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: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>
Cc: shveta malik <shveta.malik@gmail.com>, "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>, Peter Smith <smithpb2250@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: 2023-11-03T11:31:43Z
Lists: pgsql-hackers 
On Thu, Nov 2, 2023 at 2:35 PM Zhijie Hou (Fujitsu)
<houzj.fnst@fujitsu.com> wrote:
>
> Here is the new version patch set(V29) which addressed Peter comments[1][2] and
> fixed one doc compile error.
>

Few comments:
==============
1.
+       <varlistentry id="sql-createsubscription-params-with-failover">
+        <term><literal>failover</literal> (<type>boolean</type>)</term>
+        <listitem>
+         <para>
+          Specifies whether the replication slot assocaited with the
subscription
+          is enabled to be synced to the physical standbys so that logical
+          replication can be resumed from the new primary after failover.
+          The default is <literal>true</literal>.

Why do you think it is a good idea to keep the default value as true?
I think the user needs to enable standby for syncing slots which is
not a default feature, so by default, the failover property should
also be false. AFAICS, it is false for create_slot SQL API as per the
below change; so that way also keeping default true for a subscription
doesn't make sense.
@@ -479,6 +479,7 @@ CREATE OR REPLACE FUNCTION
pg_create_logical_replication_slot(
     IN slot_name name, IN plugin name,
     IN temporary boolean DEFAULT false,
     IN twophase boolean DEFAULT false,
+    IN failover boolean DEFAULT false,
     OUT slot_name name, OUT lsn pg_lsn)

BTW, the below change indicates that the code treats default as false;
so, it seems to be a documentation error.
@@ -157,6 +158,8 @@ parse_subscription_options(ParseState *pstate,
List *stmt_options,
  opts->runasowner = false;
  if (IsSet(supported_opts, SUBOPT_ORIGIN))
  opts->origin = pstrdup(LOGICALREP_ORIGIN_ANY);
+ if (IsSet(supported_opts, SUBOPT_FAILOVER))
+ opts->failover = false;

2.
-
 /*
  * Common option parsing function for CREATE and ALTER SUBSCRIPTION commands.
  *

Spurious line removal.

3.
+ else if (opts.slot_name && failover_enabled)
+ {
+ walrcv_alter_slot(wrconn, opts.slot_name, opts.failover);
+ ereport(NOTICE,
+ (errmsg("altered replication slot \"%s\" on publisher",
+ opts.slot_name)));
+ }

I think we can add a comment to describe why it makes sense to enable
the failover property of the slot in this case. Can we change the
notice message to: "enabled failover for replication slot \"%s\" on
publisher"

4.
 libpqrcv_create_slot(WalReceiverConn *conn, const char *slotname,
- bool temporary, bool two_phase, CRSSnapshotAction snapshot_action,
- XLogRecPtr *lsn)
+ bool temporary, bool two_phase, bool failover,
+ CRSSnapshotAction snapshot_action, XLogRecPtr *lsn)
 {
  PGresult   *res;
  StringInfoData cmd;
@@ -913,7 +917,14 @@ libpqrcv_create_slot(WalReceiverConn *conn, const
char *slotname,
  else
  appendStringInfoChar(&cmd, ' ');
  }
-
+ if (failover)
+ {
+ appendStringInfoString(&cmd, "FAILOVER");
+ if (use_new_options_syntax)
+ appendStringInfoString(&cmd, ", ");
+ else
+ appendStringInfoChar(&cmd, ' ');
+ }

I don't see a corresponding change in repl_gram.y. I think the
following part of the code needs to be changed:
/* CREATE_REPLICATION_SLOT slot [TEMPORARY] LOGICAL plugin [options] */
| K_CREATE_REPLICATION_SLOT IDENT opt_temporary K_LOGICAL IDENT
create_slot_options

You also need to update the docs for the same. See [1].

5.
@@ -228,6 +230,28 @@ pg_logical_slot_get_changes_guts(FunctionCallInfo
fcinfo, bool confirm, bool bin
  NameStr(MyReplicationSlot->data.plugin),
  format_procedure(fcinfo->flinfo->fn_oid))));
..
+ if (XLogRecPtrIsInvalid(upto_lsn))
+ wal_to_wait = end_of_wal;
+ else
+ wal_to_wait = Min(upto_lsn, end_of_wal);
+
+ /* Initialize standby_slot_names_list */
+ SlotSyncInitConfig();
+
+ /*
+ * Wait for specified streaming replication standby servers (if any)
+ * to confirm receipt of WAL upto wal_to_wait.
+ */
+ WalSndWaitForStandbyConfirmation(wal_to_wait);
+
+ /*
+ * The memory context used to allocate standby_slot_names_list will be
+ * freed at the end of this call. So free and nullify the list in
+ * order to avoid usage of freed list in the next call to this
+ * function.
+ */
+ SlotSyncFreeConfig();

What if there is an error in WalSndWaitForStandbyConfirmation() before
calling SlotSyncFreeConfig()? I think the problem you are trying to
avoid by freeing it here can occur. I think it is better to do this in
a logical decoding context and free the list along with it as we are
doing in commit c7256e6564(see PG15). Also, it is better to allocate
this list somewhere in WalSndWaitForStandbyConfirmation(), probably in
WalSndGetStandbySlots, that will make the code look neat and also
avoid allocating this list when failover is not enabled for the slot.

6.
+/* ALTER_REPLICATION_SLOT slot */
+alter_replication_slot:
+ K_ALTER_REPLICATION_SLOT IDENT '(' generic_option_list ')'

I think you need to update the docs for this new command. See existing docs [1].

[1] - https://www.postgresql.org/docs/devel/protocol-replication.html

-- 
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).