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: shveta malik <shveta.malik@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: 2023-09-28T04:51:38Z
Lists: pgsql-hackers 
On Wed, Sep 27, 2023 at 3:13 PM Drouvot, Bertrand
<bertranddrouvot.pg@gmail.com> wrote:
>
> On 9/19/23 6:50 AM, shveta malik wrote:
> >
> > 1) patch001: wait for physical-standby confirmation logic is now
> > integrated with WalSndWaitForWal(). Now walsender waits for physical
> > standby's confirmation to take changes upto RecentFlushPtr in
> > WalSndWaitForWal(). This allows walsender to send the changes to
> > logical subscribers one by one which are already covered in
> > RecentFlushPtr without needing to wait on every commit for physical
> > standby confirmation.
>
> +       /* XXX: Is waiting for 1 second before retrying enough or more or less? */
> +       (void) WaitLatch(MyLatch,
> +                                        WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
> +                                        1000L,
> +                                        WAIT_EVENT_WAL_SENDER_WAIT_FOR_STANDBY_CONFIRMATION);
>
> I think it would be better to let the physical walsender(s) wake up those logical
> walsender(s) (instead of waiting for 1 sec or such). Maybe we could introduce a new CV that would
> broadcast in PhysicalConfirmReceivedLocation() when restart_lsn is changed, what do you think?
>

Yes, I also think there should be some way for physical walsender to
wake up logical walsenders instead of just waiting. By the way, do you
think we need a GUC like standby_slot_names (please see discussion
[1])?

> Still regarding preventing the logical replication to go ahead of
> physical replication standbys specified in standby_slot_names: we currently don't impose this
> limitation to pg_logical_slot_get_changes and friends (that don't start a dedicated walsender).
>
> Shouldn't we also prevent them to go ahead of physical replication standbys specified in standby_slot_names?
>

Yes, I also think similar handling is required in
pg_logical_slot_get_changes_guts(). We do call GetFlushRecPtr(), so
the handling similar to what the patch is trying to do in
WalSndWaitForWal() can be done.

[1] - https://www.postgresql.org/message-id/CAJpy0uA%2Bt3XP2M0qtEmrOG1gSwHghjHPno5AtwTXM-94-%2Bc6JQ%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).