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: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>, "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>, 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: 2024-04-04T08:24:58Z
Lists: pgsql-hackers 
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:

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():

    if (remote_slot->confirmed_lsn != slot->data.confirmed_flush ||
        remote_slot->restart_lsn != slot->data.restart_lsn ||
        remote_slot->catalog_xmin != slot->data.catalog_xmin)
    {
        /*
         * We can't directly copy the remote slot's LSN or xmin unless there
         * exists a consistent snapshot at that point. Otherwise, after
         * promotion, the slots may not reach a consistent point before the
         * confirmed_flush_lsn which can lead to a data loss. To avoid data
         * loss, we let slot machinery advance the slot which ensures that
         * snapbuilder/slot statuses are updated properly.
         */
        if (SnapBuildSnapshotExists(remote_slot->restart_lsn))
        {
            /*
             * Update the slot info directly if there is a serialized snapshot
             * at the restart_lsn, as the slot can quickly reach consistency
             * at restart_lsn by restoring the snapshot.
             */
            SpinLockAcquire(&slot->mutex);
            slot->data.restart_lsn = remote_slot->restart_lsn;
            slot->data.confirmed_flush = remote_slot->confirmed_lsn;
            slot->data.catalog_xmin = remote_slot->catalog_xmin;
            slot->effective_catalog_xmin = remote_slot->catalog_xmin;
            SpinLockRelease(&slot->mutex);

            if (found_consistent_snapshot)
                *found_consistent_snapshot = true;
        }
        else
        {
            LogicalSlotAdvanceAndCheckSnapState(remote_slot->confirmed_lsn,
                                                found_consistent_snapshot);
        }

        ReplicationSlotsComputeRequiredXmin(false);
        ReplicationSlotsComputeRequiredLSN();

        slot_updated = true;

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.

It seems to me that we can skip calling
LogicalSlotAdvanceAndCheckSnapState() at least when the remote and
local have the same restart_lsn and confirmed_lsn.

I'm not sure there are other scenarios but is it worth fixing this symptom?

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).