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Improve documentation for START_REPLICATION ... LOGICAL. 

The starting point may not be exactly what the client requested; it
may be at the slot's confirmed_flush_lsn.

Also, upgrade the message from DEBUG1 to LOG when this happens.

Reviewed-by: Amit Kapila
Discussion: https://postgr.es/m/c5c861d576f2511732f8002c76245da587110b1c.camel%40j-davis.com
This commit is contained in:
Jeff Davis 2021-07-30 14:59:19 -07:00
parent 5fcf3945bd
commit 14d474e079
2 changed files with 21 additions and 5 deletions
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20
doc/src/sgml/protocol.sgml
View File

@ -2410,10 +2410,22 @@ The commands accepted in replication mode are:
<term><literal>START_REPLICATION</literal> <literal>SLOT</literal> <replaceable class="parameter">slot_name</replaceable> <literal>LOGICAL</literal> <replaceable class="parameter">XXX/XXX</replaceable> [ ( <replaceable>option_name</replaceable> [ <replaceable>option_value</replaceable> ] [, ...] ) ]</term>
<listitem>
<para>
Instructs server to start streaming WAL for logical replication, starting
at WAL location <replaceable class="parameter">XXX/XXX</replaceable>. The server can
reply with an error, for example if the requested section of WAL has already
been recycled. On success, server responds with a CopyBothResponse
Instructs server to start streaming WAL for logical replication,
starting at either WAL location <replaceable
class="parameter">XXX/XXX</replaceable> or the slot's
<literal>confirmed_flush_lsn</literal> (see <xref
linkend="view-pg-replication-slots"/>), whichever is greater. This
behavior makes it easier for clients to avoid updating their local LSN
status when there is no data to process. However, starting at a
different LSN than requested might not catch certain kinds of client
errors; so the client may wish to check that
<literal>confirmed_flush_lsn</literal> matches its expectations before
issuing <literal>START_REPLICATION</literal>.
</para>
<para>
The server can reply with an error, for example if the
slot does not exist. On success, server responds with a CopyBothResponse
message, and then starts to stream WAL to the frontend.
</para>

6
src/backend/replication/logical/logical.c
View File

@ -520,8 +520,12 @@ CreateDecodingContext(XLogRecPtr start_lsn,
* xlog records didn't result in anything relevant for logical
* decoding. Clients have to be able to do that to support synchronous
* replication.
*
* Starting at a different LSN than requested might not catch certain
* kinds of client errors; so the client may wish to check that
* confirmed_flush_lsn matches its expectations.
*/
elog(DEBUG1, "cannot stream from %X/%X, minimum is %X/%X, forwarding",
elog(LOG, "%X/%X has been already streamed, forwarding to %X/%X",
LSN_FORMAT_ARGS(start_lsn),
LSN_FORMAT_ARGS(slot->data.confirmed_flush));