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: Amit Kapila <amit.kapila16@gmail.com>
Cc: 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>
Date: 2023-06-26T05:45:28Z
Lists: pgsql-hackers 
Hi,

On 6/20/23 12:22 PM, Amit Kapila wrote:
> On Mon, Jun 19, 2023 at 9:56 PM Drouvot, Bertrand
> <bertranddrouvot.pg@gmail.com> wrote:

>> In such a case (slot valid on the primary but invalidated on the standby) then I think we
>> could drop and recreate the invalidated slot on the standby.
>>
> 
> Will it be safe? Because after recreating the slot, it will reserve
> the new WAL location and build the snapshot based on that which might
> miss some important information in the snapshot. For example, to
> update the slot's position with new information from the primary, the
> patch uses pg_logical_replication_slot_advance() which means it will
> process all records and update the snapshot via
> DecodeCommit->SnapBuildCommitTxn().

Your concern is that the slot could have been consumed on the standby?

I mean, if we suppose the "synchronized" slot can't be consumed on the standby then
drop/recreate such an invalidated slot would be ok?
  
Asking, because I'm not sure we should allow consumption of a "synchronized" slot
until the standby gets promoted.

When the patch has been initially proposed, logical decoding from a standby
was not implemented yet.
  
> The other related thing is that do we somehow need to ensure that WAL
> is replayed on standby before moving the slot's position to the target
> location received from the primary?

Yeah, will check if this is currently done that way in the patch proposal.

>>> BTW, does the patch handles drop of logical slots on standby when the
>>> same slot is dropped on publisher/primary?
>>>
>>
>> from what I've seen, yes it looks like it behaves that way (will look closer).
>>
> 
> Okay, I have asked because I don't see a call to ReplicationSlotDrop()
> in the patch.
> 

Right. I'd need to look closer to understand how it works (for the moment
the "only" thing I've done was the re-base shared up-thread).

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).