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: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, Amit Kapila <amit.kapila16@gmail.com>, "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>, Peter Smith <smithpb2250@gmail.com>, Nisha Moond <nisha.moond412@gmail.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: 2023-12-22T14:29:28Z
Lists: pgsql-hackers 
Hi,

On Fri, Dec 22, 2023 at 04:02:21PM +0530, shveta malik wrote:
> PFA v53. Changes are:

Thanks!

> patch002:
> 2) Addressed comments in [2] for v52-002.
> 3) Fixed CFBot failure. The failure was caused by an assert in
> wait_for_primary_slot_catchup() for null confirmed_lsn received. In
> wait_for_primary_slot_catchup(), we had an assumption that if
> restart_lsn is valid and 'conflicting' is also false, then we must
> have non-null confirmed_lsn. But this is not true. It is possible to
> get null values for confirmed_lsn and catalog_xmin if on the primary
> server the slot is just created with a valid restart_lsn and slot-sync
> worker has fetched the slot before the primary server could set valid
> confirmed_lsn and catalog_xmin. In
> pg_create_logical_replication_slot(), there is a small window between
> CreateInitDecodingContext-->ReplicationSlotReserveWal() which sets
> restart_lsn and DecodingContextFindStartpoint() which sets
> confirmed_lsn. If the slot-sync worker fetches the slot in this
> window, confirmed_lsn received will be NULL. Corrected the code to
> remove assert and added one additional condition that confirmed_lsn
> should be valid before moving the slot to 'r'.
> 

Looking at v53-0002 commit message:

It states:

"
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.
"

and one of the reasons mentioned is:

"
    - The primary changes wal_level to a level lower than logical.
"

I think that as long at there is still logical replication slot on the primary
that should not be possible. The primary should fail to start with messages like:

"
2023-12-22 14:06:09.281 UTC [31824] FATAL:  logical replication slot "logical_slot" exists, but wal_level < logical
"

Now, if:

- The standby is shutdown
- All the logical replication slots are removed on the primary
- wal_level is set to < logical on the primary and it is restarted

Then when the standby starts, the "synced" slots will be invalidated and later 
removed but not re-created on the next sync-cycle (because they don't exist
anymore on the primary).

Worth to reword a bit that part?

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).