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: 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: 2023-10-25T15:19:54Z
Lists: pgsql-hackers 
Hi,

On 10/9/23 12:30 PM, shveta malik wrote:
> PFA v22 patch-set. It has below changes:
> 
> patch 001:
> 1) Now physical walsender wakes up logical walsender(s) by using a new
> CV as suggested in [1]

Thanks!

I think that works fine as long as the standby is up and running and catching up.

The problem I see with the current WalSndWaitForStandbyConfirmation() implementation
is that if the standby is not running then:

+   for (;;)
+   {
+       ListCell   *l;
+       long        sleeptime = -1;

will loop until we reach the "terminating walsender process due to replication timeout" if we
explicitly want to end with SIGINT or friends.

For example a scenario like:

- standby down
- pg_recvlogical running

then CTRL-C on pg_recvlogical would not "respond" immediately but when we reach the replication timeout.

So it seems that we should use something like WalSndWait() instead of ConditionVariableTimedSleep() here:

+               /*
+                * Sleep until other physical walsenders awaken us or until a timeout
+                * occurs.
+                */
+               sleeptime = WalSndComputeSleeptime(GetCurrentTimestamp());
+
+               ConditionVariableTimedSleep(&WalSndCtl->wal_confirm_rcv_cv, sleeptime,
+                                                                       WAIT_EVENT_WAL_SENDER_WAIT_FOR_STANDBY_CONFIRMATION);

In that case I think that WalSndWait() should take care of the new CV WalSndCtl->wal_confirm_rcv_cv too.
The wait on the socket should allow us to stop waiting when, for example, CTRL-C on pg_recvlogical is triggered.

Then we would need to deal with this scenario: Standby down or not catching up and exited WalSndWait() due to the socket
to break the loop or shutdown the walsender.

Thoughts?

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).