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: 2023-10-02T06:09:01Z
Lists: pgsql-hackers 
Hi,

On 9/29/23 1:33 PM, Amit Kapila wrote:
> On Thu, Sep 28, 2023 at 6:31 PM Drouvot, Bertrand
> <bertranddrouvot.pg@gmail.com> wrote:
>>
>>
>> I think that standby_slot_names could be used to do some filtering (means
>> for which standby(s) we don't want the logical replication on the primary to go
>> ahead and for which standby(s) one would allow it).
>>
> 
> Isn't it implicit that the physical standby that has requested
> 'synchronize_slot_names' should be ahead of their corresponding
> logical walsenders? Otherwise, after the switchover to the new
> physical standby, the logical subscriptions won't work.

Right, but the idea was to let the flexibility to bypass this constraint. Use
case was to avoid a physical standby being down preventing the decoding
on the primary.

> 
>> I think that removing the GUC would:
>>
>> - remove this flexibility
>>
> 
> I think if required we can add such a GUC later as well. Asking users
> to set more parameters also makes the feature less attractive, so I am
> trying to see if we can avoid this GUC.

Agree but I think we have to address the standby being down case.
> 
>> - probably open corner cases like: what if a standby is down? would that mean
>> that synchronize_slot_names not being send to the primary would allow the decoding
>> on the primary to go ahead?
>>
> 
> Good question. BTW, irrespective of whether we have
> 'standby_slot_names' parameters or not, how should we behave if
> standby is down? Say, if 'synchronize_slot_names' is only specified on
> standby then in such a situation primary won't be even aware that some
> of the logical walsenders need to wait.

Exactly, that's why I was thinking keeping standby_slot_names to address
this scenario. In such a case one could simply decide to keep or remove
the associated physical replication slot from standby_slot_names. Keep would
mean "wait" and removing would mean allow to decode on the primary.

> OTOH, one can say that users
> should configure 'synchronize_slot_names' on both primary and standby
> but note that this value could be different for different standby's,
> so we can't configure it on primary.
> 

Yeah, I think that's a good use case for standby_slot_names, what do you think?

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).