Re: Conflict detection for update_deleted in logical replication

On Fri, Nov 29, 2024 at 4:05 PM Hayato Kuroda (Fujitsu)
<kuroda(dot)hayato(at)fujitsu(dot)com> wrote:
>
> 02. maybe_advance_nonremovable_xid
>
> ```
> + case RCI_REQUEST_PUBLISHER_STATUS:
> + request_publisher_status(data);
> + break;
> ```
>
> I think the part is not reachable because the transit
> RCI_REQUEST_PUBLISHER_STATUS->RCI_WAIT_FOR_PUBLISHER_STATUS is done in
> get_candidate_xid()->request_publisher_status().
> Can we remove this?
>

After changing phase to RCI_REQUEST_PUBLISHER_STATUS, we directly
invoke request_publisher_status, and similarly, after changing phase
to RCI_WAIT_FOR_LOCAL_FLUSH, we call wait_for_local_flush. Won't it be
better that in both cases and other similar cases, we instead invoke
maybe_advance_nonremovable_xid()? This will make
maybe_advance_nonremovable_xid() the only function with the knowledge
to take action based on phase rather than spreading the knowledge of
phase-related actions to various functions. Then we should also add a
comment at the end in request_publisher_status() where we change the
phase but don't do anything. The comment should explain the reason for
the same.

One more point, it seems on a busy server, the patch won't be able to
advance nonremovable_xid. We should call
maybe_advance_nonremovable_xid() at all the places where we call
send_feedback() and additionally, we should also call it after
applying some threshold number (say 100) of messages. The latter is to
avoid the cases where we won't invoke the required functionality on a
busy server with a large value of sender/receiver timeouts.

--
With Regards,
Amit Kapila.