Horton

RE: Synchronizing slots from primary to standby

Zhijie Hou (Fujitsu) <houzj.fnst@fujitsu.com>

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From: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>
To: shveta malik <shveta.malik@gmail.com>
Cc: Peter Smith <smithpb2250@gmail.com>, "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>, "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>, Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>, Peter Eisentraut <peter.eisentraut@enterprisedb.com>, Amit Kapila <amit.kapila16@gmail.com>, Bruce Momjian <bruce@momjian.us>, Ashutosh Sharma <ashu.coek88@gmail.com>, Andres Freund <andres@anarazel.de>, pgsql-hackers <pgsql-hackers@postgresql.org>, Ajin Cherian <itsajin@gmail.com>, Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2023-10-24T05:21:13Z
Lists: pgsql-hackers 
On Friday, October 20, 2023 11:27 AM shveta malik <shveta.malik@gmail.com> wrote:
> 
> The changes are mostly in patch001 and a very few in patch002.
> 
> Thank You Ajin for working on alter-subscription changes and adding more
> TAP-tests for 'failover'

Thanks for updating the patch. Here are few things I noticed when testing the patch.
1)
+++ b/src/backend/replication/logical/logical.c
@@ -602,6 +602,9 @@ CreateDecodingContext(XLogRecPtr start_lsn,
 		SnapBuildSetTwoPhaseAt(ctx->snapshot_builder, start_lsn);
 	}
 
+	/* set failover in the slot, as requested */
+	slot->data.failover = ctx->failover;
+

I noticed others also commented this change. I found this will over-write the
slot's failover value to a wrong one in the case when we have acquired the slot and
call CreateDecodingContext after that. (e.g. it can be a problem if user call
pg_logical_slot_get_changes for a logical slot).

2)

WalSndWaitForStandbyConfirmation
...
+void
+WalSndWaitForStandbyConfirmation(XLogRecPtr wait_for_lsn)
+{
+	List	   *standby_slot_cpy;
+
+	if (!MyReplicationSlot->data.failover)
+		return;
+
+	standby_slot_cpy = list_copy(standby_slot_names_list);
+

The standby list could be un-initialized when calling from a non-walsender
backend (e.g. via pg_logical_slot_get_changes()). So, we need to call
SlotSyncInitConfig somewhere in this case.

Best Regards,
Hou zj

Commits

  1. Doc: Add the new section "Logical Replication Failover".

  2. Fix the review comments and a bug in the slot sync code.

  3. Fix a test in failover slots regression test.

  4. Fix the intermittent buildfarm failures in 040_standby_failover_slots_sync.

  5. Split XLogCtl->LogwrtResult into separate struct members

  6. Ensure that the sync slots reach a consistent state after promotion without losing data.

  7. Introduce a new GUC 'standby_slot_names'.

  8. Fix BF failure introduced by commit b3f6b14cf4.

  9. Fixups for commit 93db6cbda0.

  10. Fix BF failure in commit 93db6cbda0.

  11. Add a new slot sync worker to synchronize logical slots.

  12. Improve ERROR/LOG messages added by commits ddd5f4f54a and 7a424ece48.

  13. Disable autovacuum on primary in 040_standby_failover_slots_sync test.

  14. Fix the incorrect format specifier used in commit 7a424ece48.

  15. Change the LOG level in 040_standby_failover_slots_sync.pl to DEBUG2.

  16. Add more LOG and DEBUG messages for slot synchronization.

  17. Another try to fix BF failure introduced in commit ddd5f4f54a.

  18. Fix BF introduced in commit ddd5f4f54a.

  19. Add a slot synchronization function.

  20. Clean-ups for 776621a5e4 and 7329240437.

  21. Enhance libpqrcv APIs to support slot synchronization.

  22. Add a failover option to subscriptions.

  23. Allow setting failover property in the replication command.

  24. Allow to enable failover property for replication slots via SQL API.

  25. Add macros for looping through a List without a ListCell.

  26. Track conflict_reason in pg_replication_slots.

  27. Allow upgrades to preserve the full subscription's state.

  28. Add support for incremental backup.

  29. Simplify some logic in CreateReplicationSlot()

  30. Fix uninitialized access to InitialRunningXacts during decoding after ERROR.

  31. Flexible options for CREATE_REPLICATION_SLOT.

  32. Empty search_path in logical replication apply worker and walsender.

  33. Fix oldest xmin and LSN computation across repslots after advancing

  34. Require C99 (and thus MSCV 2013 upwards).