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: shveta malik <shveta.malik@gmail.com>, Amit Kapila <amit.kapila16@gmail.com>
Cc: 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: 2023-11-13T09:53:17Z
Lists: pgsql-hackers 
Hi,

On 11/13/23 5:24 AM, shveta malik wrote:
> On Thu, Nov 9, 2023 at 8:56 AM Amit Kapila <amit.kapila16@gmail.com> wrote:
>>
>> Apart from the above, I would like to discuss the slot sync work
>> distribution strategy of this patch. The current implementation as
>> explained in the commit message [1] works well if the slots belong to
>> multiple databases. It is clear from the data in emails [2][3][4] that
>> having more workers really helps if the slots belong to multiple
>> databases. But I think if all the slots belong to one or very few
>> databases then such a strategy won't be as good. Now, on one hand, we
>> get very good numbers for a particular workload with the strategy used
>> in the patch but OTOH it may not be adaptable to various different
>> kinds of workloads. So, I have a question whether we should try to
>> optimize this strategy for various kinds of workloads or for the first
>> version let's use a single-slot sync-worker and then we can enhance
>> the functionality in later patches either in PG17 itself or in PG18 or
>> later versions.
> 
> I can work on separating the patch. We can first focus on single
> worker design and then we can work on multi-worker design either
> immediately (if needed) or we can target it in the second draft of the
> patch. I would like to know the thoughts of others on this.

If we need to put more thoughts on the workers distribution strategy
then I also think it's better to focus on a single worker and then
improve/discuss a distribution design later on.

> 
> One thing to note is that a lot of the complexity of
>> the patch is attributed to the multi-worker strategy which may still
>> not be efficient, so there is an argument to go with a simpler
>> single-slot sync-worker strategy and then enhance it in future
>> versions as we learn more about various workloads. It will also help
>> to develop this feature incrementally instead of doing all the things
>> in one go and taking a much longer time than it should.
> 
> Agreed. With multi-workers, a lot of complexity (dsa, locks etc) have
> come into play. We can decide better on our workload distribution
> strategy among workers once we have more clarity on different types of
> workloads.
> 

Agreed.

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).