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Allow logical decoding on standbys

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Unsurprisingly, this requires wal_level = logical to be set on the primary and
standby. The infrastructure added in 26669757b6 ensures that slots are
invalidated if the primary's wal_level is lowered.

Creating a slot on a standby waits for a xl_running_xact record to be
processed. If the primary is idle (and thus not emitting xl_running_xact
records), that can take a while.  To make that faster, this commit also
introduces the pg_log_standby_snapshot() function. By executing it on the
primary, completion of slot creation on the standby can be accelerated.

Note that logical decoding on a standby does not itself enforce that required
catalog rows are not removed. The user has to use physical replication slots +
hot_standby_feedback or other measures to prevent that. If catalog rows
required for a slot are removed, the slot is invalidated.

See 6af1793954 for an overall design of logical decoding on a standby.

Bumps catversion, for the addition of the pg_log_standby_snapshot() function.

Author: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>
Author: Andres Freund <andres@anarazel.de> (in an older version)
Author: Amit Khandekar <amitdkhan.pg@gmail.com> (in an older version)
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: FabrÌzio de Royes Mello <fabriziomello@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Reviewed-By: Robert Haas <robertmhaas@gmail.com>
This commit is contained in:
Andres Freund
2023-04-08 02:20:01 -07:00
parent e101dfac3a
commit 0fdab27ad6
12 changed files with 203 additions and 62 deletions
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30
src/backend/replication/logical/decode.c
View File

@@ -152,11 +152,39 @@ xlog_decode(LogicalDecodingContext *ctx, XLogRecordBuffer *buf)
* can restart from there.
*/
break;
case XLOG_PARAMETER_CHANGE:
{
xl_parameter_change *xlrec =
(xl_parameter_change *) XLogRecGetData(buf->record);
/*
* If wal_level on the primary is reduced to less than
* logical, we want to prevent existing logical slots from
* being used. Existing logical slots on the standby get
* invalidated when this WAL record is replayed; and further,
* slot creation fails when wal_level is not sufficient; but
* all these operations are not synchronized, so a logical
* slot may creep in while the wal_level is being
* reduced. Hence this extra check.
*/
if (xlrec->wal_level < WAL_LEVEL_LOGICAL)
{
/*
* This can occur only on a standby, as a primary would
* not allow to restart after changing wal_level < logical
* if there is pre-existing logical slot.
*/
Assert(RecoveryInProgress());
ereport(ERROR,
(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
errmsg("logical decoding on a standby requires wal_level to be at least logical on the primary")));
}
break;
}
case XLOG_NOOP:
case XLOG_NEXTOID:
case XLOG_SWITCH:
case XLOG_BACKUP_END:
case XLOG_PARAMETER_CHANGE:
case XLOG_RESTORE_POINT:
case XLOG_FPW_CHANGE:
case XLOG_FPI_FOR_HINT:

36
src/backend/replication/logical/logical.c
View File

@@ -124,23 +124,21 @@ CheckLogicalDecodingRequirements(void)
(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
errmsg("logical decoding requires a database connection")));
/* ----
* TODO: We got to change that someday soon...
*
* There's basically three things missing to allow this:
* 1) We need to be able to correctly and quickly identify the timeline a
* LSN belongs to
* 2) We need to force hot_standby_feedback to be enabled at all times so
* the primary cannot remove rows we need.
* 3) support dropping replication slots referring to a database, in
* dbase_redo. There can't be any active ones due to HS recovery
* conflicts, so that should be relatively easy.
* ----
*/
if (RecoveryInProgress())
ereport(ERROR,
(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
errmsg("logical decoding cannot be used while in recovery")));
{
/*
* This check may have race conditions, but whenever
* XLOG_PARAMETER_CHANGE indicates that wal_level has changed, we
* verify that there are no existing logical replication slots. And to
* avoid races around creating a new slot,
* CheckLogicalDecodingRequirements() is called once before creating
* the slot, and once when logical decoding is initially starting up.
*/
if (GetActiveWalLevelOnStandby() < WAL_LEVEL_LOGICAL)
ereport(ERROR,
(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
errmsg("logical decoding on a standby requires wal_level to be at least logical on the primary")));
}
}
/*
@@ -342,6 +340,12 @@ CreateInitDecodingContext(const char *plugin,
LogicalDecodingContext *ctx;
MemoryContext old_context;
/*
* On a standby, this check is also required while creating the
* slot. Check the comments in the function.
*/
CheckLogicalDecodingRequirements();
/* shorter lines... */
slot = MyReplicationSlot;