Horton

Re: Synchronizing slots from primary to standby

Peter Eisentraut <peter.eisentraut@enterprisedb.com>

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From: Peter Eisentraut <peter.eisentraut@enterprisedb.com>
To: pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2021-12-14T22:12:44Z
Lists: pgsql-hackers

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On 31.10.21 11:08, Peter Eisentraut wrote:
> I want to reactivate $subject.  I took Petr Jelinek's patch from [0], 
> rebased it, added a bit of testing.  It basically works, but as 
> mentioned in [0], there are various issues to work out.
> 
> The idea is that the standby runs a background worker to periodically 
> fetch replication slot information from the primary.  On failover, a 
> logical subscriber would then ideally find up-to-date replication slots 
> on the new publisher and can just continue normally.

> So, again, this isn't anywhere near ready, but there is already a lot 
> here to gather feedback about how it works, how it should work, how to 
> configure it, and how it fits into an overall replication and HA 
> architecture.

Here is an updated patch.  The main changes are that I added two 
configuration parameters.  The first, synchronize_slot_names, is set on 
the physical standby to specify which slots to sync from the primary. 
By default, it is empty.  (This also fixes the recovery test failures 
that I had to disable in the previous patch version.)  The second, 
standby_slot_names, is set on the primary.  It holds back logical 
replication until the listed physical standbys have caught up.  That 
way, when failover is necessary, the promoted standby is not behind the 
logical replication consumers.

In principle, this works now, I think.  I haven't made much progress in 
creating more test cases for this; that's something that needs more 
attention.

It's worth pondering what the configuration language for 
standby_slot_names should be.  Right now, it's just a list of slots that 
all need to be caught up.  More complicated setups are conceivable. 
Maybe you have standbys S1 and S2 that are potential failover targets 
for logical replication consumers L1 and L2, and also standbys S3 and S4 
that are potential failover targets for logical replication consumers L3 
and L4.  Viewed like that, this setting could be a replication slot 
setting.  The setting might also have some relationship with 
synchronous_standby_names.  Like, if you have synchronous_standby_names 
set, then that's a pretty good indication that you also want some or all 
of those standbys in standby_slot_names.  (But note that one is slots 
and one is application names.)  So there are a variety of possibilities.

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).