Horton

RE: Synchronizing slots from primary to standby

Zhijie Hou (Fujitsu) <houzj.fnst@fujitsu.com>

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From: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>
To: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>, 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: 2023-11-24T03:35:02Z
Lists: pgsql-hackers 
On Thursday, November 23, 2023 11:45 PM Drouvot, Bertrand <bertranddrouvot.pg@gmail.com> wrote:

Hi,

> 
> On 11/23/23 6:13 AM, Amit Kapila wrote:
> > 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 don't think so thanks to the fact that we ensure that logical walsenders on the
> primary wait for the physical standby.
> 
> Indeed that should prevent any decoding activity on the primary while the
> promotion is in progress on the standby (at least as soon as the walreceiver is
> shutdown).
> 
> So that I think that a promotion flow like:
> 
> - walreceiver shutdown
> - last round of sync
> - sync-worker shutdown
> 
> Should ensure that slots are in sync (as logical slots on the primary should not
> be able to advance as soon as the walreceiver is shutdown during the
> promotion).
> 

I think it could not ensure the slots are in sync, because there is no
guarantee that the logical slot has caught up to the physical standby on
promotion and logical publisher and subscriber both could still be active
during promotion. IOW, the logical slot's LSN can still be advanced after the
walreceiver shutdown if it was far bebind the physical slot's LSN.


Best Regards,
Hou zj

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).