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: shveta malik <shveta.malik@gmail.com>
Cc: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>, Amit Kapila <amit.kapila16@gmail.com>, "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, Dilip Kumar <dilipbalaut@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>
Date: 2024-01-19T05:05:06Z
Lists: pgsql-hackers 
On Wed, Jan 17, 2024 at 7:30 PM shveta malik <shveta.malik@gmail.com> wrote:
>
> On Wed, Jan 17, 2024 at 3:08 PM Bertrand Drouvot
> <bertranddrouvot.pg@gmail.com> wrote:
> >
> > Hi,
> >
> > On Tue, Jan 16, 2024 at 05:27:05PM +0530, shveta malik wrote:
> > > PFA v62. Details:
> >
> > Thanks!
> >
> > > v62-003:
> > > It is a new patch which attempts to implement slot-sync worker as a
> > > special process which is neither a bgworker nor an Auxiliary process.
> > > Here we get the benefit of converting enable_syncslot to a PGC_SIGHUP
> > > Guc rather than PGC_POSTMASTER. We launch the slot-sync worker only if
> > > it is hot-standby and 'enable_syncslot' is ON.
> >
> > The implementation looks reasonable to me (from what I can see some parts is
> > copy/paste from an already existing "special" process and some parts are
> > "sync slot" specific) which makes fully sense.
> >
> > A few remarks:
> >
> > 1 ===
> > +                * Was it the slot sycn worker?
> >
> > Typo: sycn
> >
> > 2 ===
> > +                * ones), and no walwriter, autovac launcher or bgwriter or slot sync
> >
> > Instead? "* ones), and no walwriter, autovac launcher, bgwriter or slot sync"
> >
> > 3 ===
> > + * restarting slot slyc worker. If stopSignaled is set, the worker will
> >
> > Typo: slyc
> >
> > 4 ===
> > +/* Flag to tell if we are in an slot sync worker process */
> >
> > s/an/a/ ?
> >
> > 5 === (coming from v62-0002)
> > +       Assert(tuplestore_tuple_count(res->tuplestore) == 1);
> >
> > Is it even possible for the related query to not return only one row? (I think the
> > "count" ensures it).
> >
> > 6 ===
> >         if (conninfo_changed ||
> >                 primary_slotname_changed ||
> > +               old_enable_syncslot != enable_syncslot ||
> >                 (old_hot_standby_feedback != hot_standby_feedback))
> >         {
> >                 ereport(LOG,
> >                                 errmsg("slot sync worker will restart because of"
> >                                            " a parameter change"));
> >
> > I don't think "slot sync worker will restart" is true if one change enable_syncslot
> > from on to off.
> >
> > IMHO, v62-003 is in good shape and could be merged in v62-002 (that would ease
> > the review). But let's wait to see if others think differently.
> >
> > Regards,
> >
> > --
> > Bertrand Drouvot
> > PostgreSQL Contributors Team
> > RDS Open Source Databases
> > Amazon Web Services: https://aws.amazon.com
>
>
> PFA v63.
>
> --It addresses comments by Peter given in [1], [2], comment by Nisha
> given in [3], comments by Bertrand given in [4]
> --It also moves race-condition fix from patch003 to patch002 as
> suggested by Swada-san offlist. Race-condition is mentioned in [5]
>

Thank you for updating the patch. I have some comments:

---
+        latestWalEnd = GetWalRcvLatestWalEnd();
+        if (remote_slot->confirmed_lsn > latestWalEnd)
+        {
+                elog(ERROR, "exiting from slot synchronization as the
received slot sync"
+                         " LSN %X/%X for slot \"%s\" is ahead of the
standby position %X/%X",
+                         LSN_FORMAT_ARGS(remote_slot->confirmed_lsn),
+                         remote_slot->name,
+                         LSN_FORMAT_ARGS(latestWalEnd));
+        }

IIUC GetWalRcvLatestWalEnd () returns walrcv->latestWalEnd, which is
typically the primary server's flush position and doesn't mean the LSN
where the walreceiver received/flushed up to. Does it really happen
that the slot's confirmed_flush_lsn is higher than the primary's flush
lsn?

---
After dropping a database on the primary, I got the following LOG (PID
2978463 is the slotsync worker on the standby):

LOG:  still waiting for backend with PID 2978463 to accept ProcSignalBarrier
CONTEXT:  WAL redo at 0/301CE00 for Database/DROP: dir 1663/16384

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).