Horton

RE: Synchronizing slots from primary to standby

Zhijie Hou (Fujitsu) <houzj.fnst@fujitsu.com>

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From: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: Peter Smith <smithpb2250@gmail.com>, shveta malik <shveta.malik@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>, Ajin Cherian <itsajin@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>, Nisha Moond <nisha.moond412@gmail.com>, "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>, Bertrand Drouvot <bertranddrouvot.pg@gmail.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>, Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2024-03-03T07:56:32Z
Lists: pgsql-hackers

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On Saturday, March 2, 2024 6:55 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> 
> On Sat, Mar 2, 2024 at 9:21 AM Zhijie Hou (Fujitsu) <houzj.fnst@fujitsu.com>
> wrote:
> >
> > Apart from the comments, the code in WalSndWaitForWal was refactored a
> > bit to make it neater. Thanks Shveta for helping writing the code and doc.
> >
> 
> A few more comments:

Thanks for the comments.

> ==================
> 1.
> +# Wait until the primary server logs a warning indicating that it is
> +waiting # for the sb1_slot to catch up.
> +$primary->wait_for_log(
> + qr/replication slot \"sb1_slot\" specified in parameter
> standby_slot_names does not have active_pid/,
> + $offset);
> 
> Shouldn't we wait for such a LOG even in the first test as well which involves two
> standbys and two logical subscribers?

Yes, we should. Added.

> 
> 2.
> +##################################################
> +# Test that logical replication will wait for the user-created inactive
> +# physical slot to catch up until we remove the slot from standby_slot_names.
> +##################################################
> 
> 
> I don't see anything different tested in this test from what we already tested in
> the first test involving two standbys and two logical subscribers. Can you
> please clarify if I am missing something?

I think the intention was to test that the wait loop is ended due to GUC config
reload, while the first test is for the case when the loop is ended due to
restart_lsn movement. But it seems we tested the config reload with xx_get_changes() as
well, so I can remove it if you agree.

> 
> 3.
> Note that after receiving the shutdown signal, an ERROR
> + * is reported if any slots are dropped, invalidated, or inactive. This
> + * measure is taken to prevent the walsender from waiting indefinitely.
> + */
> + if (NeedToWaitForStandby(target_lsn, flushed_lsn, wait_event))
> 
> Isn't this part of the comment should be moved inside
> NeedToWaitForStandby()?

Moved.

> 
> 4.
> + /*
> + * Update our idea of the currently flushed position only if we are
> + * not waiting for standbys to catch up, otherwise the standby would
> + * have to catch up to a newer WAL location in each cycle.
> + */
> + if (wait_event != WAIT_EVENT_WAIT_FOR_STANDBY_CONFIRMATION)
> + {
> 
> This functionality (in function WalSndWaitForWal()) seems to ensure that we
> first wait for the required WAL to be flushed and then wait for standbys. If true,
> we should cover that point in the comments here or somewhere in the function
> WalSndWaitForWal().
> 
> Apart from this, I have made a few modifications in the comments.

Thanks. I have reviewed and merged them.

Here is the V104 patch which addressed above and Peter's comments.

Best Regards,
Hou zj

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).