Horton

Re: Synchronizing slots from primary to standby

Bertrand Drouvot <bertranddrouvot.pg@gmail.com>

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From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
To: shveta malik <shveta.malik@gmail.com>
Cc: Amit Kapila <amit.kapila16@gmail.com>, Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>, "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, Ajin Cherian <itsajin@gmail.com>, Peter Smith <smithpb2250@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>, Nisha Moond <nisha.moond412@gmail.com>, "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>, Peter Eisentraut <peter.eisentraut@enterprisedb.com>, Bruce Momjian <bruce@momjian.us>, Ashutosh Sharma <ashu.coek88@gmail.com>, Andres Freund <andres@anarazel.de>, pgsql-hackers <pgsql-hackers@postgresql.org>, Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2024-04-05T03:52:19Z
Lists: pgsql-hackers 
Hi,

On Thu, Apr 04, 2024 at 05:31:45PM +0530, shveta malik wrote:
> On Thu, Apr 4, 2024 at 2:59 PM shveta malik <shveta.malik@gmail.com> wrote:
> >
> >
> > Prior to commit 2ec005b, this check was okay, as we did not expect
> > restart_lsn of the synced slot to be ahead of remote since we were
> > directly copying the lsns. But now when we use 'advance' to do logical
> > decoding on standby, there is a possibility that restart lsn of the
> > synced slot is ahead of remote slot, if there are running txns records
> > found after reaching consistent-point while consuming WALs from
> > restart_lsn till confirmed_lsn. In such a case, slot-sync's advance
> > may end up serializing snapshots and setting restart_lsn to the
> > serialized snapshot point, ahead of remote one.
> >
> > Fix:
> > The sanity check needs to be corrected. Attached a patch to address the issue.
> 

Thanks for reporting, explaining the issue and providing a patch.

Regarding the patch:

1 ===

+        * Attempt to sync lsns and xmins only if remote slot is ahead of local

s/lsns/LSNs/?

2 ===

+                       if (slot->data.confirmed_flush != remote_slot->confirmed_lsn)
+                               elog(LOG,
+                                        "could not synchronize local slot \"%s\" LSN(%X/%X)"
+                                        " to remote slot's LSN(%X/%X) ",
+                                        remote_slot->name,
+                                        LSN_FORMAT_ARGS(slot->data.confirmed_flush),
+                                        LSN_FORMAT_ARGS(remote_slot->confirmed_lsn));

I don't think that the message is correct here. Unless I am missing something
there is nothing in the following code path that would prevent the slot to be
sync during this cycle. 

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com

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).