Horton

Re: Synchronizing slots from primary to standby

Bertrand Drouvot <bertranddrouvot.pg@gmail.com>

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From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: Masahiko Sawada <sawada.mshk@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>, shveta malik <shveta.malik@gmail.com>, "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.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: 2024-01-03T11:26:57Z
Lists: pgsql-hackers 
Hi,

On Wed, Jan 03, 2024 at 04:20:03PM +0530, Amit Kapila wrote:
> On Fri, Dec 29, 2023 at 12:32 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > On Fri, Dec 29, 2023 at 6:59 AM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> > >
> > > On Wed, Dec 27, 2023 at 7:43 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> > > >
> > > > >
> > > > > 3) The slotsync worker uses primary_conninfo but also uses a new GUC
> > > > > parameter, say slot_sync_dbname, to specify the database to connect.
> > > > > The slot_sync_dbname overwrites the dbname if primary_conninfo also
> > > > > specifies it. If both don't have a dbname, raise an error.
> > > > >
> > > >
> > > > Would the users prefer to provide a value for a separate GUC instead
> > > > of changing primary_conninfo? It is possible that we can have some
> > > > users prefer to use one GUC and others prefer a separate GUC but we
> > > > should add a new GUC if we are sure that is what users would prefer.
> > > > Also, even if have to consider this option, I think we can easily
> > > > later add a new GUC to provide a dbname in addition to having the
> > > > provision of giving it in primary_conninfo.
> > >
> > > I think having two separate GUCs is more flexible for example when
> > > users want to change the dbname to connect. It makes sense that the
> > > slotsync worker wants to use the same connection string as the
> > > walreceiver uses. But I guess today most primary_conninfo settings
> > > that are set manually or are generated by tools such as pg_basebackup
> > > don't have dbname. If we require a dbname in primary_conninfo, many
> > > tools will need to be changed. Once the connection string is
> > > generated, it would be tricky to change the dbname in it, as Shveta
> > > mentioned. The users will have to carefully select the database to
> > > connect when taking a base backup.
> > >
> >
> > I see your point and agree that users need to be careful. I was trying
> > to compare it with other places like the conninfo used with a
> > subscription where no separate dbname needs to be provided. Now, here
> > the situation is not the same because the same conninfo is used for
> > different purposes (walreceiver doesn't require dbname (dbname is
> > ignored even if present) whereas slotsyncworker requires dbname). I
> > was just trying to see if we can avoid having a new GUC for this
> > purpose. Does anyone else have an opinion on this matter?
> >
> 
> Bertrand, Dilip, and others involved in this thread or otherwise, see
> if you can share an opinion on the above point because it would be
> good to get some more opinions before we decide to add a new GUC (for
> dbname) for slotsync worker.
> 

I think that as long as enable_syncslot is off then there is no need to add the
dbname in primary_conninfo (means there is no need to change an existing primary_conninfo
for the ones that don't use the sync slot feature).

So given that primary_conninfo does not necessary need to be changed (for ones that
don't use the sync slot feature) and that adding a new GUC looks more a one-way door
change to me, I'd vote to keep the patch as it is (we can still revisit this later
on and add a new GUC if we feel the need based on user's feedback).

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
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).