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: 2023-10-03T14:26:31Z
Lists: pgsql-hackers 
Hi,

On 10/3/23 12:54 PM, Amit Kapila wrote:
> On Mon, Oct 2, 2023 at 11:39 AM Drouvot, Bertrand
> <bertranddrouvot.pg@gmail.com> wrote:
>>
>> 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:
>>>>
>>>
>>>> - 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?
>>
> 
> But, even if we keep 'standby_slot_names' for this purpose, the
> primary doesn't know the value of 'synchronize_slot_names' once the
> standby is down and or the primary is restarted. So, how will we know
> which logical WAL senders needs to wait for 'standby_slot_names'?
> 

Yeah right, I also think we'd need:

- synchronize_slot_names on both primary and standby

But now we would need to take care of different standby having different values (
as you said up-thread)....

Thinking out loud: What about a single GUC on the primary (not standby_slot_names nor
synchronize_slot_names) but say logical_slots_wait_for_standby that could be a list of say
"logical_slot_name:physical_slot".

I think this GUC would help us define each walsender behavior (should the standby(s)
be up or down):

- don't wait if its associated logical_slot is not listed in this GUC
- or wait based on its associated "list" of mapped physical slots (would probably
have to deal with the min restart_lsn for all the corresponding mapped ones).

I don't think we can avoid having to define at least one GUC on the primary (at least to
handle the case of standby(s) being down).

Thoughts?

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).