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>
Cc: Amit Kapila <amit.kapila16@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: 2023-11-10T11:00:36Z
Lists: pgsql-hackers 
Hi,

On 11/10/23 4:31 AM, shveta malik wrote:
> On Thu, Nov 9, 2023 at 9:15 PM Drouvot, Bertrand
> <bertranddrouvot.pg@gmail.com> wrote:
>> Yeah I think so, because there is a time window when one could "use" the slot
>> after the promotion and before it is removed. Producing things like:
>>
>> "
>> 2023-11-09 15:16:50.294 UTC [2580462] LOG:  dropped replication slot "logical_slot2" of dbid 5 as it was not sync-ready
>> 2023-11-09 15:16:50.295 UTC [2580462] LOG:  dropped replication slot "logical_slot3" of dbid 5 as it was not sync-ready
>> 2023-11-09 15:16:50.297 UTC [2580462] LOG:  dropped replication slot "logical_slot4" of dbid 5 as it was not sync-ready
>> 2023-11-09 15:16:50.297 UTC [2580462] ERROR:  replication slot "logical_slot5" is active for PID 2594628
>> "
>>
>> After the promotion one was able to use logical_slot5 and now we can now drop it.
> 
> Yes, I was suspicious about this small window which may allow others
> to use this slot, that is why I was thinking of putting it in the
> promotion flow and thus asked that question earlier. But the slot-sync
> worker may end up creating it again in case it has not exited.

Sorry, there is a typo up-thread, I meant "After the promotion one was able to
use logical_slot5 and now we can NOT drop it.". We can not drop it because it
is in use.

> So we
> need to carefully decide at what all places we need to put  'not-in
> recovery' checks in slot-sync workers. In the previous version,
> synchronize_one_slot() had that check and it was skipping sync if
> '!RecoveryInProgress'. But I have removed that check in v32 thinking
> that the slots which the worker has already fetched from the primary,
> let them all get synced and exit after that  nstead of syncing half
> and leaving rest. But now on rethinking, was the previous behaviour
> correct i.e. skip sync at that point onward where we see it is no
> longer in standby-mode while few of the slots have already been synced
> in that sync-cycle. Thoughts?
> 

I think we still need to think/discuss the promotion flow. I think we would need
to have the slot sync worker shutdown during the promotion (as suggested by Amit in [1])
but before that let the sync slot worker knows it is now acting during promotion.

Something like:

- let the sync worker know it is now acting under promotion
- do what needs to be done while acting under promotion
- shutdown the sync worker

That way we would avoid any "risk" of having the sync worker doing something
we don't expect while not in recovery anymore.

Regarding "do what needs to be done while acting under promotion":

- Ensure all slots in 'r' state are synced
- drop slots that are in 'i' state

Thoughts?

[1]: https://www.postgresql.org/message-id/CAA4eK1J2Pc%3D5TOgty5u4bp--y7ZHaQx3_2eWPL%3DVPJ7A_0JF2g%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).