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>, "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.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: 2023-11-23T05:13:47Z
Lists: pgsql-hackers 
On Tue, Nov 21, 2023 at 4:35 PM Drouvot, Bertrand
<bertranddrouvot.pg@gmail.com> wrote:
>
> On 11/21/23 10:32 AM, shveta malik wrote:
> > On Tue, Nov 21, 2023 at 2:02 PM shveta malik <shveta.malik@gmail.com> wrote:
> >>
>
> > v37 fails to apply to HEAD due to a recent commit e83aa9f92fdd,
> > rebased the patches.  PFA v37_2 patches.
>
> Thanks!
>
> Regarding the promotion flow: If the primary is available and reachable I don't
> think we currently try to ensure that slots are in sync. I think we'd miss the
> activity since the last sync and the promotion request or am I missing something?
>
> If the primary is available and reachable shouldn't we launch a last round of
> synchronization (skipping all the slots that are not in 'r' state)?
>

We may miss the last round but there is no guarantee that we can
ensure to sync of everything if the primary is available. Because
after our last sync, there could probably be some more activity. I
think it is the user's responsibility to promote a new primary when
the old one is not required for some reason. It is not only slots that
can be out of sync but even we can miss fetching some of the data. I
think this is quite similar to what we do for WAL where on finding the
promotion signal, we shut down Walreceiver and just replay any WAL
that was already received by walreceiver. Also, the promotion
shouldn't create any problem w.r.t subscribers connecting to the new
primary because the slot's position is slightly behind what could be
requested by subscribers which means the corresponding data will be
available on the new primary.

Do you have something in mind that can create any problem if we don't
attempt additional fetching round after the promotion signal is
received?

-- 
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).