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: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Cc: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, Bertrand Drouvot <bertranddrouvot.pg@gmail.com>, shveta malik <shveta.malik@gmail.com>, Ajin Cherian <itsajin@gmail.com>, Peter Smith <smithpb2250@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>, Nisha Moond <nisha.moond412@gmail.com>, "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.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: 2024-04-03T05:43:10Z
Lists: pgsql-hackers

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On Wed, Apr 3, 2024 at 9:36 AM Bharath Rupireddy
<bharath.rupireddyforpostgres@gmail.com> wrote:
>
> On Wed, Apr 3, 2024 at 9:04 AM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > > I'd just rename LogicalSlotAdvanceAndCheckSnapState(XLogRecPtr
> > > moveto, bool *found_consistent_snapshot) to
> > > pg_logical_replication_slot_advance(XLogRecPtr moveto, bool
> > > *found_consistent_snapshot) and use it. If others don't like this, I'd
> > > at least turn pg_logical_replication_slot_advance(XLogRecPtr moveto) a
> > > static inline function.
> > >
> > Yeah, we can do that but it is not a performance-sensitive routine so
> > don't know if it is worth it.
>
> Okay for what the patch has right now. No more bikeshedding from me on this.
>
> > > > The slotsync worker needs to advance the slots from different databases in
> > > > fast_forward. So, we need to skip this check in fast_forward mode. The analysis can
> > > > be found in [3].
> > > -    if (slot->data.database != MyDatabaseId)
> > > +    /*
> > > +     * We need to access the system tables during decoding to build the
> > > +     * logical changes unless we are in fast_forward mode where no changes are
> > > +     * generated.
> > > +     */
> > > +    if (slot->data.database != MyDatabaseId && !fast_forward)
> > >          ereport(ERROR,
> > >
> > > It's not clear from the comment that we need it for a slotsync worker
> > > to advance the slots from different databases. Can this be put into
> > > the comment? Also, specify in the comment, why this is safe?
> > >
> > It is not specific to slot sync worker but specific to fast_forward
> > mode. There is already a comment "We need to access the system tables
> > during decoding to build the logical changes unless we are in
> > fast_forward mode where no changes are generated." telling why it is
> > safe. The point is we need database access to access system tables
> > while generating the logical changes and in fast-forward mode, we
> > don't generate logical changes so this check is not required. Do let
> > me if you have a different understanding or if my understanding is
> > incorrect.
>
> Understood. Thanks. Just curious, why isn't a problem for the existing
> fast_forward mode callers pg_replication_slot_advance and
> LogicalReplicationSlotHasPendingWal?
>

We call those after connecting to the database and the slot also
belongs to that database whereas during synchronization of slots
standby. the slots could be from different databases.

> I quickly looked at v8, and have a nit, rest all looks good.
>
> +        if (DecodingContextReady(ctx) && found_consistent_snapshot)
> +            *found_consistent_snapshot = true;
>
> Can the found_consistent_snapshot be checked first to help avoid the
> function call DecodingContextReady() for pg_replication_slot_advance
> callers?
>

Okay, changed. Additionally, I have updated the comments and commit
message. I'll push this patch after some more testing.

-- 
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).