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: 2023-06-16T09:56:05Z
Lists: pgsql-hackers 
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?

2. It seems that the initially synchronized list of slots is only used
to launch a per-database worker to synchronize all the slots
corresponding to that database. If so, then why do we need to fetch
all the slot-related information via LIST_SLOTS command?

3. As mentioned in the initial email, I think it would be better to
replace LIST_SLOTS command with a SELECT query.

4. How the limit of sync_slot workers is decided? Can we document such
a piece of information? Do we need a new GUC to decide the number of
workers? Ideally, it would be better to avoid GUC, can we use any
existing logical replication workers related GUC?

5. Can we separate out the functionality related to standby_slot_names
in a separate patch, probably the first one? I think that will patch
easier to review.

6. In libpqrcv_list_slots(), two-phase related slot information is not
retrieved. Is there a reason for the same?

7.
+static void
+wait_for_standby_confirmation(XLogRecPtr commit_lsn)

Some comments atop this function would make it easier to review.

8.
+/*-------------------------------------------------------------------------
+ * slotsync.c
+ *    PostgreSQL worker for synchronizing slots to a standby from primary
+ *
+ * Copyright (c) 2016-2018, PostgreSQL Global Development Group
+ *

The copyright notice is out-of-date.

9. Why synchronize_one_slot() compares
MyReplicationSlot->data.restart_lsn with the value of
confirmed_flush_lsn passed to it? Also, why it does only for new slots
but not existing slots?

10. Can we somehow test if the restart_lsn is advanced properly after
sync? I think it is important to ensure that because otherwise after
standby's promotion, the subscriber can start syncing from the wrong
position.

-- 
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).