Horton

Re: Synchronizing slots from primary to standby

Ajin Cherian <itsajin@gmail.com>

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From: Ajin Cherian <itsajin@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>, 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>, Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2023-11-23T10:06:03Z
Lists: pgsql-hackers 
On Tue, Nov 21, 2023 at 8:32 PM shveta malik <shveta.malik@gmail.com> wrote:
>
> v37 fails to apply to HEAD due to a recent commit e83aa9f92fdd,
> rebased the patches.  PFA v37_2 patches.

Thanks for the patch. Some comments:
subscriptioncmds.c:
CreateSubscription()
and tablesync.c:
process_syncing_tables_for_apply()
                 walrcv_create_slot(wrconn, opts.slot_name, false,
twophase_enabled,
-                                   CRS_NOEXPORT_SNAPSHOT, NULL);
-
-                if (twophase_enabled)
-                    UpdateTwoPhaseState(subid,
LOGICALREP_TWOPHASE_STATE_ENABLED);
-
+                                   failover_enabled,
CRS_NOEXPORT_SNAPSHOT, NULL);

either here or in libpqrcv_create_slot(), shouldn't you check the
remote server version if it supports the failover flag?


+
+            /*
+             * If only the slot_name is specified, it is possible
that the user intends to
+             * use an existing slot on the publisher, so here we
enable failover for the
+             * slot if requested.
+             */
+            else if (opts.slot_name && failover_enabled)
+            {
+                walrcv_alter_slot(wrconn, opts.slot_name, opts.failover);
+                ereport(NOTICE,
+                        (errmsg("enabled failover for replication
slot \"%s\" on publisher",
+                                opts.slot_name)));
+            }

Here, the code only alters the slot if failover = true. You could use
"else if (opts.slot_name && IsSet(opts.specified_opts,
SUBOPT_FAILOVER)" to check if the failover flag is specified and alter
for failover=false as well. Also, shouldn't you check for the server
version if the command ALTER_REPLICATION_SLOT is supported?

slot.c:
ReplicationSlotAlter()

+void
+ReplicationSlotAlter(const char *name, bool failover)
+{
+    Assert(MyReplicationSlot == NULL);
+
+    ReplicationSlotAcquire(name, true);
+
+    if (SlotIsPhysical(MyReplicationSlot))
+        ereport(ERROR,
+                errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+                errmsg("cannot use %s with a physical replication slot",
+                       "ALTER_REPLICATION_SLOT"));

shouldn't you release the slot by calling ReplicationSlotRelease
before erroring out?

slot.c:
+/*
+ * A helper function to validate slots specified in standby_slot_names GUCs.
+ */
+static bool
+validate_standby_slots(char **newval)
+{
+    char       *rawname;
+    List       *elemlist;
+    ListCell   *lc;
+
+    /* Need a modifiable copy of string */
+    rawname = pstrdup(*newval);

rawname is not always freed.

launcher.c:

+    SlotSyncWorker->hdr.proc = MyProc;
+
+    before_shmem_exit(slotsync_worker_detach, (Datum) 0);
+
+    LWLockRelease(SlotSyncWorkerLock);
+}

before_shmem_exit() can error out leaving the lock acquired. Maybe you
should release the lock prior to calling before_shmem_exit() because
you don't need the lock there.

regards,
Ajin Cherian
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).