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: 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>
Date: 2023-06-29T10:36:39Z
Lists: pgsql-hackers 
On Wed, Jun 28, 2023 at 12:19 PM Drouvot, Bertrand
<bertranddrouvot.pg@gmail.com> wrote:
>
> On 6/26/23 12:34 PM, Amit Kapila wrote:
> > On Mon, Jun 26, 2023 at 11:15 AM Drouvot, Bertrand
> > <bertranddrouvot.pg@gmail.com> wrote:
> >>
> >> On 6/20/23 12:22 PM, Amit Kapila wrote:
> >>> On Mon, Jun 19, 2023 at 9:56 PM Drouvot, Bertrand
> >>> <bertranddrouvot.pg@gmail.com> wrote:
> >>
> >>>> In such a case (slot valid on the primary but invalidated on the standby) then I think we
> >>>> could drop and recreate the invalidated slot on the standby.
> >>>>
> >>>
> >>> Will it be safe? Because after recreating the slot, it will reserve
> >>> the new WAL location and build the snapshot based on that which might
> >>> miss some important information in the snapshot. For example, to
> >>> update the slot's position with new information from the primary, the
> >>> patch uses pg_logical_replication_slot_advance() which means it will
> >>> process all records and update the snapshot via
> >>> DecodeCommit->SnapBuildCommitTxn().
> >>
> >> Your concern is that the slot could have been consumed on the standby?
> >>
> >> I mean, if we suppose the "synchronized" slot can't be consumed on the standby then
> >> drop/recreate such an invalidated slot would be ok?
> >>
> >
> > That also may not be sufficient because as soon as the slot is
> > invalidated/dropped, the required WAL could be removed on standby.
> >
>
> Yeah, I think once the slot is dropped we just have to wait for the slot to
> be re-created on the standby according to the new synchronize_slot_names GUC.
>
> Assuming the initial slot "creation" on the standby (coming from the synchronize_slot_names usage)
> is working "correctly" then it should also work "correctly" once the slot is dropped.
>

I also think so.

> If we agree that a synchronized slot can not/should not be consumed (will implement this behavior) then
> I think the proposed scenario above should make sense, do you agree?
>

Yeah, I also can't think of a use case for this. So, we can probably
disallow it and document the same. I guess if we came across a use
case for this, we can rethink allowing to consume the changes from
synchronized slots.

-- 
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).