Horton

Re: Synchronizing slots from primary to standby

Bertrand Drouvot <bertranddrouvot.pg@gmail.com>

View thread View on postgr.es
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: 2023-10-04T06:38:49Z
Lists: pgsql-hackers 
Hi,

On 10/4/23 7:00 AM, shveta malik wrote:
> On Wed, Oct 4, 2023 at 9:56 AM shveta malik <shveta.malik@gmail.com> wrote:

> The most simplistic approach would be:
> 
> 1) maintain standby_slot_names GUC on primary
> 2) maintain synchronize_slot_names GUC on physical standby alone.
> 
> On primary, let all logical-walsenders wait on physical-standbys
> configured in standby_slot_names GUC. This will work and will avoid
> all the complexity involved in designs discussed above. But  this
> simplistic approach comes with disadvantages like below:
> 
> 1) Even if the associated slot of logical-walsender is not part of
> synchronize_slot_names of any of the physical-standbys, it is still
> waiting for all the configured standbys to finish.

That's right. Currently (with walsender waiting an arbitrary amount of time)
that sounds like a -1. But if we're going with a new CV approach (like proposed
in [1], that might not be so terrible). Though I don't feel comfortable with
waiting for no reasons (even if this is for a short amount of time possible).

> 2) If associated slot of logical walsender is part of
> synchronize_slot_names of standby1, it is still waiting on standby2,3
> etc to finish i.e. waiting on rest of the standbys configured in
> standby_slot_names which have not even marked that logical slot in
> their synchronize_slot_names.
> 

Same thoughts as above for 1)

> So we need to weigh our options here.
> 

With the simplistic approach, if a standby goes down that would impact non related
walsenders on the primary until the standby's associated physical slot is removed
from standby_slot_names and I don't feel comfortable wit this behavior.

So, I'm +1 for the ReplicationSlotPersistentData approach proposed by Amit.

[1]: https://www.postgresql.org/message-id/CAA4eK1LNjgL6Lghgu1PcDfuoOfa8Ug4J7Uv-H%3DBPP8Wgf1%2BpOw%40mail.gmail.com

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).