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Add option to control snapshot export to CREATE_REPLICATION_SLOT

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We used to export snapshots unconditionally in CREATE_REPLICATION_SLOT
in the replication protocol, but several upcoming patches want more
control over what happens.

Suppress snapshot export in pg_recvlogical, which neither needs nor can
use the exported snapshot.  Since snapshot exporting can fail this
improves reliability.

This also paves the way for allowing the creation of replication slots
on standbys, which cannot export snapshots because they cannot allocate
new XIDs.

Author: Petr Jelinek <petr.jelinek@2ndquadrant.com>
This commit is contained in:
Peter Eisentraut
2017-03-14 17:13:56 -04:00
parent 7150402655
commit eb4da3e380
10 changed files with 141 additions and 26 deletions
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11
doc/src/sgml/logicaldecoding.sgml
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@ -271,8 +271,9 @@ $ pg_recvlogical -d postgres --slot test --drop-slot
<sect2>
<title>Exported Snapshots</title>
<para>
When a new replication slot is created using the streaming replication interface,
a snapshot is exported
When a new replication slot is created using the streaming replication
interface (see <xref linkend="protocol-replication-create-slot">), a
snapshot is exported
(see <xref linkend="functions-snapshot-synchronization">), which will show
exactly the state of the database after which all changes will be
included in the change stream. This can be used to create a new replica by
@ -282,6 +283,12 @@ $ pg_recvlogical -d postgres --slot test --drop-slot
database's state at that point in time, which afterwards can be updated
using the slot's contents without losing any changes.
</para>
<para>
Creation of a snapshot is not always possible. In particular, it will
fail when connected to a hot standby. Applications that do not require
snapshot export may suppress it with the <literal>NOEXPORT_SNAPSHOT</>
option.
</para>
</sect2>
</sect1>