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: 2023-11-09T15:45:57Z
Lists: pgsql-hackers 
Hi,

On 11/9/23 11:54 AM, shveta malik wrote:
> 
> PFA v32 patches which has below changes:

Thanks!
   
> 7) Added warning for cases where a user-slot with the same name is
> already present which slot-sync worker is trying to create. Sync for
> such slots is skipped.

I'm seeing assertion and segfault in this case due to ReplicationSlotRelease()
in synchronize_one_slot().

Adding this extra check prior to it:

-       ReplicationSlotRelease();
+       if (!(found && s->data.sync_state == SYNCSLOT_STATE_NONE))
+               ReplicationSlotRelease();

make them disappear.

> 
> Open Question:
> 1) Currently I have put drop slot logic for slots with 'sync_state=i'
> in slot-sync worker. Do we need to put it somewhere in promotion-logic
> as well? 

Yeah I think so, because there is a time window when one could "use" the slot
after the promotion and before it is removed. Producing things like:

"
2023-11-09 15:16:50.294 UTC [2580462] LOG:  dropped replication slot "logical_slot2" of dbid 5 as it was not sync-ready
2023-11-09 15:16:50.295 UTC [2580462] LOG:  dropped replication slot "logical_slot3" of dbid 5 as it was not sync-ready
2023-11-09 15:16:50.297 UTC [2580462] LOG:  dropped replication slot "logical_slot4" of dbid 5 as it was not sync-ready
2023-11-09 15:16:50.297 UTC [2580462] ERROR:  replication slot "logical_slot5" is active for PID 2594628
"

After the promotion one was able to use logical_slot5 and now we can now drop it.

> Perhaps in WaitForWALToBecomeAvailable() where we call
> XLogShutdownWalRcv after checking 'CheckForStandbyTrigger'. Thoughts?
> 

You mean here?

/*
  * Check to see if promotion is requested. Note that we do
  * this only after failure, so when you promote, we still
  * finish replaying as much as we can from archive and
  * pg_wal before failover.
  */
if (StandbyMode && CheckForStandbyTrigger())
{
  	XLogShutdownWalRcv();
         return XLREAD_FAIL;
}

If so, that sounds like a good place to me.

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).