the <i>listening descriptor</i> returning a <i>connected descriptor</i> with the client. <code name="code" class="python">redis.c:createClient</code> adds a file event on the <i>connected descriptor</i> through a call to <code name="code" class="python">ae.c:aeCreateFileEvent</code> like below:<br/><br/><pre class="codeblock python python python python" name="code">
if (aeCreateFileEvent(server.el, c-&gt;fd, AE_READABLE,
readQueryFromClient, c) == AE_ERR) {
freeClient(c);
return NULL;
}
</pre><code name="code" class="python">c</code> is the <code name="code" class="python">redisClient</code> structure variable and <code name="code" class="python">c-&gt;fd</code> is the connected descriptor.<br/><br/>Next the <code name="code" class="python">ae.c:aeProcessEvent</code> calls <code name="code" class="python">ae.c:processTimeEvents</code><h3><a name="processTimeEvents">processTimeEvents</a></h3><code name="code" class="python">ae.processTimeEvents</code> iterates over list of time events starting at <code name="code" class="python">eventLoop-&gt;timeEventHead</code>.<br/><br/>For every timed event that has elapsed <code name="code" class="python">processTimeEvents</code> calls the registered callback. In this case it calls the only timed event callback registered, that is, <code name="code" class="python">redis.c:serverCron</code>. The callback returns the time in milliseconds after which the callback must be called again. This change is recorded via a call to <code name="code" class="python">ae.c:aeAddMilliSeconds</code> and will be handled on the next iteration of <code name="code" class="python">ae.c:aeMain</code> while loop.<br/><br/>That's all.
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&iuml;&raquo;&iquest;= Redis Guides and Howtos=
<ul><li> <a href="QuickStart.html">Redis Quick Start</a></li><li> <a href="VirtualMemoryUserGuide.html">Virutal Memory User Guide</a></li><li> <a href="IntroductionToRedisDataTypes.html">A Fifteen Minutes Introduction to the Redis Data Types</a></li><li> <a href="ReplicationHowto.html">The Redis Replication HOWTO</a></li><li> <a href="AppendOnlyFileHowto.html">The Append Only File HOWTO</a></li></ul>
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<b>RedisInternals: Contents</b><br>&nbsp;&nbsp;<a href="#Redis Internals">Redis Internals</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Redis STRINGS">Redis STRINGS</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Redis Virtual Memory">Redis Virtual Memory</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Redis Event Library">Redis Event Library</a>
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<h1><a name="Redis Internals">Redis Internals</a></h1>This is a source code level documentation of Redis.<h2><a name="Redis STRINGS">Redis STRINGS</a></h2>String is the basic building block of Redis types. <br/><br/>Redis is a key-value store.
All Redis keys are strings and its also the simplest value type.<br/><br/><blockquote></blockquote>Lists, sets, sorted sets and hashes are other more complex value types and even
these are composed of strings.<br/><br/><a href="HackingStrings.html">Hacking Strings</a> documents the Redis String implementation details.<h2><a name="Redis Virtual Memory">Redis Virtual Memory</a></h2>A technical specification full of details about the <a href="VirtualMemorySpecification.html">Redis Virtual Memory subsystem</a><h2><a name="Redis Event Library">Redis Event Library</a></h2>Read <a href="EventLibray.html">event library</a> to understand what an event library does and why its needed.<br/><br/><a href="RedisEventLibrary.html">Redis event library</a> documents the implementation details of the event library used by Redis
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<b>Redis_1_2_0_Changelog: Contents</b><br>&nbsp;&nbsp;<a href="#What's new in Redis 1.2">What's new in Redis 1.2</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#New persistence mode: Append Only File">New persistence mode: Append Only File</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#New data type: sorted sets">New data type: sorted sets</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Specialized integer objects encoding">Specialized integer objects encoding</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#MSET and MSETNX">MSET and MSETNX</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Better Performances">Better Performances</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Solaris Support">Solaris Support</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Support for the new generation protocol">Support for the new generation protocol</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#A few new commands about already supported data types">A few new commands about already supported data types</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Bug fixing">Bug fixing</a><br>&nbsp;&nbsp;<a href="#CHANGELOG for Redis 1.1.90">CHANGELOG for Redis 1.1.90</a>
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<h1><a name="What's new in Redis 1.2">What's new in Redis 1.2</a></h1><h2><a name="New persistence mode: Append Only File">New persistence mode: Append Only File</a></h2>The Append Only File is an alternative way to save your data in Redis that is fully durable! Unlike the snapshotting (default) persistence mode, where the database is saved asynchronously from time to time, the Append Only File saves every change ASAP in a text-only file that works like a journal. Redis will play back this file again at startup reloading the whole dataset back in memory. Redis Append Only File supports background Log compaction. For more info read the <a href="AppendOnlyFileHowto.html">Append Only File HOWTO</a>.<h2><a name="New data type: sorted sets">New data type: sorted sets</a></h2>Sorted sets are collections of elements (like Sets) with an associated score (in the form of a double precision floating point number). Elements in a sorted set are taken in order, so for instance to take the greatest element is an O(1) operation. Insertion and deletion is O(log(N)). Sorted sets are implemented using a dual ported data structure consisting of an hash table and a skip list. For more information please read the <a href="IntroductionToRedisDataTypes.html">Introduction To Redis Data Types</a>.<h2><a name="Specialized integer objects encoding">Specialized integer objects encoding</a></h2>Redis 1.2 will use less memory than Redis 1.0 for values in Strings, Lists or Sets elements that happen to be representable as 32 or 64 bit signed integers (it depends on your arch bits for the long C type). This is totally transparent form the point of view of the user, but will safe a lot of memory (30% less in datasets where there are many integers).<h2><a name="MSET and MSETNX">MSET and MSETNX</a></h2>That is, setting multiple keys in one command, atomically. For more information see the <a href="MsetCommand.html">MSET command</a> wiki page.<h2><a name="Better Performances">Better Performances</a></h2><ul><li> 100x times faster SAVE and BGSAVE! There was a problem in the LZF lib configuration that is now resolved. The effect is this impressive speedup. Also the saving child will no longer use 100% of CPU.</li><li> Glue output buffer and writev(). Many commands producing large outputs, like LRANGE, will now be even 10 times faster, thanks to the new output buffer gluing algorithm and the (optional) use of writev(2) syscall.</li><li> Support for epool and kqueue / kevent. 10,000 clients scalability.</li><li> Much better EXPIRE support, now it's possible to work with very large sets of keys expiring in very short time without to incur in memory problems (the new algorithm expires keys in an adaptive way, so will get more aggressive if there are a lot of expiring keys)</li></ul>
<h2><a name="Solaris Support">Solaris Support</a></h2>Redis will now compile and work on Solaris without problems. Warning: the Solaris user base is very little, so Redis running on Solaris may not be as tested and stable as it is on Linux and Mac OS X.<h2><a name="Support for the new generation protocol">Support for the new generation protocol</a></h2><ul><li> Redis is now able to accept commands in a new fully binary safe way: with the new protocol keys are binary safe, not only values, and there is no distinction between bulk commands and inline commands. This new protocol is currently used only for MSET and MSETNX but at some point it will hopefully replace the old one. See the Multi Bulk Commands section in the <a href="ProtocolSpecification.html">Redis Protocol Specification</a> for more information.</li></ul>
<h2><a name="A few new commands about already supported data types">A few new commands about already supported data types</a></h2><ul><li> <a href="SrandmemberCommand.html">SRANDMEMBER</a></li><li> The <a href="SortCommand.html">SortCommand</a> is now supprots the <b>STORE</b> and <b>GET #</b> forms, the first can be used to save sorted lists, sets or sorted sets into keys for caching. Check the manual page for more information about the <b>GET #</b> form.</li><li> The new <a href="RpoplpushCommand.html">RPOPLPUSH command</a> can do many interesting magics, and a few of this are documented in the wiki page of the command.</li></ul>
<h2><a name="Bug fixing">Bug fixing</a></h2>Of course, many bugs are now fixed, and I bet, a few others introduced: this is how software works after all, so make sure to report issues in the Redis mailing list or in the Google Code issues tracker.<br/><br/>Enjoy!
antirez<h1><a name="CHANGELOG for Redis 1.1.90">CHANGELOG for Redis 1.1.90</a></h1><ul><li> 2009-09-10 in-memory specialized object encoding. (antirez)</li><li> 2009-09-17 maxmemory fixed in 64 systems for values &gt; 4GB. (antirez)</li><li> 2009-10-07 multi-bulk protocol implemented. (antriez)</li><li> 2009-10-16 MSET and MSETNX commands implemented (antirez)</li><li> 2009-10-21 SRANDMEMBER added (antirez)</li><li> 2009-10-23 Fixed compilation in mac os x snow leopard when compiling a 32 bit binary. (antirez)</li><li> 2009-10-23 New data type: Sorted sets and Z-commands (antirez)</li><li> 2009-10-26 Solaris fixed (Alan Harder)</li><li> 2009-10-29 Fixed Issue a number of open issues (antirez)</li><li> 2009-10-30 New persistence mode: append only file (antirez)</li><li> 2009-11-01 SORT STORE option (antirez)</li><li> 2009-11-03 redis-cli now accepts a -r (repeat) switch. (antirez)</li><li> 2009-11-04 masterauth option merged (Anthony Lauzon)</li><li> 2009-11-04 redis-test is now a better Redis citizen, testing everything against DB 9 and 10 and only if this DBs are empty. (antirez)</li><li> 2009-11-10 Implemented a much better lazy expiring algorithm for EXPIRE (antirez)</li><li> 2009-11-11 RPUSHLPOP (antirez from an idea of @ezmobius)</li><li> 2009-11-12 Merge git://github.com/ianxm/redis (Can't remmber what this implements, sorry)</li><li> 2009-11-17 multi-bulk reply support for redis-bench, LRANGE speed tests (antirez)</li><li> 2009-11-17 support for writev implemented. (Stefano Barbato)</li><li> 2009-11-19 debug mode (-D) in redis-bench (antirez)</li><li> 2009-11-21 SORT GET # implemented (antirez)</li><li> 2009-11-23 ae.c made modular, with support for epoll. (antirez)</li><li> 2009-11-26 background append log rebuilding (antirez)</li><li> 2009-11-28 Added support for kqueue. (Harish Mallipeddi)</li><li> 2009-11-29 SORT support for sorted sets (antirez, thanks to @tobi for the idea)</li></ul>
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<b>RenameCommand: Contents</b><br>&nbsp;&nbsp;<a href="#RENAME _oldkey_ _newkey_">RENAME _oldkey_ _newkey_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<i>Time complexity: O(1)</i><blockquote>Atomically renames the key <i>oldkey</i> to <i>newkey</i>. If the source anddestination name are the same an error is returned. If <i>newkey</i>already exists it is overwritten.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Status code repy</a>
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<b>RenamenxCommand: Contents</b><br>&nbsp;&nbsp;<a href="#RENAMENX _oldkey_ _newkey_">RENAMENX _oldkey_ _newkey_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<i>Time complexity: O(1)</i><blockquote>Rename <i>oldkey</i> into <i>newkey</i> but fails if the destination key <i>newkey</i> already exists.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Integer reply</a>, specifically:<br/><br/><pre class="codeblock python" name="code">
1 if the key was renamed
0 if the target key already exist
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<b>ReplicationHowto: Contents</b><br>&nbsp;&nbsp;<a href="#Redis Replication Howto">Redis Replication Howto</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#General Information">General Information</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#How Redis replication works">How Redis replication works</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Configuration">Configuration</a>
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<h1><a name="Redis Replication Howto">Redis Replication Howto</a></h1><h2><a name="General Information">General Information</a></h2>Redis replication is a very simple to use and configure master-slave replication that allows slave Redis servers to be exact copies of master servers. The following are some very important facts about Redis replication:<br/><br/><ul><li> A master can have multiple slaves.</li><li> Slaves are able to accept other slaves connections, so instead to connect a number of slaves against the same master it is also possible to connect some of the slaves to other slaves in a graph-alike structure.</li><li> Redis replication is non-blocking on the master side, this means that the master will continue to serve queries while one or more slaves are performing the first synchronization. Instead replication is blocking on the slave side: while the slave is performing the first synchronization it can't reply to queries.</li><li> Replications can be used both for scalability, in order to have multiple slaves for read-only queries (for example heavy <a href="SortCommand.html">SORT</a> operations can be launched against slaves), or simply for data redundancy.</li><li> It is possible to use replication to avoid the saving process on the master side: just configure your master redis.conf in order to avoid saving at all (just comment al the &quot;save&quot; directives), then connect a slave configured to save from time to time.</li></ul>
<h2><a name="How Redis replication works">How Redis replication works</a></h2>In order to start the replication, or after the connection closes in order resynchronize with the master, the slave connects to the master and issues the SYNC command.<br/><br/>The master starts a background saving, and at the same time starts to collect all the new commands received that had the effect to modify the dataset. When the background saving completed the master starts the transfer of the database file to the slave, that saves it on disk, and then load it in memory. At this point the master starts to send all the accumulated commands, and all the new commands received from clients that had the effect of a dataset modification, to the slave, as a stream of commands, in the same format of the Redis protocol itself.<br/><br/>You can try it yourself via telnet. Connect to the Redis port while the server is doing some work and issue the SYNC command. You'll see a bulk transfer and then every command received by the master will be re-issued in the telnet session.<br/><br/>Slaves are able to automatically reconnect when the master <code name="code" class="python">&lt;-&gt;</code> slave link goes down for some reason. If the master receives multiple concurrent slave synchronization requests it performs a single background saving in order to serve all them.<h2><a name="Configuration">Configuration</a></h2>To configure replication is trivial: just add the following line to the slave configuration file:
<pre class="codeblock python" name="code">
slaveof 192.168.1.1 6379
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Of course you need to replace 192.168.1.1 6379 with your master ip address (or hostname) and port.
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<b>ReplyTypes: Contents</b><br>&nbsp;&nbsp;<a href="#Redis Reply Types">Redis Reply Types</a><br>&nbsp;&nbsp;<a href="#Status code reply">Status code reply</a><br>&nbsp;&nbsp;<a href="#Error reply">Error reply</a><br>&nbsp;&nbsp;<a href="#Integer reply">Integer reply</a><br>&nbsp;&nbsp;<a href="#Bulk reply">Bulk reply</a><br>&nbsp;&nbsp;<a href="#Multi bulk reply">Multi bulk reply</a>
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<h1><a name="Redis Reply Types">Redis Reply Types</a></h1>Redis commands can reply to the client with four different kind of replies, you can find the protocol level specification of this replies in the <a href="ProtocolSpecification.html">Redis Protocol Specification</a>. This page is instead an higher level description of the four types of replies from the point of view of the final user.<h1><a name="Status code reply">Status code reply</a></h1>
Status code replies are single line strings having the <b>+</b> character as first byte. The string to return to the client is simply verything that follows the first <b>+</b> character. For example the <a href="PingCommand.html">PING</a> command returns <b>+PONG</b>, that is the string &quot;PONG&quot;.<h1><a name="Error reply">Error reply</a></h1>
This is like a status code reply but the first character is <b>-</b> instead of <b>+</b>. The client library should raise an error for error replies and stop the execution of the program if the exception is not trapped, showing the error message (everything following the first <b>-</b> character). An example of error is &quot;-Error no such key&quot; or &quot;-foobar&quot;. Note that error replies will not collide with negative integer replies since integer replies are prefixed with the <b>:</b> character.<h1><a name="Integer reply">Integer reply</a></h1>
At protocol level integer replies are single line replies in form of a decimal singed number prefixed by a <b>:</b> character. For example <b>:10</b> is an integer reply. Redis commands returning <i>true</i> or <i>false</i> will use an integer reply with 0 or 1 as values where 0 is false and 1 is true.<br/><br/>Integer replies are usually passed by client libraries as integer values.<h1><a name="Bulk reply">Bulk reply</a></h1>
A bulk reply is a binary-safe reply that is used to return a binary safe single string value (string is not limited to alphanumerical strings, it may contain binary data of any kind). Client libraries will usually return a string as return value of Redis commands returning bulk replies. There is a special bulk reply that signal that the element does not exist. When this happens the client library should return 'nil', 'false', or some other special element that can be distinguished by an empty string.<h1><a name="Multi bulk reply">Multi bulk reply</a></h1>
While a bulk reply returns a single string value, multi bulk replies are used to return multiple values: lists, sets, and so on. Elements of a bulk reply can be missing. Client libraries should return 'nil' or 'false' in order to make this elements distinguishable from empty strings. Client libraries should return multi bulk replies that are about ordered elements like list ranges as lists, and bulk replies about sets as hashes or Sets if the implementation language has a Set type.
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<b>RoadMap: Contents</b><br>&nbsp;&nbsp;<a href="#Road Map (ROUGH DRAFT)">Road Map (ROUGH DRAFT)</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Features added in past versions">Features added in past versions</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#1.1 / 1.2">1.1 / 1.2</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#0.x / 1.0">0.x / 1.0</a>
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<h1><a name="Road Map (ROUGH DRAFT)">Road Map (ROUGH DRAFT)</a></h1>The up to date, raw Road Map for Redis is part of the source code, you can find it here: <a href="http://github.com/antirez/redis/raw/master/TODO" target="_blank">http://github.com/antirez/redis/raw/master/TODO</a><h2><a name="Features added in past versions">Features added in past versions</a></h2><h2><a name="1.1 / 1.2">1.1 / 1.2</a></h2><ul><li> <a href="DataTypes.html">Ordered Set (ZSET)</a></li><li> <a href="MultiBulkCommands.html">Multibulk Commands</a></li><li> In memory integer encoding of <a href="DataTypes.html">integers</a>. Memory saving of 20% or more with datasets using high number of integer IDs. </li><li> Enhanced <a href="ExpireCommand.html">EXPIRE</a> algorithm.</li></ul>
<h2><a name="0.x / 1.0">0.x / 1.0</a></h2><ul><li> TODO: Add 1.0 Features. This is important for clarity in <a href="SupportedLanguages.html">SupportedLanguages</a></li></ul>
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<b>RpoplpushCommand: Contents</b><br>&nbsp;&nbsp;<a href="#RPOPLPUSH _srckey_ _dstkey_ (Redis &gt;">RPOPLPUSH _srckey_ _dstkey_ (Redis &gt;</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Programming patterns: safe queues">Programming patterns: safe queues</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Programming patterns: server-side O(N) list traversal">Programming patterns: server-side O(N) list traversal</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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&iuml;&raquo;&iquest;#sidebar <a href="ListCommandsSidebar.html">ListCommandsSidebar</a><h1><a name="RPOPLPUSH _srckey_ _dstkey_ (Redis &gt;">RPOPLPUSH _srckey_ _dstkey_ (Redis &gt;</a></h1> 1.1) =
<i>Time complexity: O(1)</i><blockquote>Atomically return and remove the last (tail) element of the <i>srckey</i> list,and push the element as the first (head) element of the <i>dstkey</i> list. Forexample if the source list contains the elements &quot;a&quot;,&quot;b&quot;,&quot;c&quot; and thedestination list contains the elements &quot;foo&quot;,&quot;bar&quot; after an RPOPLPUSH commandthe content of the two lists will be &quot;a&quot;,&quot;b&quot; and &quot;c&quot;,&quot;foo&quot;,&quot;bar&quot;.</blockquote>
<blockquote>If the <i>key</i> does not exist or the list is already empty the specialvalue 'nil' is returned. If the <i>srckey</i> and <i>dstkey</i> are the same theoperation is equivalent to removing the last element from the list and pusingit as first element of the list, so it's a &quot;list rotation&quot; command.</blockquote>
<h2><a name="Programming patterns: safe queues">Programming patterns: safe queues</a></h2><blockquote>Redis lists are often used as queues in order to exchange messages betweendifferent programs. A program can add a message performing an <a href="RpushCommand.html">LPUSH</a> operationagainst a Redis list (we call this program a Producer), while another program(that we call Consumer) can process the messages performing an <a href="LpopCommand.html">RPOP</a> commandin order to start reading the messages from the oldest.</blockquote>
<blockquote>Unfortunately if a Consumer crashes just after an <a href="LpopCommand.html">RPOP</a> operation the messagegets lost. RPOPLPUSH solves this problem since the returned message isadded to another &quot;backup&quot; list. The Consumer can later remove the messagefrom the backup list using the <a href="LremCommand.html">LREM</a> command when the message was correctlyprocessed.</blockquote>
<blockquote>Another process, called Helper, can monitor the &quot;backup&quot; list to check fortimed out entries to repush against the main queue.</blockquote>
<h2><a name="Programming patterns: server-side O(N) list traversal">Programming patterns: server-side O(N) list traversal</a></h2><blockquote>Using RPOPPUSH with the same source and destination key a process canvisit all the elements of an N-elements List in O(N) without to transferthe full list from the server to the client in a single <a href="LrangeCommand.html">LRANGE</a> operation.Note that a process can traverse the list even while other processesare actively RPUSHing against the list, and still no element will be skipped.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Bulk reply</a>
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<b>RpushCommand: Contents</b><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="#RPUSH _key_ _string_">RPUSH _key_ _string_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="#LPUSH _key_ _string_">LPUSH _key_ _string_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h3><a name="LPUSH _key_ _string_">LPUSH _key_ _string_</a></h3>
<i>Time complexity: O(1)</i><blockquote>Add the <i>string</i> value to the head (RPUSH) or tail (LPUSH) of the liststored at <i>key</i>. If the key does not exist an empty list is created just beforethe append operation. If the key exists but is not a List an erroris returned.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Status code reply</a>
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</a>
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<b>SaddCommand: Contents</b><br>&nbsp;&nbsp;<a href="#SADD _key_ _member_">SADD _key_ _member_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
</div>
<h1 class="wikiname">SaddCommand</h1>
<div class="summary">
</div>
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&iuml;&raquo;&iquest;#sidebar <a href="SetCommandsSidebar.html">SetCommandsSidebar</a><h1><a name="SADD _key_ _member_">SADD _key_ _member_</a></h1>
<i>Time complexity O(1)</i><blockquote>Add the specified <i>member</i> to the set value stored at <i>key</i>. If <i>member</i>is already a member of the set no operation is performed. If <i>key</i>does not exist a new set with the specified <i>member</i> as sole member iscreated. If the key exists but does not hold a set value an error isreturned.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Integer reply</a>, specifically:<br/><br/><pre class="codeblock python" name="code">
1 if the new element was added
0 if the element was already a member of the set
</pre>
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<b>SaveCommand: Contents</b><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="#SAVE">SAVE</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1 class="wikiname">SaveCommand</h1>
<div class="summary">
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&iuml;&raquo;&iquest;#sidebar <a href="ControlCommandsSidebar.html">ControlCommandsSidebar</a><h3><a name="SAVE">SAVE</a></h3>
<blockquote>Save the whole dataset on disk (this means that all the databases are saved, as well as keys with an EXPIRE set (the expire is preserved). The server hangs while the saving is notcompleted, no connection is served in the meanwhile. An OK codeis returned when the DB was fully stored in disk.</blockquote>
<blockquote>The background variant of this command is <a href="BgsaveCommand.html">BGSAVE</a> that is able to perform the saving in the background while the server continues serving other clients.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Status code reply</a>
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<b>ScardCommand: Contents</b><br>&nbsp;&nbsp;<a href="#SCARD _key_">SCARD _key_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1 class="wikiname">ScardCommand</h1>
<div class="summary">
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&iuml;&raquo;&iquest;#sidebar <a href="SetCommandsSidebar.html">SetCommandsSidebar</a><h1><a name="SCARD _key_">SCARD _key_</a></h1>
<i>Time complexity O(1)</i><blockquote>Return the set cardinality (number of elements). If the <i>key</i> does notexist 0 is returned, like for empty sets.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Integer reply</a>, specifically:<br/><br/><pre class="codeblock python" name="code">
the cardinality (number of elements) of the set as an integer.
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<b>SdiffCommand: Contents</b><br>&nbsp;&nbsp;<a href="#SDIFF _key1_ _key2_ ... _keyN_">SDIFF _key1_ _key2_ ... _keyN_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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&iuml;&raquo;&iquest;#sidebar <a href="SetCommandsSidebar.html">SetCommandsSidebar</a><h1><a name="SDIFF _key1_ _key2_ ... _keyN_">SDIFF _key1_ _key2_ ... _keyN_</a></h1>
<i>Time complexity O(N) with N being the total number of elements of all the sets</i><blockquote>Return the members of a set resulting from the difference between the firstset provided and all the successive sets. Example:</blockquote>
<pre class="codeblock python" name="code">
key1 = x,a,b,c
key2 = c
key3 = a,d
SDIFF key1,key2,key3 =&gt; x,b
</pre><blockquote>Non existing keys are considered like empty sets.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Multi bulk reply</a>, specifically the list of common elements.
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<b>SdiffstoreCommand: Contents</b><br>&nbsp;&nbsp;<a href="#SDIFFSTORE _dstkey_ _key1_ _key2_ ... _keyN_">SDIFFSTORE _dstkey_ _key1_ _key2_ ... _keyN_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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&iuml;&raquo;&iquest;#sidebar <a href="SetCommandsSidebar.html">SetCommandsSidebar</a><h1><a name="SDIFFSTORE _dstkey_ _key1_ _key2_ ... _keyN_">SDIFFSTORE _dstkey_ _key1_ _key2_ ... _keyN_</a></h1>
<i>Time complexity O(N) where N is the total number of elements in all the provided sets</i><blockquote>This command works exactly like SDIFF but instead of being returned the resulting set is stored in <i>dstkey</i>.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Status code reply</a>
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<b>SelectCommand: Contents</b><br>&nbsp;&nbsp;<a href="#SELECT _index_">SELECT _index_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1 class="wikiname">SelectCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="GenericCommandsSidebar.html">GenericCommandsSidebar</a><h1><a name="SELECT _index_">SELECT _index_</a></h1>
<blockquote>Select the DB with having the specified zero-based numeric index.For default every new client connection is automatically selectedto DB 0.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Status code reply</a>
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<img style="border:none" alt="Redis Documentation" src="redis.png">
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<b>SetCommand: Contents</b><br>&nbsp;&nbsp;<a href="#SET _key_ _value_">SET _key_ _value_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1 class="wikiname">SetCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="StringCommandsSidebar.html">StringCommandsSidebar</a><h1><a name="SET _key_ _value_">SET _key_ _value_</a></h1>
<i>Time complexity: O(1)</i><blockquote>Set the string <i>value</i> as value of the <i>key</i>.The string can't be longer than 1073741824 bytes (1 GB).</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Status code reply</a>
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<img style="border:none" alt="Redis Documentation" src="redis.png">
</a>
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<b>SetCommandsSidebar: Contents</b>
</div>
<h1 class="wikiname">SetCommandsSidebar</h1>
<div class="summary">
</div>
<div class="narrow">
&iuml;&raquo;&iquest;== Set Commands ==<br/><br/><ul><li> <a href="SaddCommand.html">SADD</a></li><li> <a href="SremCommand.html">SREM</a></li><li> <a href="SpopCommand.html">SPOP</a></li><li> <a href="SmoveCommand.html">SMOVE</a></li><li> <a href="ScardCommand.html">SCARD</a></li><li> <a href="SismemberCommand.html">SISMEMBER</a></li><li> <a href="SinterCommand.html">SINTER</a></li><li> <a href="SinterstoreCommand.html">SINTERSTORE</a></li><li> <a href="SunionCommand.html">SUNION</a></li><li> <a href="SunionstoreCommand.html">SUNIONSTORE</a></li><li> <a href="SdiffCommand.html">SDIFF</a></li><li> <a href="SdiffstoreCommand.html">SDIFFSTORE</a></li><li> <a href="SmembersCommand.html">SMEMBERS</a></li><li> <a href="SrandmemberCommand.html">SRANDMEMBER</a></li><li> <a href="SortCommand.html">SORT</a></li></ul>
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<b>SetexCommand: Contents</b><br>&nbsp;&nbsp;<a href="#SETEX _key_ _time_ _value_">SETEX _key_ _time_ _value_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1 class="wikiname">SetexCommand</h1>
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</div>
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&iuml;&raquo;&iquest;#sidebar <a href="StringCommandsSidebar.html">StringCommandsSidebar</a><h1><a name="SETEX _key_ _time_ _value_">SETEX _key_ _time_ _value_</a></h1>
<i>Time complexity: O(1)</i><blockquote>The command is exactly equivalent to the following group of commands:</blockquote><pre class="codeblock python" name="code">
SET _key_ _value_
EXPIRE _key_ _time_
</pre>
<blockquote>The operation is atomic. An atomic <a href="SetCommand.html">SET</a>+<a href="ExpireCommand.html">EXPIRE</a> operation was already providedusing <a href="MultiExecCommand.html">MULTI/EXEC</a>, but SETEX is a faster alternative providedbecause this operation is very common when Redis is used as a Cache.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Status code reply</a>
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<b>SetnxCommand: Contents</b><br>&nbsp;&nbsp;<a href="#SETNX _key_ _value_">SETNX _key_ _value_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Design pattern: Implementing locking with SETNX">Design pattern: Implementing locking with SETNX</a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Handling deadlocks">Handling deadlocks</a>
</div>
<h1 class="wikiname">SetnxCommand</h1>
<div class="summary">
</div>
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&iuml;&raquo;&iquest;#sidebar <a href="StringCommandsSidebar.html">StringCommandsSidebar</a><h1><a name="SETNX _key_ _value_">SETNX _key_ _value_</a></h1>
<i>Time complexity: O(1)</i><blockquote>SETNX works exactly like <a href="SetCommand.html">SET</a> with the only difference thatif the key already exists no operation is performed.SETNX actually means &quot;SET if Not eXists&quot;.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Integer reply</a>, specifically:<br/><br/><pre class="codeblock python" name="code">
1 if the key was set
0 if the key was not set
</pre><h2><a name="Design pattern: Implementing locking with SETNX">Design pattern: Implementing locking with SETNX</a></h2><blockquote>SETNX can also be seen as a locking primitive. For instance to acquirethe lock of the key <b>foo</b>, the client could try the following:</blockquote>
<pre class="codeblock python python" name="code">
SETNX lock.foo &lt;current UNIX time + lock timeout + 1&gt;
</pre><blockquote>If SETNX returns 1 the client acquired the lock, setting the <b>lock.foo</b>key to the UNIX time at witch the lock should no longer be considered valid.The client will later use <b>DEL lock.foo</b> in order to release the lock.</blockquote>
<blockquote>If SETNX returns 0 the key is already locked by some other client. We caneither return to the caller if it's a non blocking lock, or enter aloop retrying to hold the lock until we succeed or some kind of timeoutexpires.</blockquote>
<h3><a name="Handling deadlocks">Handling deadlocks</a></h3><blockquote>In the above locking algorithm there is a problem: what happens if a clientfails, crashes, or is otherwise not able to release the lock?It's possible to detect this condition because the lock key contains aUNIX timestamp. If such a timestamp is &lt;= the current Unix time the lockis no longer valid.</blockquote>
<blockquote>When this happens we can't just call DEL against the key to remove the lockand then try to issue a SETNX, as there is a race condition here, whenmultiple clients detected an expired lock and are trying to release it.</blockquote>
<ul><li> C1 and C2 read lock.foo to check the timestamp, because SETNX returned 0 to both C1 and C2, as the lock is still hold by C3 that crashed after holding the lock.</li><li> C1 sends DEL lock.foo</li><li> C1 sends SETNX =&gt; success!</li><li> C2 sends DEL lock.foo</li><li> C2 sends SETNX =&gt; success!</li><li> ERROR: both C1 and C2 acquired the lock because of the race condition.</li></ul>
<blockquote>Fortunately it's possible to avoid this issue using the following algorithm.Let's see how C4, our sane client, uses the good algorithm:</blockquote>
<ul><li> C4 sends SETNX lock.foo in order to acquire the lock</li><li> The crashed C3 client still holds it, so Redis will reply with 0 to C4.</li><li> C4 GET lock.foo to check if the lock expired. If not it will sleep one second (for instance) and retry from the start.</li><li> If instead the lock is expired because the UNIX time at lock.foo is older than the current UNIX time, C4 tries to perform GETSET lock.foo &lt;current unix timestamp + lock timeout + 1&gt;</li><li> Thanks to the <a href="GetsetCommand.html">GETSET</a> command semantic C4 can check if the old value stored at key is still an expired timestamp. If so we acquired the lock!</li><li> Otherwise if another client, for instance C5, was faster than C4 and acquired the lock with the GETSET operation, C4 GETSET operation will return a non expired timestamp. C4 will simply restart from the first step. Note that even if C4 set the key a bit a few seconds in the future this is not a problem.</li></ul>
IMPORTANT NOTE: In order to make this locking algorithm more robust, a client holding a lock should always check the timeout didn't expired before to unlock the key with DEL because client failures can be complex, not just crashing but also blocking a lot of time against some operation and trying to issue DEL after a lot of time (when the LOCK is already hold by some other client).
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<b>Sets: Contents</b><br>&nbsp;&nbsp;<a href="#Redis Set Type">Redis Set Type</a><br>&nbsp;&nbsp;<a href="#Implementation details">Implementation details</a>
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<h1 class="wikiname">Sets</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="SetCommandsSidebar.html">SetCommandsSidebar</a><h1><a name="Redis Set Type">Redis Set Type</a></h1>Redis Sets are unordered collections of <a href="Strings.html">Redis Strings</a>. It's possible to add, remove, and test for existence of members in O(1).<br/><br/>Redis Sets have the desirable property of not allowing repeated members. Adding the same element multiple times will result in a set having a single copy of this element. Practically speaking this means that adding an members does not require a &quot;check if exists then add&quot; operation.<br/><br/>Commands operating on sets try to make a good use of the return value in order to signal the application about previous existence of members. For instance the <a href="SaddCommand.html">SADD</a> command will return 1 if the element added was not already a member of the set, otherwise will return 0.<br/><br/>The max number of members in a set is 232-1 (4294967295, more than 4 billion of members per set).<br/><br/>Redis Sets support a wide range of operations, like union, intersection, difference. Intersection is optimized in order to perform the smallest number of lookups. For instance if you try to intersect a 10000 members set with a 2 members set Redis will iterate the 2 members set testing for members existence in the other set, performing 2 lookups instead of 10000.<h1><a name="Implementation details">Implementation details</a></h1>Redis Sets are implemented using hash tables, so adding, removing and testing for members is O(1) in the average. The hash table will automatically resize when new elements are added or removed into a Set.<br/><br/>The hash table resizing is a blocking operation performed synchronously so working with huge sets (consisting of many millions of elements) care should be taken when mass-inserting a very big amount of elements in a Set while other clients are querying Redis at high speed.<br/><br/>It is possible that in the near future Redis will switch to skip lists (already used in sorted sets) in order to avoid such a problem.
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<b>ShutdownCommand: Contents</b><br>&nbsp;&nbsp;<a href="#SHUTDOWN">SHUTDOWN</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1 class="wikiname">ShutdownCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="ControlCommandsSidebar.html">ControlCommandsSidebar</a><h1><a name="SHUTDOWN">SHUTDOWN</a></h1>
<blockquote>Stop all the clients, save the DB, then quit the server. This commandsmakes sure that the DB is switched off without the lost of any data.This is not guaranteed if the client uses simply &quot;SAVE&quot; and then&quot;QUIT&quot; because other clients may alter the DB data between the twocommands.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Status code reply</a> on error. On success nothing is returned since the server quits and the connection is closed.
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<b>SideBar: Contents</b>
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<h1 class="wikiname">SideBar</h1>
<div class="summary">
</div>
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DRAFT<br/><br/><ul><li> <a href="Features.html">Features</a></li><li> <a href="QuickStart.html">Quick Start</a></li><li> <a href="DataTypes.html">Data Types</a></li><li> <a href="SupportedLanguages.html">Supported Languages</a></li><li> <a href="ObjectHashMappers.html">Object Hash Mappers</a></li><li> <a href="ProgrammingExamples.html">Programming Examples</a></li><li> <a href="RoadMap.html">Road Map</a></li><li> <a href="Speed.html">Speed</a></li><li> <a href="FromSqlToDataStructures.html">FromSqlToDataStructures</a></li></ul><blockquote></blockquote>
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<b>SinterCommand: Contents</b><br>&nbsp;&nbsp;<a href="#SINTER _key1_ _key2_ ... _keyN_">SINTER _key1_ _key2_ ... _keyN_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1 class="wikiname">SinterCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="SetCommandsSidebar.html">SetCommandsSidebar</a><h1><a name="SINTER _key1_ _key2_ ... _keyN_">SINTER _key1_ _key2_ ... _keyN_</a></h1>
<i>Time complexity O(NM) worst case where N is the cardinality of the smallest set and M the number of sets</i><blockquote>Return the members of a set resulting from the intersection of all thesets hold at the specified keys. Like in LRANGE the result is sent tothe client as a multi-bulk reply (see the protocol specification formore information). If just a single key is specified, then this commandproduces the same result as SMEMBERS. Actually SMEMBERS is just syntaxsugar for SINTERSECT.</blockquote>
<blockquote>Non existing keys are considered like empty sets, so if one of the keys ismissing an empty set is returned (since the intersection with an emptyset always is an empty set).</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Multi bulk reply</a>, specifically the list of common elements.
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<b>SinterstoreCommand: Contents</b><br>&nbsp;&nbsp;<a href="#SINTERSTORE _dstkey_ _key1_ _key2_ ... _keyN_">SINTERSTORE _dstkey_ _key1_ _key2_ ... _keyN_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1 class="wikiname">SinterstoreCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="SetCommandsSidebar.html">SetCommandsSidebar</a><h1><a name="SINTERSTORE _dstkey_ _key1_ _key2_ ... _keyN_">SINTERSTORE _dstkey_ _key1_ _key2_ ... _keyN_</a></h1>
<i>Time complexity O(NM) worst case where N is the cardinality of the smallest set and M the number of sets</i><blockquote>This commnad works exactly like SINTER but instead of being returned the resulting set is sotred as <i>dstkey</i>.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Status code reply</a>
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<b>SismemberCommand: Contents</b><br>&nbsp;&nbsp;<a href="#SISMEMBER _key_ _member_">SISMEMBER _key_ _member_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1 class="wikiname">SismemberCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="SetCommandsSidebar.html">SetCommandsSidebar</a><h1><a name="SISMEMBER _key_ _member_">SISMEMBER _key_ _member_</a></h1>
<i>Time complexity O(1)</i><blockquote>Return 1 if <i>member</i> is a member of the set stored at <i>key</i>, otherwise0 is returned.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Integer reply</a>, specifically:<br/><br/><pre class="codeblock python" name="code">
1 if the element is a member of the set
0 if the element is not a member of the set OR if the key does not exist
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<b>SlaveofCommand: Contents</b><br>&nbsp;&nbsp;<a href="#SLAVEOF _host_ _port_">SLAVEOF _host_ _port_</a><br>&nbsp;&nbsp;<a href="#SLAVEOF no one">SLAVEOF no one</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1 class="wikiname">SlaveofCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="ControlCommandsSidebar.html">ControlCommandsSidebar</a><h1><a name="SLAVEOF _host_ _port_">SLAVEOF _host_ _port_</a></h1>
<h1><a name="SLAVEOF no one">SLAVEOF no one</a></h1><blockquote>The SLAVEOF command can change the replication settings of a slave on the fly.If a Redis server is arleady acting as slave, the command <code name="code" class="python">SLAVEOF NO ONE</code>will turn off the replicaiton turning the Redis server into a MASTER.In the proper form <code name="code" class="python">SLAVEOF hostname port</code> will make the server a slave of thespecific server listening at the specified hostname and port.</blockquote>
<blockquote>If a server is already a slave of some master, <code name="code" class="python">SLAVEOF hostname port</code> willstop the replication against the old server and start the synchrnonizationagainst the new one discarding the old dataset.</blockquote>
<blockquote>The form <code name="code" class="python">SLAVEOF no one</code> will stop replication turning the server into aMASTER but will not discard the replication. So if the old master stop workingit is possible to turn the slave into a master and set the application touse the new master in read/write. Later when the other Redis server will befixed it can be configured in order to work as slave.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Status code reply</a>
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<b>SmembersCommand: Contents</b><br>&nbsp;&nbsp;<a href="#SMEMBERS _key_">SMEMBERS _key_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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&iuml;&raquo;&iquest;#sidebar <a href="SetCommandsSidebar.html">SetCommandsSidebar</a><h1><a name="SMEMBERS _key_">SMEMBERS _key_</a></h1>
<i>Time complexity O(N)</i><blockquote>Return all the members (elements) of the set value stored at <i>key</i>. Thisis just syntax glue for <a href="SintersectCommand.html">SINTER</a>.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Multi bulk reply</a>
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