Horton

Re: Synchronizing slots from primary to standby

Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>

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From: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
To: James Coleman <jtc331@gmail.com>
Cc: Andres Freund <andres@anarazel.de>, Peter Eisentraut <peter.eisentraut@enterprisedb.com>, pgsql-hackers <pgsql-hackers@postgresql.org>, kato-sho@fujitsu.com
Date: 2022-02-28T05:04:25Z
Lists: pgsql-hackers 
On Thu, Feb 24, 2022 at 12:46 AM James Coleman <jtc331@gmail.com> wrote:
> I've been working on adding test coverage to prove this out, but I've
> encountered the problem reported in [1].
>
> My assumption, but Andres please correct me if I'm wrong, that we
> should see issues with the following steps (given the primary,
> physical replica, and logical subscriber already created in the test):
>
> 1. Ensure both logical subscriber and physical replica are caught up
> 2. Disable logical subscription
> 3. Make a catalog change on the primary (currently renaming the
> primary key column)
> 4. Vacuum pg_class
> 5. Ensure physical replication is caught up
> 6. Stop primary and promote the replica
> 7. Write to the changed table
> 8. Update subscription to point to promoted replica
> 9. Re-enable logical subscription
>
> I'm attaching my test as an additional patch in the series for
> reference. Currently I have steps 3 and 4 commented out to show that
> the issues in [1] occur without any attempt to trigger the catalog
> xmin problem.
>
> Given this error seems pretty significant in terms of indicating
> fundamental lack of test coverage (the primary stated benefit of the
> patch is physical failover), and it currently is a blocker to testing
> more deeply.

Few of my initial concerns specified at [1] are this:

1) Instead of a new LIST_SLOT command, can't we use
READ_REPLICATION_SLOT (slight modifications needs to be done to make
it support logical replication slots and to get more information from
the subscriber).

2) How frequently the new bg worker is going to sync the slot info?
How can it ensure that the latest information exists say when the
subscriber is down/crashed before it picks up the latest slot
information?

4) IIUC, the proposal works only for logical replication slots but do
you also see the need for supporting some kind of synchronization of
physical replication slots as well? IMO, we need a better and
consistent way for both types of replication slots. If the walsender
can somehow push the slot info from the primary (for physical
replication slots)/publisher (for logical replication slots) to the
standby/subscribers, this will be a more consistent and simplistic
design. However, I'm not sure if this design is doable at all.

Can anyone help clarify these?

[1] https://www.postgresql.org/message-id/CALj2ACUGNGfWRtwwZwT-Y6feEP8EtOMhVTE87rdeY14mBpsRUA%40mail.gmail.com

Regards,
Bharath Rupireddy.

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).