Horton

Re: Synchronizing slots from primary to standby

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>
Cc: Ajin Cherian <itsajin@gmail.com>, 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>, Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2023-10-25T04:57:15Z
Lists: pgsql-hackers 
On Tue, Oct 24, 2023 at 3:35 PM Drouvot, Bertrand
<bertranddrouvot.pg@gmail.com> wrote:
>
> On 10/24/23 7:44 AM, Ajin Cherian wrote:
> > On Mon, Oct 23, 2023 at 11:22 PM Drouvot, Bertrand
> > <bertranddrouvot.pg@gmail.com> wrote:
>
> 2) When we create a subscription, another slot is created during the subscription synchronization, namely
> like "pg_16397_sync_16388_7293447291374081805" (coming from ReplicationSlotNameForTablesync()).
>
> This extra slot appears to have failover also set to true.
>
> So, If the standby refresh the list of slot to sync when the subscription is still synchronizing we'd see things like
> on the standby:
>
> LOG:  waiting for remote slot "mysub" LSN (0/C0034808) and catalog xmin (756) to pass local slot LSN (0/C0034840) and and catalog xmin (756)
> LOG:  wait over for remote slot "mysub" as its LSN (0/C00368B0) and catalog xmin (756) has now passed local slot LSN (0/C0034840) and catalog xmin (756)
> LOG:  waiting for remote slot "pg_16397_sync_16388_7293447291374081805" LSN (0/C0034808) and catalog xmin (756) to pass local slot LSN (0/C00368E8) and and catalog xmin (756)
> WARNING:  slot "pg_16397_sync_16388_7293447291374081805" disappeared from the primary, aborting slot creation
>
> I'm not sure this "pg_16397_sync_16388_7293447291374081805" should have failover set to true. If there is a failover
> during the subscription creation, better to re-launch the subscription instead?
>

But note that the subscription doesn't wait for the completion of
tablesync. So, how will we deal with that? Also, this situation is the
same for non-tablesync slots as well. I have given another option in
the email [1] which is to enable failover even for the main slot after
all tables are in ready state, something similar to what we do for
two_phase.

[1] - https://www.postgresql.org/message-id/CAA4eK1J6BqO5%3DueFAQO%2BaYyHLaU-oCHrrVFJqHS-i0Ce9aPY2w%40mail.gmail.com

-- 
With Regards,
Amit Kapila.

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).