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: shveta malik <shveta.malik@gmail.com>
Cc: Peter Smith <smithpb2250@gmail.com>, "Drouvot, Bertrand" <bertranddrouvot.pg@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: 2023-10-18T10:54:10Z
Lists: pgsql-hackers 
On Tue, Oct 17, 2023 at 2:01 PM shveta malik <shveta.malik@gmail.com> wrote:
>
> On Tue, Oct 17, 2023 at 12:44 PM Peter Smith <smithpb2250@gmail.com> wrote:
> >
> > FYI - the latest patch failed to apply.
> >
> > [postgres@CentOS7-x64 oss_postgres_misc]$ git apply
> > ../patches_misc/v24-0001-Allow-logical-walsenders-to-wait-for-the-physica.patch
> > error: patch failed: src/include/utils/guc_hooks.h:160
> > error: src/include/utils/guc_hooks.h: patch does not apply
>
> Rebased v24. PFA.
>

Few comments:
==============
1.
+        List of physical replication slots that logical replication
with failover
+        enabled waits for.

/logical replication/logical replication slots

2.
 If
+        <varname>enable_syncslot</varname> is not enabled on the
+        corresponding standbys, then it may result in indefinite waiting
+        on the primary for physical replication slots configured in
+        <varname>standby_slot_names</varname>
+       </para>

Why the above leads to indefinite wait? I think we should just ignore
standby_slot_names and probably LOG a message in the server for the
same.

3.
+++ b/src/backend/replication/logical/tablesync.c
@@ -1412,7 +1412,8 @@ LogicalRepSyncTableStart(XLogRecPtr *origin_startpos)
  */
  walrcv_create_slot(LogRepWorkerWalRcvConn,
     slotname, false /* permanent */ , false /* two_phase */ ,
-    CRS_USE_SNAPSHOT, origin_startpos);
+    false /* enable_failover */ , CRS_USE_SNAPSHOT,
+    origin_startpos);

As per this code, we won't enable failover for tablesync slots. So,
what happens if we need to failover to new node after the tablesync
worker has reached SUBREL_STATE_FINISHEDCOPY or SUBREL_STATE_DATASYNC?
I think we won't be able to continue replication from failed over
node. If this theory is correct, we have two options (a) enable
failover for sync slots as well, if it is enabled for main slot; but
then after we drop the slot on primary once sync is complete, same
needs to be taken care at standby. (b) enable failover even for the
main slot after all tables are in ready state, something similar to
what we do for two_phase.

4.
+ /* Verify syntax */
+ if (!validate_slot_names(newval, &elemlist))
+ return false;
+
+ /* Now verify if these really exist and have correct type */
+ if (!validate_standby_slots(elemlist))

These two functions serve quite similar functionality which makes
their naming quite confusing. Can we directly move the functionality
of validate_slot_names() into validate_standby_slots()?

5.
+SlotSyncInitConfig(void)
+{
+ char    *rawname;
+
+ /* Free the old one */
+ list_free(standby_slot_names_list);
+ standby_slot_names_list = NIL;
+
+ if (strcmp(standby_slot_names, "") != 0)
+ {
+ rawname = pstrdup(standby_slot_names);
+ SplitIdentifierString(rawname, ',', &standby_slot_names_list);

How does this handle the case where '*' is specified for standby_slot_names?


-- 
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).