Horton

Re: Synchronizing slots from primary to standby

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>
Cc: Masahiko Sawada <sawada.mshk@gmail.com>, shveta malik <shveta.malik@gmail.com>, Peter Smith <smithpb2250@gmail.com>, Ajin Cherian <itsajin@gmail.com>, Bertrand Drouvot <bertranddrouvot.pg@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: 2024-02-15T11:30:18Z
Lists: pgsql-hackers 
On Thu, Feb 15, 2024 at 4:29 PM Zhijie Hou (Fujitsu)
<houzj.fnst@fujitsu.com> wrote:
>
> On Thursday, February 15, 2024 5:20 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> > On Thu, Feb 15, 2024 at 9:05 AM Zhijie Hou (Fujitsu) <houzj.fnst@fujitsu.com>
> > wrote:
> > >
> > > On Thursday, February 15, 2024 10:49 AM Amit Kapila
> > <amit.kapila16@gmail.com> wrote:
> > > >
> > > > On Wed, Feb 14, 2024 at 7:26 PM Bertrand Drouvot
> > > >
> > > > 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.
> > >
> > > Here is the patch that attempts the re-create sub idea.
> > >
> >
> > Pushed this.
> >
> > >
> >  I also think that a LOG/DEBUG
> > > would be useful for such analysis, so the 0002 is to add such a log.
> > >
> >
> > I feel such a LOG would be useful.
> >
> > + ereport(LOG,
> > + errmsg("waiting for remote slot \"%s\" LSN (%X/%X) and catalog xmin"
> > +    " (%u) to pass local slot LSN (%X/%X) and catalog xmin (%u)",
> >
> > I think waiting is a bit misleading here, how about something like:
> > "could not sync slot information as remote slot precedes local slot:
> > remote slot \"%s\": LSN (%X/%X), catalog xmin (%u) local slot: LSN (%X/%X),
> > catalog xmin (%u)"
>
> Changed.
>
> Attach the v2 patch here.
>
> Apart from the new log message. I think we can add one more debug message in
> reserve_wal_for_local_slot, this could be useful to analyze the failure.

Yeah, that can also be helpful, but the added message looks naive to me.
+ elog(DEBUG1, "segno: %ld oldest_segno: %ld", oldest_segno, segno);

Instead of the above, how about something like: "segno: %ld of
purposed restart_lsn for the synced slot, oldest_segno: %ld
available"?

> And we
> can also enable the DEBUG log in the 040 tap-test, I see we have similar
> setting in 010_logical_decoding_timline and logging debug1 message doesn't
> increase noticable time on my machine. These are done in 0002.
>

I haven't tested it but I think this can help in debugging BF
failures, if any. I am not sure if to keep it always like that but
till the time these tests are stabilized, this sounds like a good
idea. So, how, about just making test changes as a separate patch so
that later if required we can revert/remove it easily? Bertrand, do
you have any thoughts on this?

-- 
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).