Horton

RE: Synchronizing slots from primary to standby

Zhijie Hou (Fujitsu) <houzj.fnst@fujitsu.com>

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From: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>
To: Masahiko Sawada <sawada.mshk@gmail.com>, Amit Kapila <amit.kapila16@gmail.com>
Cc: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>, Bertrand Drouvot <bertranddrouvot.pg@gmail.com>, shveta malik <shveta.malik@gmail.com>, Ajin Cherian <itsajin@gmail.com>, Peter Smith <smithpb2250@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: 2024-04-09T13:00:49Z
Lists: pgsql-hackers

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On Thursday, April 4, 2024 4:25 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:

Hi,

> On Wed, Apr 3, 2024 at 7:06 PM Amit Kapila <amit.kapila16@gmail.com>
> wrote:
> >
> > On Wed, Apr 3, 2024 at 11:13 AM Amit Kapila <amit.kapila16@gmail.com>
> wrote:
> > >
> > > On Wed, Apr 3, 2024 at 9:36 AM Bharath Rupireddy
> > > <bharath.rupireddyforpostgres@gmail.com> wrote:
> > >
> > > > 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.
> > >
> >
> > Pushed!
> 
> While testing this change, I realized that it could happen that the server logs
> are flooded with the following logical decoding logs that are written every 200
> ms:

Thanks for reporting!

> 
> 2024-04-04 16:15:19.270 JST [3838739] LOG:  starting logical decoding for slot
> "test_sub"
> 2024-04-04 16:15:19.270 JST [3838739] DETAIL:  Streaming transactions
> committing after 0/50006F48, reading WAL from 0/50006F10.
> 2024-04-04 16:15:19.270 JST [3838739] LOG:  logical decoding found
> consistent point at 0/50006F10
> 2024-04-04 16:15:19.270 JST [3838739] DETAIL:  There are no running
> transactions.
> 2024-04-04 16:15:19.477 JST [3838739] LOG:  starting logical decoding for slot
> "test_sub"
> 2024-04-04 16:15:19.477 JST [3838739] DETAIL:  Streaming transactions
> committing after 0/50006F48, reading WAL from 0/50006F10.
> 2024-04-04 16:15:19.477 JST [3838739] LOG:  logical decoding found
> consistent point at 0/50006F10
> 2024-04-04 16:15:19.477 JST [3838739] DETAIL:  There are no running
> transactions.
> 
> For example, I could reproduce it with the following steps:
> 
> 1. create the primary and start.
> 2. run "pgbench -i -s 100" on the primary.
> 3. run pg_basebackup to create the standby.
> 4. configure slotsync setup on the standby and start.
> 5. create a publication for all tables on the primary.
> 6. create the subscriber and start.
> 7. run "pgbench -i -Idtpf" on the subscriber.
> 8. create a subscription on the subscriber (initial data copy will start).
> 
> The logical decoding logs were written every 200 ms during the initial data
> synchronization.
> 
> Looking at the new changes for update_local_synced_slot():
...
> We call LogicalSlotAdvanceAndCheckSnapState() if one of confirmed_lsn,
> restart_lsn, and catalog_xmin is different between the remote slot and the local
> slot. In my test case, during the initial sync performing, only catalog_xmin was
> different and there was no serialized snapshot at restart_lsn, and the slotsync
> worker called LogicalSlotAdvanceAndCheckSnapState(). However no slot
> properties were changed even after the function and it set slot_updated = true.
> So it starts the next slot synchronization after 200ms.

I was trying to reproduce this and check why the catalog_xmin is different
among synced slot and remote slot, but I was not able to reproduce the case
where there are lots of logical decoding logs. The script I used is attached.

Would it be possible for you to share the script you used to reproduce this
issue? Alternatively, could you please share the log files from both the
primary and standby servers after reproducing the problem (it would be greatly
helpful if you could set the log level to DEBUG2).

Best Regards,
Hou zj

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).