Horton

Re: Synchronizing slots from primary to standby

Bertrand Drouvot <bertranddrouvot.pg@gmail.com>

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From: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: shveta malik <shveta.malik@gmail.com>, Peter Smith <smithpb2250@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: 2023-11-08T07:02:34Z
Lists: pgsql-hackers 
Hi,

On 11/8/23 4:50 AM, Amit Kapila wrote:
> On Tue, Nov 7, 2023 at 7:58 PM Drouvot, Bertrand
> <bertranddrouvot.pg@gmail.com> wrote:
>>
>> If we think this window is too short we could:
>>
>> - increase it
>> or
>> - don't drop the slot once created (even if there is no activity
>> on the primary during PrimaryCatchupWaitAttempt attempts) so that
>> the next loop of attempts will compare with "older" LSN/xmin (as compare to
>> dropping and re-creating the slot). That way the window would be since the
>> initial slot creation.
>>
> 
> Yeah, this sounds reasonable but we can't mark such slots to be
> synced/available for use after failover.

Yeah, currently we are fine as slots are dropped in wait_for_primary_slot_catchup() if
we are not in recovery anymore.

> I think if we want to follow
> this approach then we need to also monitor these slots for any change
> in the consecutive cycles and if we are able to sync them then
> accordingly we enable them to use after failover.

What about to add a new field in ReplicationSlotPersistentData
indicating that we are waiting for "sync" and drop such slots during promotion and
/or if not in recovery?

> Another somewhat related point is that right now, we just wait for the
> change on the first slot (the patch refers to it as the monitoring
> slot) for computing nap_time before which we will recheck all the
> slots. I think we can improve that as well such that even if any
> slot's information is changed, we don't consider changing naptime.
> 

Yeah, that sounds reasonable to me.

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).