Horton

Re: Synchronizing slots from primary to standby

Masahiko Sawada <sawada.mshk@gmail.com>

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From: Masahiko Sawada <sawada.mshk@gmail.com>
To: shveta malik <shveta.malik@gmail.com>
Cc: Ajin Cherian <itsajin@gmail.com>, "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, Amit Kapila <amit.kapila16@gmail.com>, Peter Smith <smithpb2250@gmail.com>, Bertrand Drouvot <bertranddrouvot.pg@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: 2024-02-05T14:25:17Z
Lists: pgsql-hackers 
On Mon, Feb 5, 2024 at 8:26 PM shveta malik <shveta.malik@gmail.com> wrote:
>
> On Mon, Feb 5, 2024 at 10:57 AM Ajin Cherian <itsajin@gmail.com> wrote:
> >
> > Just noticed that doc/src/sgml/config.sgml still refers to enable_synclot instead of sync_replication_slots:
> >
> > The standbys corresponding to the physical replication slots in
> > <varname>standby_slot_names</varname> must configure
> > <literal>enable_syncslot = true</literal> so they can receive
> >  failover logical slots changes from the primary.
>
> Thanks Ajin for pointing this out. Here are v78 patches, corrected there.
>
> Other changes are:
>
> 1)  Rebased the patches as the v77-001 is now pushed.
> 2)  Enabled executing pg_sync_replication_slots() on cascading-standby.
> 3)  Rearranged the code around parameter validity checks. Changed
> function names and changed the way how dbname is extracted as
> suggested by Amit offlist.
> 4)  Rearranged the code around check_primary_info(). Removed output args.
> 5)  Few other trivial changes.
>

Thank you for updating the patch! Here are some comments:

---
Since Two processes (e.g. the slotsync worker and
pg_sync_replication_slots()) concurrently fetch and update the slot
information, there is a race condition where slot's
confirmed_flush_lsn goes backward. . We have the following check but
it doesn't prevent the slot's confirmed_flush_lsn from moving backward
if the restart_lsn does't change:

            /*
             * Sanity check: As long as the invalidations are handled
             * appropriately as above, this should never happen.
             */
            if (remote_slot->restart_lsn < slot->data.restart_lsn)
                elog(ERROR,
                     "cannot synchronize 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));

---
+     It is recommended that subscriptions are first disabled before promoting
f+     the standby and are enabled back after altering the connection string.

I think it's better to describe the reason why it's recommended to
disable subscriptions before the standby promotion.

---
+/* Slot sync worker objects */
+extern PGDLLIMPORT char *PrimaryConnInfo;
+extern PGDLLIMPORT char *PrimarySlotName;

These two variables are declared also in xlogrecovery.h. Is it
intentional? If so, I think it's better to write comments.

---
Global functions and variables used by the slotsync worker are
declared in logicalworker.h and worker_internal.h. But is it really
okay to make a dependency between the slotsync worker and logical
replication workers? IIUC the slotsync worker is conceptually a
separate feature from the logical replication. I think the slotsync
worker can have its own header file.

---
+               SELECT r.srsubid AS subid, CONCAT('pg_' || srsubid ||
'_sync_' || srrelid || '_' || ctl.system_identifier) AS slotname

and

+               SELECT (CASE WHEN r.srsubstate = 'f' THEN
pg_replication_origin_progress(CONCAT('pg_' || r.srsubid || '_' ||
r.srrelid), false)

If we use CONCAT function, we can replace '||' with ','.

---
+     Confirm that the standby server is not lagging behind the subscribers.
+     This step can be skipped if
+     <link linkend="guc-standby-slot-names"><varname>standby_slot_names</varname></link>
+     has been correctly configured.

How can the user confirm if standby_slot_names is correctly configured?

Regards,

-- 
Masahiko Sawada
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).