Horton

Re: Synchronizing slots from primary to standby

Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>

View thread View on postgr.es
From: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
To: shveta malik <shveta.malik@gmail.com>
Cc: Amit Kapila <amit.kapila16@gmail.com>, "Drouvot, Bertrand" <bertranddrouvot.pg@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>
Date: 2023-08-02T18:57:00Z
Lists: pgsql-hackers 
On Tue, Aug 1, 2023 at 5:01 PM shveta malik <shveta.malik@gmail.com> wrote:
>
> > The work division amongst the sync workers can
> > be simple, the logical replication launcher builds a shared memory
> > structure based on number of slots to sync and starts the sync workers
> > dynamically, and each sync worker picks {dboid, slot name, conninfo}
> > from the shared memory, syncs it and proceeds with other slots.
>
> Do you mean  the logical replication launcher builds a shared memory
> structure based
> on the number of 'dbs' to sync as I understood from your initial comment?

Yes. I haven't looked at the 0003 patch posted upthread. However, the
standby must do the following at a minimum:

- Make GUCs synchronize_slot_names and max_slot_sync_workers of
PGC_POSTMASTER type needing postmaster restart when changed as they
affect the number of slot sync workers.
- LR (logical replication) launcher connects to primary to fetch the
logical slots specified in synchronize_slot_names. This is a one-time
task.
- LR launcher prepares a dynamic shared memory (created via
dsm_create) with some state like locks for IPC and an array of
{slot_name, dboid_associated_with_slot, is_sync_in_progress} - maximum
number of elements in the array is the number of slots specified in
synchronize_slot_names. This is a one-time task.
- LR launcher decides the *best* number of slot sync workers - (based
on some perf numbers) it can just launch, say, one worker per 2 or 4
or 8 etc. slots.
- Each slot sync worker then picks up a slot from the DSM, connects to
primary using primary conn info, syncs it, and moves to another slot.

Not having the capability of on-demand stop/launch of slot sync
workers makes the above design simple IMO.

Thoughts?

--
Bharath Rupireddy
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).