Horton

Re: Synchronizing slots from primary to standby

Amit Kapila <amit.kapila16@gmail.com>

View thread View on postgr.es
From: Amit Kapila <amit.kapila16@gmail.com>
To: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Cc: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, Masahiko Sawada <sawada.mshk@gmail.com>, shveta malik <shveta.malik@gmail.com>, Peter Smith <smithpb2250@gmail.com>, Ajin Cherian <itsajin@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>, Nisha Moond <nisha.moond412@gmail.com>, "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>, Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.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: 2024-02-15T02:48:59Z
Lists: pgsql-hackers 
On Wed, Feb 14, 2024 at 7:26 PM Bertrand Drouvot
<bertranddrouvot.pg@gmail.com> wrote:
>
> On Wed, Feb 14, 2024 at 10:40:11AM +0000, Zhijie Hou (Fujitsu) wrote:
> > On Wednesday, February 14, 2024 6:05 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> > >
> > > To ensure that restart_lsn has been moved to a recent position, we need to log
> > > XLOG_RUNNING_XACTS and make sure the same is processed as well by
> > > walsender. The attached patch does the required change.
> > >
> > > Hou-San can reproduce this problem by adding additional checkpoints in the
> > > test and after applying the attached it fixes the problem. Now, this patch is
> > > mostly based on the theory we formed based on LOGs on BF and a reproducer
> > > by Hou-San, so still, there is some chance that this doesn't fix the BF failures in
> > > which case I'll again look into those.
> >
> > I have verified that the patch can fix the issue on my machine(after adding few
> > more checkpoints before slot invalidation test.) I also added one more check in
> > the test to confirm the synced slot is not temp slot. Here is the v2 patch.
>
> Thanks!
>
> +# To ensure that restart_lsn has moved to a recent WAL position, we need
> +# to log XLOG_RUNNING_XACTS and make sure the same is processed as well
> +$primary->psql('postgres', "CHECKPOINT");
>
> Instead of "CHECKPOINT" wouldn't a less heavy "SELECT pg_log_standby_snapshot();"
> be enough?
>

Yeah, that would be enough. However, the test still fails randomly due
to the same reason. See [1]. So, as mentioned yesterday, now, I feel
it is better to recreate the subscription/slot so that it can get the
latest restart_lsn rather than relying on pg_log_standby_snapshot() to
move it.

> Not a big deal but maybe we could do the change while modifying
> 040_standby_failover_slots_sync.pl in the next patch "Add a new slotsync worker".
>

Right, we can do that or probably this test would have made more sense
with a worker patch where we could wait for the slot to be synced.
Anyway, let's try to recreate the slot/subscription idea. BTW, do you
think that adding a LOG when we are not able to sync will help in
debugging such problems? I think eventually we can change it to DEBUG1
but for now, it can help with stabilizing BF and or some other
reported issues.

[1] - https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=skink&dt=2024-02-15%2000%3A14%3A38

-- 
With Regards,
Amit Kapila.

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).