Re: Synchronizing slots from primary to standby

On Fri, Mar 29, 2024 at 6:36 AM Zhijie Hou (Fujitsu)
<houzj(dot)fnst(at)fujitsu(dot)com> wrote:
>
>
> Attach a new version patch which fixed an un-initialized variable issue and
> added some comments.
>

The other approach to fix this issue could be that the slotsync worker
get the serialized snapshot using pg_read_binary_file() corresponding
to restart_lsn and writes those at standby. But there are cases when
we won't have such a file like (a) when we initially create the slot
and reach the consistent_point, or (b) also by the time the slotsync
worker starts to read the remote snapshot file, the snapshot file
could have been removed by the checkpointer on the primary (if the
restart_lsn of the remote has been advanced in this window). So, in
such cases, we anyway need to advance the slot. I think these could be
optimizations that we could do in the future.

Few comments:
=============
1.
- if (slot->data.database != MyDatabaseId)
+ if (slot->data.database != MyDatabaseId && !fast_forward)
ereport(ERROR,
(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
errmsg("replication slot \"%s\" was not created in this database",
@@ -526,7 +527,7 @@ CreateDecodingContext(XLogRecPtr start_lsn,
* Do not allow consumption of a "synchronized" slot until the standby
* gets promoted.
*/
- if (RecoveryInProgress() && slot->data.synced)
+ if (RecoveryInProgress() && slot->data.synced && !IsSyncingReplicationSlots())

Add comments at both of the above places.

2.
+extern XLogRecPtr pg_logical_replication_slot_advance(XLogRecPtr moveto,
+ bool *found_consistent_point);
+

This API looks a bit awkward as the functionality doesn't match the
name. How about having a function with name
LogicalSlotAdvanceAndCheckReadynessForDecoding(moveto,
ready_for_decoding) with the same functionality as your patch has for
pg_logical_replication_slot_advance() and then invoke it both from
pg_logical_replication_slot_advance and slotsync.c. The function name
is too big, we can think of a shorter name. Any ideas?

--
With Regards,
Amit Kapila.