From 0c40ab3a88edf654165e562deee0c303a6ebef5e Mon Sep 17 00:00:00 2001
From: Simon Riggs <simon@2ndQuadrant.com>
Date: Sat, 3 Sep 2016 16:19:11 +0100
Subject: [PATCH] Fix wording of logical decoding concepts
Be specific about conditions under which we emit >1 copy of message
Craig Ringer
---
doc/src/sgml/logicaldecoding.sgml | 18 +++++++++++++++---
1 file changed, 15 insertions(+), 3 deletions(-)
diff --git a/doc/src/sgml/logicaldecoding.sgml b/doc/src/sgml/logicaldecoding.sgml
index c42082002e0..484915d0422 100644
--- a/doc/src/sgml/logicaldecoding.sgml
+++ b/doc/src/sgml/logicaldecoding.sgml
@@ -12,7 +12,6 @@
<para>
Changes are sent out in streams identified by logical replication slots.
- Each stream outputs each change exactly once.
</para>
<para>
@@ -204,8 +203,7 @@ $ pg_recvlogical -d postgres --slot test --drop-slot
In the context of logical replication, a slot represents a stream of
changes that can be replayed to a client in the order they were made on
the origin server. Each slot streams a sequence of changes from a single
- database, sending each change exactly once (except when peeking forward
- in the stream).
+ database.
</para>
<note>
@@ -221,6 +219,20 @@ $ pg_recvlogical -d postgres --slot test --drop-slot
independently of the connection using them and are crash-safe.
</para>
+ <para>
+ A logical slot will emit each change just once in normal operation.
+ The current position of each slot is persisted only at checkpoint, so in
+ the case of a crash the slot may return to an earlier LSN, which will
+ then cause recent changes to be resent when the server restarts.
+ Logical decoding clients are responsible for avoiding ill effects from
+ handling the same message more than once. Clients may wish to record
+ the last LSN they saw when decoding and skip over any repeated data or
+ (when using the replication protocol) request that decoding start from
+ that LSN rather than letting the server determine the start point.
+ The Replication Progress Tracking feature is designed for this purpose,
+ refer to <link linkend="replication-origins">replication origins</link>.
+ </para>
+
<para>
Multiple independent slots may exist for a single database. Each slot has
its own state, allowing different consumers to receive changes from
--
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