From 1f8d9319ef9c934c414cebf1f5223c3b3023bf7f Mon Sep 17 00:00:00 2001 From: Craig Ringer Date: Wed, 16 Mar 2016 15:45:16 +0800 Subject: [PATCH 2/2] Correct incorrect claim that slots output a change "exactly once" Replication slots may actually emit a change more than once if the master crashes before flushing the slot. See http://www.postgresql.org/message-id/CAMsr+YGSaTRGqPcx9qx4eOcizWsa27XjKEiPSOtAJE8OfiXT-g@mail.gmail.com for details. --- doc/src/sgml/logicaldecoding.sgml | 16 ++++++++++++---- 1 file changed, 12 insertions(+), 4 deletions(-) diff --git a/doc/src/sgml/logicaldecoding.sgml b/doc/src/sgml/logicaldecoding.sgml index c7b43ed..a971918 100644 --- a/doc/src/sgml/logicaldecoding.sgml +++ b/doc/src/sgml/logicaldecoding.sgml @@ -12,7 +12,6 @@ Changes are sent out in streams identified by logical replication slots. - Each stream outputs each change exactly once. @@ -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. @@ -218,7 +216,17 @@ $ pg_recvlogical -d postgres --slot test --drop-slot A replication slot has an identifier that is unique across all databases in a PostgreSQL cluster. Slots persist - independently of the connection using them and are crash-safe. + independently of the connection using them. Slot creation and drop is + crash-safe, and slots will never be corrupted by a crash. + + + + A logical slot outputs each database change at least once. A slot will + usually only emit a change once, but recently-sent changes may be sent + again if the server server crashes and restarts. Clients should remember + 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. -- 2.1.0