Horton

Re: Synchronizing slots from primary to standby

Bertrand Drouvot <bertranddrouvot.pg@gmail.com>

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From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
To: shveta malik <shveta.malik@gmail.com>
Cc: Amit Kapila <amit.kapila16@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>, "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, Peter Smith <smithpb2250@gmail.com>, Ajin Cherian <itsajin@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>, Nisha Moond <nisha.moond412@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>, Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2024-02-02T06:02:51Z
Lists: pgsql-hackers 
Hi,

On Thu, Feb 01, 2024 at 05:29:15PM +0530, shveta malik wrote:
> Attached v75 patch-set. Changes are:
> 
> 1) Re-arranged the patches:
> 1.1) 'libpqrc' related changes (from v74-001 and v74-004) are
> separated out in v75-001 as those are independent changes.
> 1.2) 'Add logical slot sync capability', 'Slot sync worker as special
> process' and 'App-name changes' are now merged to single patch which
> makes v75-002.
> 1.3) 'Wait for physical Standby confirmation' and 'Failover Validation
> Document' patches are maintained as is (v75-003 and v75-004 now).

Thanks!

I only looked at the commit message for v75-0002 and see that it has changed
since the comment done in [1], but it still does not look correct to me.

"
If a logical slot on the primary is valid but is invalidated on the standby,
then that slot is dropped and recreated on the standby in next sync-cycle
provided the slot still exists on the primary server. It is okay to recreate
such slots as long as these are not consumable on the standby (which is the
case currently). This situation may occur due to the following reasons:
- The max_slot_wal_keep_size on the standby is insufficient to retain WAL
  records from the restart_lsn of the slot.
- primary_slot_name is temporarily reset to null and the physical slot is
  removed.
- The primary changes wal_level to a level lower than logical.
"

If a logical decoding slot "still exists on the primary server" then the primary
can not change the wal_level to lower than logical, one would get something like:

"FATAL:  logical replication slot "logical_slot" exists, but wal_level < logical"

and then slots won't get invalidated on the standby. I've the feeling that the
wal_level conflict part may need to be explained separately? (I think it's not
possible that they end up being re-created on the standby for this conflict, they
will be simply removed as it would mean the counterpart one on the primary 
does not exist anymore).

[1]: https://www.postgresql.org/message-id/ZYWdSIeAMQQcLmVT%40ip-10-97-1-34.eu-west-3.compute.internal

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).