Horton

Re: Synchronizing slots from primary to standby

Masahiko Sawada <sawada.mshk@gmail.com>

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From: Masahiko Sawada <sawada.mshk@gmail.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: shveta malik <shveta.malik@gmail.com>, Ajin Cherian <itsajin@gmail.com>, "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, Peter Smith <smithpb2250@gmail.com>, Bertrand Drouvot <bertranddrouvot.pg@gmail.com>, Dilip Kumar <dilipbalaut@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>, Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2024-02-06T07:38:47Z
Lists: pgsql-hackers 
On Tue, Feb 6, 2024 at 3:19 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Mon, Feb 5, 2024 at 7:56 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> >
> > ---
> > Since Two processes (e.g. the slotsync worker and
> > pg_sync_replication_slots()) concurrently fetch and update the slot
> > information, there is a race condition where slot's
> > confirmed_flush_lsn goes backward.
> >
>
> Right, this is possible, though there shouldn't be a problem because
> anyway, slotsync is an async process. Till we hold restart_lsn, the
> required WAL won't be removed. Having said that, I can think of two
> ways to avoid it: (a) We can have some flag in shared memory using
> which we can detect whether any other process is doing slot
> syncronization and then either error out at that time or simply wait
> or may take nowait kind of parameter from user to decide what to do?
> If this is feasible, we can simply error out for the first version and
> extend it later if we see any use cases for the same (b) similar to
> restart_lsn, if confirmed_flush_lsn is getting moved back, raise an
> error, this is good for now but in future we may still have another
> similar issue, so I would prefer (a) among these but I am fine if you
> prefer (b) or have some other ideas like just note down in comments
> that this is a harmless case and can happen only very rarely.

Thank you for sharing the ideas. I would prefer (a). For (b), the same
issue still happens for other fields.

>
> >
> > ---
> > +     It is recommended that subscriptions are first disabled before promoting
> > f+     the standby and are enabled back after altering the connection string.
> >
> > I think it's better to describe the reason why it's recommended to
> > disable subscriptions before the standby promotion.
> >
>
> Agreed. The reason I see for this is that if we don't disable the
> subscription before promotion and changing the connection string there
> is a chance that the old primary comes back and the subscriber can
> have some additional data, though the chances of same are less.
>
> > ---
> > +/* Slot sync worker objects */
> > +extern PGDLLIMPORT char *PrimaryConnInfo;
> > +extern PGDLLIMPORT char *PrimarySlotName;
> >
> > These two variables are declared also in xlogrecovery.h. Is it
> > intentional? If so, I think it's better to write comments.
> >
> > ---
> > Global functions and variables used by the slotsync worker are
> > declared in logicalworker.h and worker_internal.h. But is it really
> > okay to make a dependency between the slotsync worker and logical
> > replication workers? IIUC the slotsync worker is conceptually a
> > separate feature from the logical replication. I think the slotsync
> > worker can have its own header file.
> >
>
> +1.
>
> >
> > ---
> > +     Confirm that the standby server is not lagging behind the subscribers.
> > +     This step can be skipped if
> > +     <link linkend="guc-standby-slot-names"><varname>standby_slot_names</varname></link>
> > +     has been correctly configured.
> >
> > How can the user confirm if standby_slot_names is correctly configured?
> >
>
> I think users can refer to LOGs to see if it has changed since the
> first time it was configured. I tried by existing parameter and see
> the following in LOG:
> LOG:  received SIGHUP, reloading configuration files
> 2024-02-06 11:38:59.069 IST [9240] LOG:  parameter "autovacuum" changed to "on"
>
> If the user can't confirm then it is better to follow the steps
> mentioned in the patch. Do you want something else to be written in
> docs for this? If so, what?

IIUC even if a wrong slot name is specified to standby_slot_names or
even standby_slot_names is empty, the standby server might not be
lagging behind the subscribers depending on the timing. But when
checking it the next time, the standby server might lag behind the
subscribers. So what I wanted to know is how the user can confirm if a
failover-enabled subscription is ensured not to go in front of
failover-candidate standbys (i.e., standbys using the slots listed in
standby_slot_names).

Regards,

-- 
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com

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).