Horton

Re: Synchronizing slots from primary to standby

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>
Cc: shveta malik <shveta.malik@gmail.com>, Peter Smith <smithpb2250@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: 2023-11-10T05:41:24Z
Lists: pgsql-hackers 
On Thu, Nov 9, 2023 at 7:29 PM Drouvot, Bertrand
<bertranddrouvot.pg@gmail.com> wrote:
>
> On 11/9/23 3:41 AM, Amit Kapila wrote:
> > On Wed, Nov 8, 2023 at 8:09 PM Drouvot, Bertrand
> > <bertranddrouvot.pg@gmail.com> wrote:
> >>
> >>> Unrelated to above, if there is a user slot on standby with the same
> >>> name which the slot-sync worker is trying to create, then shall it
> >>> emit a warning and skip the sync of that slot or shall it throw an
> >>> error?
> >>>
> >>
> >> I'd vote for emit a warning and move on to the next slot if any.
> >>
> >
> > But then it could take time for users to know the actual problem and
> > they probably notice it after failover.
>
> Right, that's not appealing....
>
> OTOH the slot has already been created manually on the standby so there is
> probably already a "use case" for it (that is probably unrelated to the
> failover story then).
>
> In V32, the following states have been introduced:
>
> "
> 'n': none for user slots,
> 'i': sync initiated for the slot but waiting for primary to catch up.
> 'r': ready for periodic syncs.
> "
>
> Should we introduce a new state that indicates that a sync slot creation
> has failed because the slot already existed? That would probably
> be simple to monitor instead of looking at the log file.
>

Are you saying that we change the state of the already existing slot
on standby? And, such a state would indicate that we are trying to
sync the slot with the same name from the primary. Is that what you
have in mind? If so, it appears quite odd to me to have such a state
and also set it in some unrelated slot that just has the same name.

I understand your point that we can allow other slots to proceed but
it is also important to not create any sort of inconsistency that can
surprise user after failover. Also, the current coding doesn't ensure
we will always give WARNING. If we see the below code that deals with
this WARNING,

+  /* User created slot with the same name exists, emit WARNING. */
+  else if (found && s->data.sync_state == SYNCSLOT_STATE_NONE)
+  {
+    ereport(WARNING,
+        errmsg("not synchronizing slot %s; it is a user created slot",
+             remote_slot->name));
+  }
+  /* Otherwise create the slot first. */
+  else
+  {
+    TransactionId xmin_horizon = InvalidTransactionId;
+    ReplicationSlot *slot;
+
+    ReplicationSlotCreate(remote_slot->name, true, RS_EPHEMERAL,
+                remote_slot->two_phase, false);

I think this is not a solid check to ensure that the slot existed
before. Because it could be created as soon as the slot sync worker
invokes ReplicationSlotCreate() here. So, depending on the timing, we
can either get an ERROR or WARNING. I feel giving an ERROR in this
case should be okay.

-- 
With Regards,
Amit Kapila.

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).