From 0fdab27ad68a059a1663fa5ce48d76333f1bd74c Mon Sep 17 00:00:00 2001 From: Andres Freund Date: Sat, 8 Apr 2023 02:20:01 -0700 Subject: Allow logical decoding on standbys MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Unsurprisingly, this requires wal_level = logical to be set on the primary and standby. The infrastructure added in 26669757b6a 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 6af1793954e 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" Author: Andres Freund (in an older version) Author: Amit Khandekar (in an older version) Reviewed-by: Andres Freund Reviewed-by: FabrÌzio de Royes Mello Reviewed-by: Amit Kapila Reviewed-By: Robert Haas --- src/backend/replication/logical/decode.c | 30 +++++++++++++++++++++++++++++- 1 file changed, 29 insertions(+), 1 deletion(-) (limited to 'src/backend/replication/logical/decode.c') diff --git a/src/backend/replication/logical/decode.c b/src/backend/replication/logical/decode.c index 8fe7bb65f1f..5508cc21777 100644 --- a/src/backend/replication/logical/decode.c +++ b/src/backend/replication/logical/decode.c @@ -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: -- cgit v1.2.3