Re: Synchronizing slots from primary to standby

Here are some review comments for v740001.

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src/sgml/logicaldecoding.sgml

1.
+   <sect2 id="logicaldecoding-replication-slots-synchronization">
+    <title>Replication Slot Synchronization</title>
+    <para>
+     A logical replication slot on the primary can be synchronized to the hot
+     standby by enabling the <literal>failover</literal> option during slot
+     creation and setting
+     <link linkend="guc-enable-syncslot"><varname>enable_syncslot</varname></link>
+     on the standby. For the synchronization
+     to work, it is mandatory to have a physical replication slot between the
+     primary and the standby, and
+     <link linkend="guc-hot-standby-feedback"><varname>hot_standby_feedback</varname></link>
+     must be enabled on the standby. It is also necessary to specify a valid
+     <literal>dbname</literal> in the
+     <link linkend="guc-primary-conninfo"><varname>primary_conninfo</varname></link>
+     string, which is used for slot synchronization and is ignored
for streaming.
+    </para>

IMO we don't need to repeat that last part ", which is used for slot
synchronization and is ignored for streaming." because that is a
detail about the primary_conninfo GUC, and the same information is
already described in that GUC section.

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2. ALTER_REPLICATION_SLOT slot_name ( option [, ...] ) #

          <para>
-          If true, the slot is enabled to be synced to the standbys.
+          If true, the slot is enabled to be synced to the standbys
+          so that logical replication can be resumed after failover.
          </para>

This also should have the sentence "The default is false.", e.g. the
same as the same option in CREATE_REPLICATION_SLOT says.

======
synchronize_one_slot

3.
+ /*
+ * Make sure that concerned WAL is received and flushed before syncing
+ * slot to target lsn received from the primary server.
+ *
+ * This check will never pass if on the primary server, user has
+ * configured standby_slot_names GUC correctly, otherwise this can hit
+ * frequently.
+ */
+ latestFlushPtr = GetStandbyFlushRecPtr(NULL);
+ if (remote_slot->confirmed_lsn > latestFlushPtr)

BEFORE
This check will never pass if on the primary server, user has
configured standby_slot_names GUC correctly, otherwise this can hit
frequently.

SUGGESTION (simpler way to say the same thing?)
This will always be the case unless the standby_slot_names GUC is not
correctly configured on the primary server.

~~~

4.
+ /* User created slot with the same name exists, raise ERROR. */

/User created/User-created/

~~~

5. synchronize_slots, and also drop_obsolete_slots

+ /*
+ * Use shared lock to prevent a conflict with
+ * ReplicationSlotsDropDBSlots(), trying to drop the same slot while
+ * drop-database operation.
+ */

(same code comment is in a couple of places)

SUGGESTION (while -> during, etc.)

Use a shared lock to prevent conflicting with
ReplicationSlotsDropDBSlots() trying to drop the same slot during a
drop-database operation.

~~~

6. validate_parameters_and_get_dbname

strcmp() just for the empty string "" might be overkill.

6a.
+ if (PrimarySlotName == NULL || strcmp(PrimarySlotName, "") == 0)

SUGGESTION
if (PrimarySlotName == NULL || *PrimarySlotName == '\0')

~~

6b.
+ if (PrimaryConnInfo == NULL || strcmp(PrimaryConnInfo, "") == 0)

SUGGESTION
if (PrimaryConnInfo == NULL || *PrimaryConnInfo == '\0')

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Kind Regards,
Peter Smith.
Fujitsu Australia