Re: synchronized_standby_slots behavior inconsistent with quorum-based synchronous replication - Mailing list pgsql-hackers

On Fri, Apr 3, 2026 at 2:21 PM shveta malik <shveta.malik@gmail.com> wrote:
>
> On Fri, Apr 3, 2026 at 9:46 AM shveta malik <shveta.malik@gmail.com> wrote:
> >
> > On Thu, Apr 2, 2026 at 3:55 PM Ashutosh Sharma <ashu.coek88@gmail.com> wrote:
> > >
> > > Hi Shveta,
> > >
> > > On Wed, Apr 1, 2026 at 12:06 PM shveta malik <shveta.malik@gmail.com> wrote:
> > > >
> > > > On Thu, Mar 26, 2026 at 5:23 PM Ashutosh Sharma <ashu.coek88@gmail.com> wrote:
> > > > >
> > > > >
> > > > > PFA patch addressing all the comments above and let me know for any
> > > > > further comments.
> > > > >
> > > >
> > > > Thank You Ashutosh. Doc looks good to me. Few comments:
> > > >
> > > > 3)
> > > > What is the execution time for this new test?
> > > > I ran it on my VM (which is slightly on the slower side), and the
> > > > runtime varies between ~60 seconds and ~140 seconds. I executed it
> > > > around 10–15 times. Most runs completed in about 65 seconds (which is
> > > > still more), but a few were significantly longer (100+ seconds).
> > > > During the longer runs, I noticed the following entry in pub.log
> > > > (possibly related to Test Scenario E taking more time?). Could you
> > > > please try running this on your end as well?
> > > >
> > > > 2026-03-31 19:45:45.557 IST client backend[145705]
> > > > 053_synchronized_standby_slots_quorum.pl LOG:  statement: SELECT
> > > > active_pid IS NOT NULL
> > > >   AND restart_lsn IS NOT NULL
> > > >   AND restart_lsn < '0/03000450'::pg_lsn
> > > > FROM pg_replication_slots
> > > > WHERE slot_name = 'sb1_slot';
> > > >
> > > > Just for reference, the complete  failover test
> > > > (t/040_standby_failover_slots_sync.pl) takes somewhere between 7 to
> > > > 10sec on my VM.
> > > >
> > >
> > > My concern with this new test is that it's both slow to run and prone
> > > to flakiness, which makes me question whether it's worth keeping.
> > >
> >
> > will review and share my thoughts.
> >
>
> I gave it more thought, another idea for a shorter and quicker
> testcase could be to check wait_event for that particular
> application_name in pg_stat_activity. A lagging standby will result in
> wait_event=WaitForStandbyConfirmation with backend_type=walsender.
>
> I have attached sample-code to do the same in the attached txt file,
> please have a look. I discussed with Hou-San offline, he is okay with
> this idea. Please see if it works and change it as needed.
>

More than the execution time, I'm concerned if the test-case
effectively validates what we want.

With below setup, here is what I observe:

Setup:

Primary  : psql -p 5555  (synchronous_standby_names = 'ANY 1
(standby1, standby2)'; synchronized_standby_slots = 'FIRST 1
(sb1_slot, sb2_slot)')
Standby1 : psql -p 5556  (wal_receiver_status_interval=0)
Standby2 : psql -p 5557  (wal_receiver_status_interval=10s)

--

Observations:

[local]:5555 ashu@postgres=# SELECT pg_logical_emit_message(true,
'qtest', 'first_1_lagging_blocks_1');
 pg_logical_emit_message
-------------------------
 0/04000220
(1 row)

Time: 14.378 ms

[local]:5555 ashu@postgres=# select slot_name, active_pid, restart_lsn
from pg_replication_slots where slot_type = 'physical';
 slot_name | active_pid | restart_lsn
-----------+------------+-------------
 sb1_slot  |     105328 | 0/04000250
 sb2_slot  |     105381 | 0/04000250
(2 rows)

Time: 1.370 ms

--

[local]:5555 ashu@postgres=# SELECT pg_logical_emit_message(true,
'qtest', 'first_1_lagging_blocks_2');
 pg_logical_emit_message
-------------------------
 0/040002A0
(1 row)

Time: 13.533 ms

[local]:5555 ashu@postgres=# select slot_name, active_pid, restart_lsn
from pg_replication_slots where slot_type = 'physical';
 slot_name | active_pid | restart_lsn
-----------+------------+-------------
 sb1_slot  |     105328 | 0/040002D0
 sb2_slot  |     105381 | 0/040002D0
(2 rows)

--

Takeaways:

1) In both the cases, even though wal_receiver_status_interval = 0 on
standby1, the restart_lsn of the standby1 quickly moved past the lsn
of the logical message emitted which kind of gives sense that
wal_receiver_status_interval = 0 disables periodic status packets, but
receiver/walsender still exchange feedback on other events, so slot
restart_lsn can move quickly.
2) On a fast local setup, both sb1_slot and sb2_slot can advance past
the emitted LSN before we query pg_replication_slots making the
test-case flaky/nondeterministic, it becomes time sensitive.

--
With Regards,
Ashutosh Sharma.