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Re: Synchronizing slots from primary to standby

Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>

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From: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: "Drouvot, Bertrand" <bertranddrouvot.pg@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>
Date: 2023-07-09T07:30:00Z
Lists: pgsql-hackers

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On Fri, Jun 16, 2023 at 3:26 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Mon, Apr 17, 2023 at 7:37 PM Drouvot, Bertrand
> <bertranddrouvot.pg@gmail.com> wrote:
> >
> > Please find attached V5 (a rebase of V4 posted up-thread).
> >
> > In addition to the "rebasing" work, the TAP test adds a test about conflict handling (logical slot invalidation)
> > relying on the work done in the "Minimal logical decoding on standby" patch series.
> >
> > I did not look more at the patch (than what's was needed for the rebase) but plan to do so.
> >
>
> Are you still planning to continue working on this? Some miscellaneous
> comments while going through this patch are as follows?
>
> 1. Can you please try to explain the functionality of the overall
> patch somewhere in the form of comments and or commit message?

IIUC, there are 2 core ideas of the feature:

1) It will never let the logical replication subscribers go ahead of
physical replication standbys specified in standby_slot_names. It
implements this by delaying decoding of commit records on the
walsenders corresponding to logical replication subscribers on the
primary until all the specified standbys confirm receiving the commit
LSN.

2) The physical replication standbys will synchronize data of the
specified logical replication slots (in synchronize_slot_names) from
the primary, creating the logical replication slots if necessary.
Since the logical replication subscribers will never go out of
physical replication standbys, the standbys can safely synchronize the
slots and keep the data necessary for subscribers to connect to it and
work seamlessly even after a failover.

If my understanding is right, I have few thoughts here:

1. All the logical walsenders are delayed on the primary - per
wait_for_standby_confirmation() despite the user being interested in
only a few of them via synchronize_slot_names. Shouldn't the delay be
for just the slots specified in synchronize_slot_names?
2. I think we can split the patch like this - 0001 can be the logical
walsenders delaying decoding on the primary unless standbys confirm,
0002 standby synchronizing the logical slots.
3. I think we need to change the GUC standby_slot_names to better
reflect what it is used for - wait_for_replication_slot_names or
wait_for_
4. It allows specifying logical slots in standby_slot_names, meaning,
it can disallow logical slots getting ahead of other logical slots
specified in standby_slot_names. Should we allow this case with the
thinking that if there's anyone using logical replication for failover
(well, will anybody do that in production?).
5. Similar to above, it allows specifying physical slots in
synchronize_slot_names. Should we disallow?

I'm attaching the v6 patch, a rebased version of v5.

-- 
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com

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).