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: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>
Cc: shveta malik <shveta.malik@gmail.com>, Ajin Cherian <itsajin@gmail.com>, Amit Kapila <amit.kapila16@gmail.com>, Peter Smith <smithpb2250@gmail.com>, Masahiko Sawada <sawada.mshk@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: 2024-03-28T10:04:21Z
Lists: pgsql-hackers 
Hi,

On Thu, Mar 28, 2024 at 04:38:19AM +0000, Zhijie Hou (Fujitsu) wrote:
> Hi,
> 
> When analyzing one BF error[1], we find an issue of slotsync: Since we don't
> perform logical decoding for the synced slots when syncing the lsn/xmin of
> slot, no logical snapshots will be serialized to disk. So, when user starts to
> use these synced slots after promotion, it needs to re-build the consistent
> snapshot from the restart_lsn if the WAL(xl_running_xacts) at restart_lsn
> position indicates that there are running transactions. This however could
> cause the data that before the consistent point to be missed[2].

I see, nice catch and explanation, thanks!

> This issue doesn't exist on the primary because the snapshot at restart_lsn
> should have been serialized to disk (SnapBuildProcessRunningXacts ->
> SnapBuildSerialize), so even if the logical decoding restarts, it can find
> consistent snapshot immediately at restart_lsn.

Right.

> To fix this, we could use the fast forward logical decoding to advance the synced
> slot's lsn/xmin when syncing these values instead of directly updating the
> slot's info. This way, the snapshot will be serialized to disk when decoding.
> If we could not reach to the consistent point at the remote restart_lsn, the
> slot is marked as temp and will be persisted once it reaches the consistent
> point. I am still analyzing the fix and will share once ready.

Thanks! I'm wondering about the performance impact (even in fast_forward mode),
might be worth to keep an eye on it.

Should we create a 17 open item [1]?

[1]: https://wiki.postgresql.org/wiki/PostgreSQL_17_Open_Items

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).