Horton

Re: Synchronizing slots from primary to standby

Bertrand Drouvot <bertranddrouvot.pg@gmail.com>

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From: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>
To: shveta malik <shveta.malik@gmail.com>, Amit Kapila <amit.kapila16@gmail.com>
Cc: "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: 2023-09-27T09:43:46Z
Lists: pgsql-hackers 
Hi,

On 9/19/23 6:50 AM, shveta malik wrote:
> On Wed, Sep 13, 2023 at 5:19 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>>
>> On Wed, Sep 13, 2023 at 4:54 PM shveta malik <shveta.malik@gmail.com> wrote:
>>>
>>> PFA  v17. It has below changes:
>>>
>>
>> @@ -2498,6 +2500,13 @@ ReorderBufferProcessTXN(ReorderBuffer *rb,
>> ReorderBufferTXN *txn,
>>    }
>>    else
>>    {
>> + /*
>> + * Before we send out the last set of changes to logical decoding
>> + * output plugin, wait for specified streaming replication standby
>> + * servers (if any) to confirm receipt of WAL upto commit_lsn.
>> + */
>> + WaitForStandbyLSN(commit_lsn);
>>
>> It seems the first patch has a wait logic for every commit. I think it
>> is better to integrate this wait with WalSndWaitForWal() as suggested
>> by Andres in his email[1].
>>
>> [1] - https://www.postgresql.org/message-id/20220207204557.74mgbhowydjco4mh%40alap3.anarazel.de
>>
>> --
> 
> Sure Amit. PFA  v18. It addresses below:
> 
> 1) patch001: wait for physical-standby confirmation logic is now
> integrated with WalSndWaitForWal(). Now walsender waits for physical
> standby's confirmation to take changes upto RecentFlushPtr in
> WalSndWaitForWal(). This allows walsender to send the changes to
> logical subscribers one by one which are already covered in
> RecentFlushPtr without needing to wait on every commit for physical
> standby confirmation.

+       /* XXX: Is waiting for 1 second before retrying enough or more or less? */
+       (void) WaitLatch(MyLatch,
+                                        WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
+                                        1000L,
+                                        WAIT_EVENT_WAL_SENDER_WAIT_FOR_STANDBY_CONFIRMATION);

I think it would be better to let the physical walsender(s) wake up those logical
walsender(s) (instead of waiting for 1 sec or such). Maybe we could introduce a new CV that would
broadcast in PhysicalConfirmReceivedLocation() when restart_lsn is changed, what do you think?

Still regarding preventing the logical replication to go ahead of
physical replication standbys specified in standby_slot_names: we currently don't impose this
limitation to pg_logical_slot_get_changes and friends (that don't start a dedicated walsender).

Shouldn't we also prevent them to go ahead of physical replication standbys specified in standby_slot_names?

Regards,

-- 
Bertrand Drouvot
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).