Horton

Re: Synchronizing slots from primary to standby

Peter Smith <smithpb2250@gmail.com>

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From: Peter Smith <smithpb2250@gmail.com>
To: shveta malik <shveta.malik@gmail.com>
Cc: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>, Amit Kapila <amit.kapila16@gmail.com>, "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.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: 2023-11-22T07:41:43Z
Lists: pgsql-hackers 
In addition to my recent v35-0001 comment not yet addressed [1], here
are some review comments for patch v37-0001.

======
src/backend/replication/walsender.c

1. PhysicalWakeupLogicalWalSnd
+/*
+ * Wake up logical walsenders with failover-enabled slots if the physical slot
+ * of the current walsender is specified in standby_slot_names GUC.
+ */
+void
+PhysicalWakeupLogicalWalSnd(void)
+{
+ ListCell   *lc;
+ List    *standby_slots;
+ bool slot_in_list = false;
+
+ Assert(MyReplicationSlot != NULL);
+ Assert(SlotIsPhysical(MyReplicationSlot));
+
+ standby_slots = GetStandbySlotList(false);
+
+ foreach(lc, standby_slots)
+ {
+ char    *name = lfirst(lc);
+
+ if (strcmp(name, NameStr(MyReplicationSlot->data.name)) == 0)
+ {
+ slot_in_list = true;
+ break;
+ }
+ }
+
+ if (slot_in_list)
+ ConditionVariableBroadcast(&WalSndCtl->wal_confirm_rcv_cv);
+}

1a.
Easier to have single assertion -- Assert(MyReplicationSlot &&
SlotIsPhysical(MyReplicationSlot));

~

1b.
Why bother with the 'slot_in_list' and break, when you can just call
the ConditionVariableBroadcast() and return without having the extra
variable?

======
src/test/recovery/t/050_verify_slot_order.pl

~~~

2.
Should you name the global objects with a 'regress_' prefix which
seems to be the standard for other new TAP tests?

~~~

3.
+#
+#            | ----> standby1 (connected via streaming replication)
+# | ----> standby2 (connected via streaming replication)
+# primary ----- |
+#     | ----> subscriber1 (connected via logical replication)
+#     | ----> subscriber2 (connected via logical replication)
+#
+#
+# Set up is configured in such a way that primary never lets subscriber1 ahead
+# of standby1.

3a.
Misaligned "|" in comment?

~

3b.
IMO it would be better to give an overview of how this all works
instead of just saying "configured in such a way".


~~~

4.
+# Configure primary to disallow specified logical replication slot (lsub1_slot)
+# getting ahead of specified physical replication slot (sb1_slot).
+$primary->append_conf(

It is confusing because there is no "lsub1_slot" specified anywhere
until much later. Would you be able to provide some more details?

~~~

5.
+# Create another subscriber node, wait for sync to complete
+my $subscriber2 = PostgreSQL::Test::Cluster->new('subscriber2');
+$subscriber2->init(allows_streaming => 'logical');
+$subscriber2->start;
+$subscriber2->safe_psql('postgres', "CREATE TABLE tab_int (a int
PRIMARY KEY);");
+$subscriber2->safe_psql('postgres',
+ "CREATE SUBSCRIPTION mysub2 CONNECTION '$publisher_connstr' "
+   . "PUBLICATION mypub WITH (slot_name = lsub2_slot);");
+$subscriber2->wait_for_subscription_sync;

Maybe this comment should explicitly say there is no failover enabled
here. Maybe the SUBSCRIPTION should explicitly set failover=false?

~~~

6.
+# The subscription that's up and running and is enabled for failover
+# doesn't get the data from primary and keeps waiting for the
+# standby specified in standby_slot_names.
+$result = $subscriber1->safe_psql('postgres',
+ "SELECT count(*) = 0 FROM tab_int;");
+is($result, 't', "subscriber1 doesn't get data from primary until
standby1 acknowledges changes");

Might it be better to write as "SELECT count(*) = $primary_row_count
FROM tab_int;" and expect it to return false?

======
src/test/regress/expected/subscription.out

7.
Everything here displays the "Failover" state 'd' (disabled). How
about tests for different state values?

======
[1] https://www.postgresql.org/message-id/CAHut%2BPv-yu71ogj_hRi6cCtmD55bsyw7XTxj1Nq8yVFKpY3NDQ%40mail.gmail.com

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).