Horton

Re: Synchronizing slots from primary to standby

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: Nisha Moond <nisha.moond412@gmail.com>
Cc: shveta malik <shveta.malik@gmail.com>, Peter Smith <smithpb2250@gmail.com>, "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>, "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.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>, Ajin Cherian <itsajin@gmail.com>, Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2023-12-17T02:51:40Z
Lists: pgsql-hackers 
On Fri, Dec 15, 2023 at 5:55 PM Nisha Moond <nisha.moond412@gmail.com> wrote:
>
> (1)
> When we try to create a subscription on standby using a synced slot
> that is in 'r' sync_state, the subscription will be created at the
> subscriber, and on standby, two actions will take place -
>     (i)  As copy_data is true by default, it will switch the failover
> state of the synced-slot to 'false'.
>     (ii) As we don't allow to use synced-slots, it will start giving
> the expected error in the log file -
>         ERROR:  cannot use replication slot "logical_slot" for logical decoding
>         DETAIL:  This slot is being synced from the primary server.
>         HINT:  Specify another replication slot.
>
> The first one seems an issue,  it toggles the failover to false and
> then it remains false after that. I think it should be fixed.
>

+1. If we don't allow the slot to be used, we shouldn't allow its
state to be changed as well.

> (2)
> With the patch, the 'CREATE SUBSCRIPTION' command with a 'slot_name'
> of an 'active' logical slot fails and errors out -
>     ERROR:  could not alter replication slot "logical_slot" on
> publisher: ERROR:  replication slot "logical_slot1" is active for PID
> xxxx
>
> Without the patch, the create subscription with an 'active' slot_name
> succeeds and the log file shows the error "could not start WAL
> streaming: ERROR:  replication slot "logical_slot" is active for PID
> xxxx".
>
> Given that the specified active slot_name has failover set to false
> and the create subscription command also specifies failover=false, the
> expected behavior of the "with-patch" case is anticipated to be the
> same as that of the "without-patch" scenario.
>

Currently, we first acquire the slot to change its state but I guess
if we want the behavior as you mentioned we first need to check the
slot's 'failover' state without acquiring the slot. I am not sure if
that is any better because anyway we are going to fail in the very
next step as the slot is busy.

-- 
With Regards,
Amit Kapila.

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).