Horton

Re: Synchronizing slots from primary to standby

Peter Smith <smithpb2250@gmail.com>

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From: Peter Smith <smithpb2250@gmail.com>
To: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>
Cc: Amit Kapila <amit.kapila16@gmail.com>, shveta malik <shveta.malik@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>, Ajin Cherian <itsajin@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>, Nisha Moond <nisha.moond412@gmail.com>, "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>, Bertrand Drouvot <bertranddrouvot.pg@gmail.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>, Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2024-02-29T02:59:19Z
Lists: pgsql-hackers 
On Wed, Feb 28, 2024 at 1:23 PM Zhijie Hou (Fujitsu)
<houzj.fnst@fujitsu.com> wrote:
>
> On Tuesday, February 27, 2024 3:18 PM Peter Smith <smithpb2250@gmail.com> wrote:
...
> > 20.
> > +#
> > +# | ----> standby1 (primary_slot_name = sb1_slot) # | ----> standby2
> > +(primary_slot_name = sb2_slot) # primary ----- | # | ----> subscriber1
> > +(failover = true) # | ----> subscriber2 (failover = false)
> >
> > In the diagram, the "--->" means a mixture of physical standbys and logical
> > pub/sub replication. Maybe it can be a bit clearer?
> >
> > SUGGESTION
> >
> > # primary (publisher)
> > #
> > #     (physical standbys)
> > #     | ----> standby1 (primary_slot_name = sb1_slot)
> > #     | ----> standby2 (primary_slot_name = sb2_slot)
> > #
> > #     (logical replication)
> > #     | ----> subscriber1 (failover = true, slot_name = lsub1_slot)
> > #     | ----> subscriber2 (failover = false, slot_name = lsub2_slot)
> >
>
> I think one can distinguish it based on the 'standby' and 'subscriber' as well, because
> 'standby' normally refer to physical standby while the other refer to logical.
>

Ok, but shouldn't it at least include info about the logical slot
names associated with the subscribers (slot_name = lsub1_slot,
slot_name = lsub2_slot) like suggested above?

======

Here are some more review comments for v100-0001

======
doc/src/sgml/config.sgml

1.
+       <para>
+        Lists the streaming replication standby server slot names that logical
+        WAL sender processes will wait for. Logical WAL sender processes will
+        send decoded changes to plugins only after the specified replication
+        slots confirm receiving WAL. This guarantees that logical replication
+        slots with failover enabled do not consume changes until those changes
+        are received and flushed to corresponding physical standbys. If a
+        logical replication connection is meant to switch to a physical standby
+        after the standby is promoted, the physical replication slot for the
+        standby should be listed here. Note that logical replication will not
+        proceed if the slots specified in the standby_slot_names do
not exist or
+        are invalidated.
+       </para>

Is that note ("Note that logical replication will not proceed if the
slots specified in the standby_slot_names do not exist or are
invalidated") meant only for subscriptions marked for 'failover' or
any subscription? Maybe wording can be modified to help clarify it?

======
src/backend/replication/slot.c

2.
+/*
+ * A helper function to validate slots specified in GUC standby_slot_names.
+ */
+static bool
+validate_standby_slots(char **newval)
+{
+ char    *rawname;
+ List    *elemlist;
+ bool ok;
+
+ /* Need a modifiable copy of string */
+ rawname = pstrdup(*newval);
+
+ /* Verify syntax and parse string into a list of identifiers */
+ ok = SplitIdentifierString(rawname, ',', &elemlist);
+
+ if (!ok)
+ {
+ GUC_check_errdetail("List syntax is invalid.");
+ }
+
+ /*
+ * If the replication slots' data have been initialized, verify if the
+ * specified slots exist and are logical slots.
+ */
+ else if (ReplicationSlotCtl)
+ {
+ foreach_ptr(char, name, elemlist)
+ {
+ ReplicationSlot *slot;
+
+ slot = SearchNamedReplicationSlot(name, true);
+
+ if (!slot)
+ {
+ GUC_check_errdetail("replication slot \"%s\" does not exist",
+ name);
+ ok = false;
+ break;
+ }
+
+ if (!SlotIsPhysical(slot))
+ {
+ GUC_check_errdetail("\"%s\" is not a physical replication slot",
+ name);
+ ok = false;
+ break;
+ }
+ }
+ }
+
+ pfree(rawname);
+ list_free(elemlist);
+ return ok;
+}

2a.
I didn't mention this previously because I thought this function was
not going to change anymore, but since Bertrand suggested some changes
[1], I will say IMO the { } are fine here for the single statement,
but I think it will be better to rearrange this code to be like below.
Having a 2nd NOP 'else' gives a much better place where you can put
your ReplicationSlotCtl comment.

if (!ok)
{
  GUC_check_errdetail("List syntax is invalid.");
}
else if (!ReplicationSlotCtl)
{
  <Insert big comment here about why it is OK to skip when
ReplicationSlotCtl is NULL>
}
else
{
  foreach_ptr ...
}

~

2b.
In any case even if don't refactor anything I still think you need to
extend that comment to explain how skipping ReplicationSlotCtl is NULL
is only OK because there will be some later checking also done in the
FilterStandbySlots() function to catch any config problems.

------
[1] https://www.postgresql.org/message-id/Zd8ahZXw82ieFxX/%40ip-10-97-1-34.eu-west-3.compute.internal

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).