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: Amit Kapila <amit.kapila16@gmail.com>
Cc: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, shveta malik <shveta.malik@gmail.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>, 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: 2024-03-29T12:21:32Z
Lists: pgsql-hackers 
Hi,

On Fri, Mar 29, 2024 at 02:35:22PM +0530, Amit Kapila wrote:
> On Fri, Mar 29, 2024 at 1:08 PM Bertrand Drouvot
> <bertranddrouvot.pg@gmail.com> wrote:
> >
> > On Fri, Mar 29, 2024 at 07:23:11AM +0000, Zhijie Hou (Fujitsu) wrote:
> > > On Friday, March 29, 2024 2:48 PM Bertrand Drouvot <bertranddrouvot.pg@gmail.com> wrote:
> > > >
> > > > Hi,
> > > >
> > > > On Fri, Mar 29, 2024 at 01:06:15AM +0000, Zhijie Hou (Fujitsu) wrote:
> > > > > Attach a new version patch which fixed an un-initialized variable
> > > > > issue and added some comments. Also, temporarily enable DEBUG2 for the
> > > > > 040 tap-test so that we can analyze the possible CFbot failures easily.
> > > > >
> > > >
> > > > Thanks!
> > > >
> > > > +       if (remote_slot->confirmed_lsn != slot->data.confirmed_flush)
> > > > +       {
> > > > +               /*
> > > > +                * By advancing the restart_lsn, confirmed_lsn, and xmin using
> > > > +                * fast-forward logical decoding, we ensure that the required
> > > > snapshots
> > > > +                * are saved to disk. This enables logical decoding to quickly
> > > > reach a
> > > > +                * consistent point at the restart_lsn, eliminating the risk of
> > > > missing
> > > > +                * data during snapshot creation.
> > > > +                */
> > > > +
> > > > pg_logical_replication_slot_advance(remote_slot->confirmed_lsn,
> > > > +
> > > > found_consistent_point);
> > > > +               ReplicationSlotsComputeRequiredLSN();
> > > > +               updated_lsn = true;
> > > > +       }
> > > >
> > > > Instead of using pg_logical_replication_slot_advance() for each synced slot and
> > > > during sync cycles what about?:
> > > >
> > > > - keep sync slot synchronization as it is currently (not using
> > > > pg_logical_replication_slot_advance())
> > > > - create "an hidden" logical slot if sync slot feature is on
> > > > - at the time of promotion use pg_logical_replication_slot_advance() on this
> > > > hidden slot only to advance to the max lsn of the synced slots
> > > >
> > > > I'm not sure that would be enough, just asking your thoughts on this (benefits
> > > > would be to avoid calling pg_logical_replication_slot_advance() on each sync
> > > > slots and during the sync cycles).
> > >
> > > Thanks for the idea !
> > >
> > > I considered about this. I think advancing the "hidden" slot on promotion may be a
> > > bit late, because if we cannot reach the consistent point after advancing the
> > > "hidden" slot, then it means we may need to remove all the synced slots as we
> > > are not sure if they are usable(will not loss data) after promotion.
> >
> > What about advancing the hidden slot during the sync cycles then?
> >
> > > The current approach is to mark such un-consistent slot as temp and persist
> > > them once it reaches consistent point, so that user can ensure the slot can be
> > > used after promotion once persisted.
> >
> > Right, but do we need to do so for all the sync slots? Would a single hidden
> > slot be enough?
> >
> 
> Even if we mark one of the synced slots as persistent without reaching
> a consistent state, it could create a problem after promotion. And,
> how a single hidden slot would serve the purpose, different synced
> slots will have different restart/confirmed_flush LSN and we won't be
> able to perform advancing for those using a single slot. For example,
> say for first synced slot, it has not reached a consistent state and
> then how can it try for the second slot? This sounds quite tricky to
> make work. We should go with something simple where the chances of
> introducing bugs are lesser.

Yeah, better to go with something simple.

+       if (remote_slot->confirmed_lsn != slot->data.confirmed_flush)
+       {
+               /*
+                * By advancing the restart_lsn, confirmed_lsn, and xmin using
+                * fast-forward logical decoding, we ensure that the required snapshots
+                * are saved to disk. This enables logical decoding to quickly reach a
+                * consistent point at the restart_lsn, eliminating the risk of missing
+                * data during snapshot creation.
+                */
+               pg_logical_replication_slot_advance(remote_slot->confirmed_lsn,
+                                                                                       found_consistent_point);

In our case, what about skipping WaitForStandbyConfirmation() in
pg_logical_replication_slot_advance()? (It could go until the
RecoveryInProgress() check in StandbySlotsHaveCaughtup() if we don't skip it).

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).