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: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Cc: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, Masahiko Sawada <sawada.mshk@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>, shveta malik <shveta.malik@gmail.com>, Peter Smith <smithpb2250@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>, Ajin Cherian <itsajin@gmail.com>, Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2024-01-13T04:35:52Z
Lists: pgsql-hackers 
On Fri, Jan 12, 2024 at 12:07 PM Bertrand Drouvot
<bertranddrouvot.pg@gmail.com> wrote:
>
> On Fri, Jan 12, 2024 at 08:42:39AM +0530, Amit Kapila wrote:
> > On Thu, Jan 11, 2024 at 9:11 PM Bertrand Drouvot
> > <bertranddrouvot.pg@gmail.com> wrote:
> > >
> > > I'm not sure to follow here. If the remote slot is re-created then it would
> > > be also dropped / re-created locally, or am I missing something?
> > >
> >
> > As our slot-syncing mechanism is asynchronous (from time to time we
> > check the slot information on primary), isn't it possible that the
> > same name slot is dropped and recreated between slot-sync worker's
> > checks?
> >
>
> Yeah, I should have thought harder ;-) So for this case, let's imagine that If we
> had an easy way to detect that a remote slot has been drop/re-created then I think
> we would also drop and re-create it on the standby too.
>
> If so, I think we should then update all the fields (that we're currently updating
> in the "create locally" case) when we detect that (at least) one of the following differs:
>
> - dboid
> - plugin
> - two_phase
>

Right, I think even if any of restart/confirmed LSN's or xmin has
changed then also there is no harm in simply copying all the fields
from remote_slot as done by Hou-San in latest patch.

> Maybe the "best" approach would be to have a way to detect that a slot has been
> re-created on the primary (but that would mean rely on more than the slot name
> to "identify" a slot and probably add a new member to the struct to do so).
>

Right, I also thought so but not sure further complicating the slot
machinery is worth detecting this case explicitly. If we see any
problem with the idea discussed then we may need to think something
along those lines.

-- 
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).