Horton

Re: Synchronizing slots from primary to standby

Peter Smith <smithpb2250@gmail.com>

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From: Peter Smith <smithpb2250@gmail.com>
To: shveta malik <shveta.malik@gmail.com>
Cc: Amit Kapila <amit.kapila16@gmail.com>, "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>, Nisha Moond <nisha.moond412@gmail.com>, "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.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: 2023-12-20T06:31:38Z
Lists: pgsql-hackers 
Here are some comments for the patch v50-0002.

======
GENERAL

(I made a short study of all the ereports in this patch -- here are
some findings)

~~~

0.1 Don't need the parentheses.

Checking all the ereports I see that half of them have the redundant
parentheses and half of them do not; You might as well make them all
use the new style where the extra parentheses are not needed.

e.g.
+ ereport(LOG,
+ (errmsg("skipping slot synchronization"),
+ errdetail("enable_syncslot is disabled.")));

e.g.
+ ereport(ERROR,
+ (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+ errmsg("cannot drop replication slot \"%s\"", name),
+ errdetail("This slot is being synced from the primary server.")));

and many more like this. Search for all the ereports.

~~~

0.2
+ ereport(LOG,
+ (errmsg("dropped replication slot \"%s\" of dbid %d as it "
+ "was not sync-ready", NameStr(s->data.name),
+ s->data.database)));

I felt maybe that could be:

errmsg("dropped replication slot \"%s\" of dbid %d", ...
errdetail("It was not sync-ready.")

(now this shares the same errmsg with another ereport)

~~~

0.3.
+ ereport(ERROR,
+ (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+ errmsg("skipping sync of slot \"%s\" as it is a user created"
+ " slot", remote_slot->name),
+ errdetail("This slot has failover enabled on the primary and"
+    " thus is sync candidate but user created slot with"
+    " the same name already exists on the standby.")));

This seemed too wordy. Can't it be shortened (maybe like below)
without losing any of the vital information?

errmsg("skipping sync of slot \"%s\"", ...)
errdetail("A user-created slot with the same name already exists on
the standby.")

~~~

0.4
+ ereport(ERROR,
+ (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+ errmsg("exiting from slot synchronization due to bad configuration"),
+ /* translator: second %s is a GUC variable name */
+ errdetail("The primary slot \"%s\" specified by %s is not valid.",
+    PrimarySlotName, "primary_slot_name")));

/The primary slot/The primary server slot/

~~~

0.5
+ ereport(ERROR,
+ (errmsg("could not fetch primary_slot_name \"%s\" info from the "
+ "primary: %s", PrimarySlotName, res->err)));

/primary:/primary server:/

~~~

0.6
The continuations for long lines are inconsistent. Sometimes there are
trailing spaces and sometimes there are leading spaces. And sometimes
there are both at the same time which would cause double-spacing in
the message! Please make them all the same. I think using leading
spaces is easier but YMMV.

e.g.
+ elog(ERROR,
+ "not synchronizing local slot \"%s\" LSN(%X/%X)"
+ " to remote slot's LSN(%X/%X) as synchronization "
+ " would move it backwards", remote_slot->name,
+ LSN_FORMAT_ARGS(slot->data.restart_lsn),
+ LSN_FORMAT_ARGS(remote_slot->restart_lsn));

======
src/backend/replication/logical/slotsync.c

1. check_primary_info

+ /* No need to check further, return that we are cascading standby */
+ if (remote_in_recovery)
+ {
+ *am_cascading_standby = true;
+ ExecClearTuple(tupslot);
+ walrcv_clear_result(res);
+ CommitTransactionCommand();
+ return;
+ }
+
+ valid = DatumGetBool(slot_getattr(tupslot, 2, &isnull));
+ Assert(!isnull);
+
+ if (!valid)
+ ereport(ERROR,
+ (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+ errmsg("exiting from slot synchronization due to bad configuration"),
+ /* translator: second %s is a GUC variable name */
+ errdetail("The primary slot \"%s\" specified by %s is not valid.",
+    PrimarySlotName, "primary_slot_name")));
+ ExecClearTuple(tupslot);
+ walrcv_clear_result(res);
+ CommitTransactionCommand();
+}

Now that there is a common cleanup/return code this function be
reduced further like below:

SUGGESTION

if (remote_in_recovery)
{
  /* No need to check further, return that we are cascading standby */
  *am_cascading_standby = true;
}
else
{
  /* We are a normal standby. */

  valid = DatumGetBool(slot_getattr(tupslot, 2, &isnull));
  Assert(!isnull);

  if (!valid)
    ...
}

ExecClearTuple(tupslot);
walrcv_clear_result(res);
CommitTransactionCommand();
}

~~~

2. ReplSlotSyncWorkerMain

+ /*
+ * One can promote the standby and we can no longer be a cascading
+ * standby. So recheck here.
+ */
+ if (am_cascading_standby)
+ check_primary_info(wrconn, &am_cascading_standby);

Minor rewording of that new comment.

SUGGESTION
If the standby was promoted then what was previously a cascading
standby might no longer be one, so recheck each time.

======
src/test/recovery/t/050_verify_slot_order.pl

3.
+##################################################
+# Test that a synchronized slot can not be decoded, altered and
dropped by the user
+##################################################

/and dropped/or dropped/

~~~

4.
+
+($result, $stdout, $stderr) = $standby1->psql(
+    'postgres',
+    qq[ALTER_REPLICATION_SLOT lsub1_slot (failover);],
+    replication => 'database');
+ok($stderr =~ /ERROR:  cannot alter replication slot "lsub1_slot"/,
+ "synced slot on standby cannot be altered");
+

Add a comment for this test  part

SUGGESTION
Attempting to alter a synced slot should result in an error

~~~

5.
IMO it would be better if the tests were done in the same order
mentioned in the comment. So either change the tests or change the
comment.

======
Kind Regards,
Peter Smith.
Fujitsu Australia

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).