Horton

Re: Synchronizing slots from primary to standby

Bertrand Drouvot <bertranddrouvot.pg@gmail.com>

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From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
To: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>
Cc: Amit Kapila <amit.kapila16@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>, shveta malik <shveta.malik@gmail.com>, Peter Smith <smithpb2250@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>, 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: 2024-02-12T10:03:20Z
Lists: pgsql-hackers 
Hi,

On Sun, Feb 11, 2024 at 01:23:19PM +0000, Zhijie Hou (Fujitsu) wrote:
> On Saturday, February 10, 2024 9:10 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> > 
> > On Sat, Feb 10, 2024 at 5:31 PM Masahiko Sawada <sawada.mshk@gmail.com>
> > wrote:
> > >
> > > On Fri, Feb 9, 2024 at 4:08 PM Zhijie Hou (Fujitsu)
> > > <houzj.fnst@fujitsu.com> wrote:
> > >
> > > > Another alternative is to register the callback when calling
> > > > slotsync functions and unregister it after the function call. And
> > > > register the callback in
> > > > slotsyncworkmain() for the slotsync worker patch, although this may
> > > > adds a few more codes.
> > >
> > > Another idea is that SyncReplicationSlots() calls synchronize_slots()
> > > in PG_ENSURE_ERROR_CLEANUP() block instead of PG_TRY(), to make sure
> > > to clear the flag in case of ERROR or FATAL. And the slotsync worker
> > > uses the before_shmem_callback to clear the flag.
> > >
> > 
> > +1. This sounds like a better way to clear the flag.
> 
> Agreed. Here is the V84 patch which addressed this.
> 
> Apart from above, I removed the txn start/end codes from 0001 as they are used
> in the slotsync worker patch. And I also ran pgindent and pgperltidy for the
> patch.
> 

Thanks!

A few random comments:

001 ===

"
 For
 the synchronization to work, it is mandatory to have a physical
 replication slot between the primary and the standby,
"

Maybe mention "primary_slot_name" here?

002 ===

+       <para>
+        Synchronize the logical failover slots from the primary server to the standby server.

should we say "logical failover replication slots" instead?

003 ===

+          If, after executing the function,
+          <link linkend="guc-hot-standby-feedback">
+          <varname>hot_standby_feedback</varname></link> is disabled on
+          the standby or the physical slot configured in
+          <link linkend="guc-primary-slot-name">
+          <varname>primary_slot_name</varname></link> is
+          removed,

I think another option that could lead to slot invalidation is if primary_slot_name
is NULL or miss-configured. Indeed hot_standby_feedback would be working
(for the catalog_xmin) but only as long as the standby is up and running.

004 ===

+     on the standby. For the synchronization to work, it is mandatory to
+     have a physical replication slot between the primary and the standby,

should we mention primary_slot_name here?

005 ===

+     To resume logical replication after failover from the synced logical
+     slots, the subscription's 'conninfo' must be altered

Only in a pub/sub context but not for other ways of using the logical replication
slot(s).

006 ===

+       neither be used for logical decoding nor dropped by the user

what about "nor dropped manually"?

007 ===

+typedef struct SlotSyncCtxStruct
+{

Should we remove "Struct" from the struct name?

008 ===

+                       ereport(LOG,
+                                       errmsg("dropped replication slot \"%s\" of dbid %d",
+                                                  NameStr(local_slot->data.name),
+                                                  local_slot->data.database));

We emit a message when an "invalidated" slot is dropped but not when we create
a slot. Shouldn't we emit a message when we create a synced slot on the standby?

I think that could be confusing to see "a drop" message not followed by "a create"
one when it's expected (slot valid on the primary for example).

009 ===

Regarding 040_standby_failover_slots_sync.pl what about adding tests for?

- synced slot invalidation (and ensure it's recreated once pg_sync_replication_slots()
is called and when the slot in primary is valid)
- cannot enable failover for a temporary replication slot
- replication slots can only be synchronized from a standby server

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com

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).