Horton

Re: Synchronizing slots from primary to standby

Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>

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From: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
To: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>
Cc: Amit Kapila <amit.kapila16@gmail.com>, Bertrand Drouvot <bertranddrouvot.pg@gmail.com>, shveta malik <shveta.malik@gmail.com>, Ajin Cherian <itsajin@gmail.com>, Peter Smith <smithpb2250@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>, Nisha Moond <nisha.moond412@gmail.com>, "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.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: 2024-04-02T12:48:33Z
Lists: pgsql-hackers 
On Tue, Apr 2, 2024 at 2:11 PM Zhijie Hou (Fujitsu)
<houzj.fnst@fujitsu.com> wrote:
>
> CFbot[1] complained about one query result's order in the tap-test, so I am
> attaching a V7 patch set which fixed this. There are no changes in 0001.
>
> [1] https://cirrus-ci.com/task/6375962162495488

Thanks. Here are some comments:

1. Can we just remove pg_logical_replication_slot_advance and use
LogicalSlotAdvanceAndCheckSnapState instead? If worried about the
function naming, LogicalSlotAdvanceAndCheckSnapState can be renamed to
pg_logical_replication_slot_advance?

+ * Advance our logical replication slot forward. See
+ * LogicalSlotAdvanceAndCheckSnapState for details.
  */
 static XLogRecPtr
 pg_logical_replication_slot_advance(XLogRecPtr moveto)
 {

2.
+    if (!ready_for_decoding)
+    {
+        elog(DEBUG1, "could not find consistent point for synced
slot; restart_lsn = %X/%X",
+             LSN_FORMAT_ARGS(slot->data.restart_lsn));

Can we specify the slot name in the message?

3. Also, can the "could not find consistent point for synced slot;
restart_lsn = %X/%X" be emitted at LOG level just like other messages
in update_and_persist_local_synced_slot. Although, I see "XXX should
this be changed to elog(DEBUG1) perhaps?", these messages need to be
at LOG level as they help debug issues if at all they are hit.

4. How about using found_consistent_snapshot instead of
ready_for_decoding? A general understanding is that the synced slots
are not allowed for decoding (although with this fix, we do that for
internal purposes), ready_for_decoding looks a bit misleading.

5. As far as the test case for this issue is concerned, I'm fine with
adding one using an INJECTION point because we seem to be having no
consistent way to control postgres writing current snapshot to WAL.

6. A nit: can we use "fast_forward mode" instead of "fast-forward
mode" just to be consistent?
+     * logical changes unless we are in fast-forward mode where no changes are

7.
+    /*
+     * We need to access the system tables during decoding to build the
+     * logical changes unless we are in fast-forward mode where no changes are
+     * generated.
+     */
+    if (slot->data.database != MyDatabaseId && !fast_forward)

May I know if we need this change for this fix?

-- 
Bharath Rupireddy
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).