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: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Cc: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, Masahiko Sawada <sawada.mshk@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>, shveta malik <shveta.malik@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: 2024-01-12T03:12:39Z
Lists: pgsql-hackers 
On Thu, Jan 11, 2024 at 9:11 PM Bertrand Drouvot
<bertranddrouvot.pg@gmail.com> wrote:
>
> On Thu, Jan 11, 2024 at 04:22:56PM +0530, Amit Kapila wrote:
> > On Tue, Jan 9, 2024 at 6:39 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> > >
> > > +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.
>
> I also think so.
>
> > >
> > > 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.
>
> IIUC, this would be a sync slot (so not usable until promotion) that could
> not be used anyway (invalidated), so I'll vote for drop / re-create then.
>

No, it can happen for non-sync slots 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?
> > >
> >
>
> I'm not sure to follow here. If the remote slot is re-created then it would
> be also dropped / re-created locally, or am I missing something?
>

As our slot-syncing mechanism is asynchronous (from time to time we
check the slot information on primary), isn't it possible that the
same name slot is dropped and recreated between slot-sync worker's
checks?

-- 
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).