Horton

Re: Synchronizing slots from primary to standby

shveta malik <shveta.malik@gmail.com>

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From: shveta malik <shveta.malik@gmail.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: Peter Smith <smithpb2250@gmail.com>, "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, Masahiko Sawada <sawada.mshk@gmail.com>, Ajin Cherian <itsajin@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>, Nisha Moond <nisha.moond412@gmail.com>, "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>, Bertrand Drouvot <bertranddrouvot.pg@gmail.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>, shveta malik <shveta.malik@gmail.com>
Date: 2024-03-05T06:35:22Z
Lists: pgsql-hackers

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On Tue, Mar 5, 2024 at 9:15 AM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Tue, Mar 5, 2024 at 6:10 AM Peter Smith <smithpb2250@gmail.com> wrote:
> >
> > ======
> > src/backend/replication/walsender.c
> >
> > 5. NeedToWaitForWal
> >
> > + /*
> > + * Check if the standby slots have caught up to the flushed position. It
> > + * is good to wait up to the flushed position and then let the WalSender
> > + * send the changes to logical subscribers one by one which are already
> > + * covered by the flushed position without needing to wait on every change
> > + * for standby confirmation.
> > + */
> > + if (NeedToWaitForStandbys(flushed_lsn, wait_event))
> > + return true;
> > +
> > + *wait_event = 0;
> > + return false;
> > +}
> > +
> >
> > 5a.
> > The comment (or part of it?) seems misplaced because it is talking
> > WalSender sending changes, but that is not happening in this function.
> >
>
> I don't think so. This is invoked only by walsender and a static
> function. I don't see any other better place to mention this.
>
> > Also, isn't what this is saying already described by the other comment
> > in the caller? e.g.:
> >
>
> Oh no, here we are explaining the wait order.

I think there is a scope of improvement here. The comment inside
NeedToWaitForWal() which states that we need to wait here for standbys
on flush-position(and not on each change) should be outside of this
function. It is too embedded. And the comment which states the order
of wait (first flush and then standbys confirmation) should be outside
the for-loop in WalSndWaitForWal(), but yes we do need both the
comments. Attached a patch (.txt) for comments improvement, please
merge if appropriate.

thanks
Shveta

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).