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: 2023-10-23T12:22:13Z
Lists: pgsql-hackers 
Hi,

On 10/20/23 5:27 AM, shveta malik wrote:
> On Wed, Oct 18, 2023 at 4:24 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> 
> PFA v25 patch set. The changes are:
> 
> 1) 'enable_failover' is changed to 'failover'
> 2) Alter subscription changes to support 'failover'
> 3) Fixes a bug in patch001 wherein any change in standby_slot_names
> was not considered in the flow where logical walsenders wait for
> standby's confirmation. Now during the wait, if standby_slot_names is
> changed, wait is restarted using new standby_slot_names.
> 4) Addresses comments by Bertrand and Amit in [1],[2],[3]
> 
> The changes are mostly in patch001 and a very few in patch002.
> 
> Thank You Ajin for working on alter-subscription changes and adding
> more TAP-tests for 'failover'
> 

Thanks for updating the patch!

Looking at 0001 and doing some experiment:

Creating a logical slot with failover = true and then launching
pg_logical_slot_get_changes() or pg_recvlogical() on it results
to setting failover back to false.

It occurs while creating the decoding context here:

@@ -602,6 +602,9 @@ CreateDecodingContext(XLogRecPtr start_lsn,
         SnapBuildSetTwoPhaseAt(ctx->snapshot_builder, start_lsn);
     }

+   /* set failover in the slot, as requested */
+   slot->data.failover = ctx->failover;
+

I think we can get rid of this change in CreateDecodingContext().

Looking at 0002:

     /* Enter main loop */
     for (;;)
     {
         int         rc;
         long        wait_time = DEFAULT_NAPTIME_PER_CYCLE;

         CHECK_FOR_INTERRUPTS();

         /*
          * If it is Hot standby, then try to launch slot-sync workers else
          * launch apply workers.
          */
         if (RecoveryInProgress())
         {
             /* Launch only if we have succesfully made the connection */
             if (wrconn)
                 LaunchSlotSyncWorkers(&wait_time, wrconn);
         }

We are waiting for DEFAULT_NAPTIME_PER_CYCLE (3 minutes) before checking if there
is new synced slot(s) to be created on the standby. Do we want to keep this behavior
for V1?

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).