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: 2023-06-20T10:22:17Z
Lists: pgsql-hackers 
On Mon, Jun 19, 2023 at 9:56 PM Drouvot, Bertrand
<bertranddrouvot.pg@gmail.com> wrote:
>
> On 6/19/23 12:03 PM, Amit Kapila wrote:
> > On Mon, Jun 19, 2023 at 11:34 AM Drouvot, Bertrand
> > <bertranddrouvot.pg@gmail.com> wrote:
> >>
> >> Also I think we need to handle the case of invalidated replication slot(s): should
> >> we drop/recreate it/them? (as the main goal is to have sync slot(s) on the standby).
> >>
> >
> > Do you intend to ask what happens to logical slots invalidated (due to
> > say max_slot_wal_keep_size) on publisher? I think those should be
> > invalidated on standby too.
>
> Agree that it should behave that way.
>
> > Another thought whether there is chance
> > that the slot on standby gets invalidated due to conflict (say
> > required rows removed on primary)?
>
> That's the scenario I had in mind when asking the question above.
>
> > I think in such cases the slot on
> > primary/publisher should have been dropped/invalidated by that time.
>
> I don't think so.
>
> For example, such a scenario could occur:
>
> - there is no physical slot between the standby and the primary
> - the standby is shut down
> - logical decoding on the primary is moving forward and now there is vacuum
> operations that will conflict on the standby
> - the standby starts and reports the logical slot being invalidated (while it is
> not on the primary)
>
> 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().

The other related thing is that do we somehow need to ensure that WAL
is replayed on standby before moving the slot's position to the target
location received from the primary?

> > BTW, does the patch handles drop of logical slots on standby when the
> > same slot is dropped on publisher/primary?
> >
>
> from what I've seen, yes it looks like it behaves that way (will look closer).
>

Okay, I have asked because I don't see a call to ReplicationSlotDrop()
in the patch.

-- 
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).