Re: Time delayed LR (WAS Re: logical replication restrictions)

On Fri, Dec 9, 2022 at 10:49 AM Hayato Kuroda (Fujitsu)
<kuroda(dot)hayato(at)fujitsu(dot)com> wrote:
>
> Hi Vignesh,
>
> > In the case of physical replication by setting
> > recovery_min_apply_delay, I noticed that both primary and standby
> > nodes were getting stopped successfully immediately after the stop
> > server command. In case of logical replication, stop server fails:
> > pg_ctl -D publisher -l publisher.log stop -c
> > waiting for server to shut
> > down...............................................................
> > failed
> > pg_ctl: server does not shut down
> >
> > In case of logical replication, the server does not get stopped
> > because the walsender process is not able to exit:
> > ps ux | grep walsender
> > vignesh 1950789 75.3 0.0 8695216 22284 ? Rs 11:51 1:08
> > postgres: walsender vignesh [local] START_REPLICATION
>
> Thanks for reporting the issue. I analyzed about it.
>
>
> This issue has occurred because the apply worker cannot reply during the delay.
> I think we may have to modify the mechanism that delays applying transactions.
>
> When walsender processes are requested to shut down, it can shut down only after
> that all the sent WALs are replicated on the subscriber. This check is done in
> WalSndDone(), and the replicated position will be updated when processes handle
> the reply messages from a subscriber, in ProcessStandbyReplyMessage().
>
> In the case of physical replication, the walreciever can receive WALs and reply
> even if the application is delayed. It means that the replicated position will
> be transported to the publisher side immediately. So the walsender can exit.
>

I think it is not only the replicated positions but it also checks if
there is any pending send in WalSndDone(). Why is it a must to send
all pending WAL and confirm that it is flushed on standby before the
shutdown for physical standby? Is it because otherwise, we may lose
the required WAL? I am asking because it is better to see if those
conditions apply to logical replication as well.

--
With Regards,
Amit Kapila.