Horton

RE: Synchronizing slots from primary to standby

Hayato Kuroda (Fujitsu) <kuroda.hayato@fujitsu.com>

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From: "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>
To: 'shveta malik' <shveta.malik@gmail.com>, Amit Kapila <amit.kapila16@gmail.com>
Cc: Peter Smith <smithpb2250@gmail.com>, "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>, Nisha Moond <nisha.moond412@gmail.com>, "Zhijie Hou (Fujitsu)" <houzj.fnst@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: 2023-12-19T13:05:17Z
Lists: pgsql-hackers 
Dear Shveta,

I resumed to review the patch. I will play more about it, but I can post some
cosmetic comments.

====
walsender.c

01. WalSndWaitForStandbyConfirmation

```
+        sleeptime = WalSndComputeSleeptime(GetCurrentTimestamp());
```

It works well, but I'm not sure whether we should use WalSndComputeSleeptime()
because the function won't be called by walsender.

02.WalSndWaitForStandbyConfirmation

```
+        ConditionVariableTimedSleep(&WalSndCtl->wal_confirm_rcv_cv, sleeptime,
+                                    WAIT_EVENT_WAL_SENDER_WAIT_FOR_STANDBY_CONFIRMATION)
```

Hmm, is it OK to use the same event as WalSndWaitForWal()? IIUC it should be avoided.

03. WalSndShmemInit()

```
+
+        ConditionVariableInit(&WalSndCtl->wal_confirm_rcv_cv);
```

Unnecessary blank?

~~~~~
050_standby_failover_slots_sync.pl

04. General

My pgperltidy modified your test. Please check.

05.

```
# Create publication on the primary
```

Missing "a" before publication?

06.

```
$subscriber1->init(allows_streaming => 'logical');
...
$subscriber2->init(allows_streaming => 'logical');
```

IIUC, these settings are not needed.

07.

```
my $primary_insert_time = time();
```

The variable is not used.

08.

```
# Stop the standby associated with the specified physical replication slot so
# that the logical replication slot won't receive changes until the standby
# slot's restart_lsn is advanced or the slot is removed from the
# standby_slot_names list
```

Missing comma?

09.

```
$back_q->query_until(qr//,
	"SELECT pg_logical_slot_get_changes('test_slot', NULL, NULL);\n");
```

Not sure, should we have to close the back_q connection?

10.

```
# Remove the standby from the standby_slot_names list and reload the
# configuration
$primary->adjust_conf('postgresql.conf', 'standby_slot_names', "''");
$primary->psql('postgres', "SELECT pg_reload_conf()");
```

a.
Missing comma?

b.
I counted and reload function in perl (e.g., `$primary->reload;`) is more often to
be used. Do you have a reason to use pg_reload_conf()?

11.

```
# Now that the standby lsn has advanced, the primary must send the decoded
# changes to the subscription.
$publisher->wait_for_catchup('regress_mysub1');
```

Is the comment correct? I think primary sends data because the GUC is modified.

12.

```
# Put the standby back on the primary_slot_name for the rest of the tests
$primary->adjust_conf('postgresql.conf', 'standby_slot_names', 'sb1_slot');
$primary->restart();
```

Just to confirm - you used restart() here because we must ensure the GUC change is
propagated to all backends, right?

~~~~~
wait_event_names.txt

13.

```
+WAL_SENDER_WAIT_FOR_STANDBY_CONFIRMATION    "Waiting for the WAL to be received by physical standby in WAL sender process."
```

But there is a possibility that backend processes may wait with the event, right?

Best Regards,
Hayato Kuroda
FUJITSU LIMITED

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).