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: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>
Cc: Dilip Kumar <dilipbalaut@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>, shveta malik <shveta.malik@gmail.com>, Bertrand Drouvot <bertranddrouvot.pg@gmail.com>, Peter Smith <smithpb2250@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>, Ajin Cherian <itsajin@gmail.com>, Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2024-01-09T13:09:43Z
Lists: pgsql-hackers 
On Tue, Jan 9, 2024 at 5:44 PM Zhijie Hou (Fujitsu)
<houzj.fnst@fujitsu.com> wrote:
>
> V58-0002
>

+static bool
+synchronize_one_slot(WalReceiverConn *wrconn, RemoteSlot *remote_slot)
{
...
+ /* Slot ready for sync, so sync it. */
+ else
+ {
+ /*
+ * Sanity check: With hot_standby_feedback enabled and
+ * invalidations handled appropriately as above, this should never
+ * happen.
+ */
+ if (remote_slot->restart_lsn < slot->data.restart_lsn)
+ elog(ERROR,
+ "cannot synchronize local slot \"%s\" LSN(%X/%X)"
+ " to remote slot's LSN(%X/%X) as synchronization"
+ " would move it backwards", remote_slot->name,
+ LSN_FORMAT_ARGS(slot->data.restart_lsn),
+ LSN_FORMAT_ARGS(remote_slot->restart_lsn));
...
}

I was thinking about the above code in the patch and as far as I can
think this can only occur if the same name slot is re-created with
prior restart_lsn after the existing slot is dropped. Normally, the
newly created slot (with the same name) will have higher restart_lsn
but one can mimic it by copying some older slot by using
pg_copy_logical_replication_slot().

I don't think as mentioned in comments even if hot_standby_feedback is
temporarily set to off, the above shouldn't happen. It can only lead
to invalidated slots on standby.

To close the above race, I could think of the following ways:
1. Drop and re-create the slot.
2. Emit LOG/WARNING in this case and once remote_slot's LSN moves
ahead of local_slot's LSN then we can update it; but as mentioned in
your previous comment, we need to update all other fields as well. If
we follow this then we probably need to have a check for catalog_xmin
as well.

Now, related to this the other case which needs some handling is what
if the remote_slot's restart_lsn is greater than local_slot's
restart_lsn but it is a re-created slot with the same name. In that
case, I think the other properties like 'two_phase', 'plugin' could be
different. So, is simply copying those sufficient or do we need to do
something else as well?

-- 
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).