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: 2023-09-29T11:33:36Z
Lists: pgsql-hackers 
On Thu, Sep 28, 2023 at 6:31 PM Drouvot, Bertrand
<bertranddrouvot.pg@gmail.com> wrote:
>
> On 9/25/23 6:10 AM, shveta malik wrote:
> > On Fri, Sep 22, 2023 at 3:48 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >>
> >> On Thu, Sep 21, 2023 at 9:16 AM shveta malik <shveta.malik@gmail.com> wrote:
> >>>
> >>> On Tue, Sep 19, 2023 at 10:29 AM shveta malik <shveta.malik@gmail.com> wrote:
> >>>
> >>> Currently in patch001, synchronize_slot_names is a GUC on both primary
> >>> and physical standby. This GUC tells which all logical slots need to
> >>> be synced on physical standbys from the primary. Ideally it should be
> >>> a GUC on physical standby alone and each physical standby should be
> >>> able to communicate the value to the primary (considering the value
> >>> may vary for different physical replicas of the same primary). The
> >>> primary on the other hand should be able to take UNION of these values
> >>> and let the logical walsenders (belonging to the slots in UNION
> >>> synchronize_slots_names) wait for physical standbys for confirmation
> >>> before sending those changes to logical subscribers. The intent is
> >>> logical subscribers should never be ahead of physical standbys.
> >>>
> >>
> >> Before getting into the details of 'synchronize_slot_names', I would
> >> like to know whether we really need the second GUC
> >> 'standby_slot_names'. Can't we simply allow all the logical wal
> >> senders corresponding to 'synchronize_slot_names' to wait for just the
> >> physical standby(s) (physical slot corresponding to such physical
> >> standby) that have sent ' synchronize_slot_names'list? We should have
> >> one physical standby slot corresponding to one physical standby.
> >>
> >
> > yes, with the new approach (to be implemented next) where we plan to
> > send synchronize_slot_names from each physical standby to primary, the
> > standby_slot_names GUC should no longer be needed on primary. The
> > physical standbys sending requests should automatically become the
> > ones to be waited for confirmation on the primary.
> >
>
> I think that standby_slot_names could be used to do some filtering (means
> for which standby(s) we don't want the logical replication on the primary to go
> ahead and for which standby(s) one would allow it).
>

Isn't it implicit that the physical standby that has requested
'synchronize_slot_names' should be ahead of their corresponding
logical walsenders? Otherwise, after the switchover to the new
physical standby, the logical subscriptions won't work.

> I think that removing the GUC would:
>
> - remove this flexibility
>

I think if required we can add such a GUC later as well. Asking users
to set more parameters also makes the feature less attractive, so I am
trying to see if we can avoid this GUC.

> - probably open corner cases like: what if a standby is down? would that mean
> that synchronize_slot_names not being send to the primary would allow the decoding
> on the primary to go ahead?
>

Good question. BTW, irrespective of whether we have
'standby_slot_names' parameters or not, how should we behave if
standby is down? Say, if 'synchronize_slot_names' is only specified on
standby then in such a situation primary won't be even aware that some
of the logical walsenders need to wait. OTOH, one can say that users
should configure 'synchronize_slot_names' on both primary and standby
but note that this value could be different for different standby's,
so we can't configure it on primary.

-- 
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).