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>, 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: 2023-10-28T04:28:53Z
Lists: pgsql-hackers 
On Fri, Oct 27, 2023 at 9:00 PM Drouvot, Bertrand
<bertranddrouvot.pg@gmail.com> wrote:
>
> On 10/27/23 10:35 AM, Amit Kapila wrote:
> > On Wed, Oct 25, 2023 at 3:15 PM Drouvot, Bertrand
> >
> > I think even if we provide such an API, we need to have logic to get
> > the slots from the primary and create them.
>
> Yeah, my idea was to add an API (in addition to what is already in place).
>
> > Say, even if the user used
> > the APIs, there may still be some new slots that the sync worker needs
> > to create.
>
> Right.
>
> > I think it might be better to provide a view for users to
> > view the current state of sync. For example, in the above case, we can
> > say "waiting for the primary to advance remote LSN" or something like
> > that.
>
> We are already displaying the wait event "ReplSlotsyncPrimaryCatchup" in pg_stat_activity
> so that might already be enough?
>

I am fine if the wait is already displayed in some form.

> My main idea was to be able to manually create/sync logical_slot2 in the test case described in [1]
> without waiting for activity on logical_slot1.
>
> But another (better?) option might be to change our current algorithm during slot creation on the
> standby? (to avoid an "active" slot having to wait on a "inactive" one, like described in [1]).
>

Yeah, I guess it would be better to tweak the algorithm in this case
such that the slots can't be created immediately but can be noted in a
separate list and we can continue with other remaining slots. Once, we
are finished with all the slots, this special list can be traversed
and then we can attempt to create all the remaining slots. OTOH, the
scenario you described doesn't sound to be a frequent case to be
worried for it but if we can deal with it without adding much
complexity then it would be good.

-- 
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).