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>, "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, Dilip Kumar <dilipbalaut@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>, shveta malik <shvetamalik@gmail.com>
Date: 2024-01-16T07:28:42Z
Lists: pgsql-hackers 
On Tue, Jan 16, 2024 at 1:07 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Tue, Jan 16, 2024 at 9:03 AM shveta malik <shveta.malik@gmail.com> wrote:
> >
> > On Sat, Jan 13, 2024 at 12:54 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> > >
> > > On Fri, Jan 12, 2024 at 5:50 PM shveta malik <shveta.malik@gmail.com> wrote:
> > > >
> > > > There are multiple approaches discussed and tried when it comes to
> > > > starting a slot-sync worker. I am summarizing all here:
> > > >
> > > >  1) Make slotsync worker as an Auxiliary Process (like checkpointer,
> > > > walwriter, walreceiver etc). The benefit this approach provides is, it
> > > > can control begin and stop in a more flexible way as each auxiliary
> > > > process could have different checks before starting and can have
> > > > different stop conditions. But it needs code duplication for process
> > > > management(start, stop, crash handling, signals etc) and currently it
> > > > does not support db-connection smoothly (none of the auxiliary process
> > > > has one so far)
> > > >
> > >
> > > As slotsync worker needs to perform transactions and access syscache,
> > > we can't make it an auxiliary process as that doesn't initialize the
> > > required stuff like syscache. Also, see the comment "Auxiliary
> > > processes don't run transactions ..." in AuxiliaryProcessMain() which
> > > means this is not an option.
> > >
> > > >
> > > > 2) Make slotsync worker as a 'special' process like AutoVacLauncher
> > > > which is neither an Auxiliary process nor a bgworker one. It allows
> > > > db-connection and also provides flexibility to have start and stop
> > > > conditions for a process.
> > > >
> > >
> > > Yeah, due to these reasons, I think this option is worth considering
> > > and another plus point is that this allows us to make enable_syncslot
> > > a PGC_SIGHUP GUC rather than a PGC_POSTMASTER.
> > >
> > > >
> > > > 3) Make slotysnc worker a bgworker. Here we just need to register our
> > > > process as a bgworker (RegisterBackgroundWorker()) by providing a
> > > > relevant start_time and restart_time and then the process management
> > > > is well taken care of. It does not need any code-duplication and
> > > > allows db-connection smoothly in registered process. The only thing it
> > > > lacks is that it does not provide flexibility of having
> > > > start-condition which then makes us to have 'enable_syncslot' as
> > > > PGC_POSTMASTER parameter rather than PGC_SIGHUP. Having said this, I
> > > > feel enable_syncslot is something which will not be changed frequently
> > > > and with the benefits provided by bgworker infra, it seems a
> > > > reasonably good option to choose this approach.
> > > >
> > >
> > > I agree but it may be better to make it a PGC_SIGHUP parameter.
> > >
> > > > 4) Another option is to have Logical Replication Launcher(or a new
> > > > process) to launch slot-sync worker. But going by the current design
> > > > where we have only 1 slotsync worker, it may be an overhead to have an
> > > > additional manager process maintained.
> > > >
> > >
> > > I don't see any good reason to have an additional launcher process here.
> > >
> > > >
> > > > Thus weighing pros and cons of all these options, we have currently
> > > > implemented the bgworker approach (approach 3).  Any feedback is
> > > > welcome.
> > > >
> > >
> > > I vote to go for (2) unless we face difficulties in doing so but (3)
> > > is also okay especially if others also think so.
> >
> > I am not against any of the approaches but I still feel that when we
> > have a standard way of doing things (bgworker) we should not keep
> > adding code to do things in a special way unless there is a strong
> > reason to do so. Now we need to decide if 'enable_syncslot' being
> > PGC_POSTMASTER is a strong reason to go the non-standard way?
> >
>
> Agreed and as said earlier I think it is better to make it a
> PGC_SIGHUP. Also, not sure we can say it is a non-standard way as
> already autovacuum launcher is handled in the same way. One more minor
> thing is it will save us for having a new bgworker state
> BgWorkerStart_ConsistentState_HotStandby as introduced by this patch.

Why do we need to add a new BgWorkerStart_ConsistentState_HotStandby
for the slotsync worker? Isn't it sufficient that the slotsync worker
exits if not in hot standby mode?

Is there any technical difficulty or obstacle to make the slotsync
worker start using bgworker after reloading the config file?

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).