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

On Mon, 14 Nov 2022 at 12:14, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> Hi,
>
> The thread title doesn't really convey the topic under discussion, so
> changed it. IIRC, this has been mentioned by others as well in the
> thread.
>
> On Sat, Nov 12, 2022 at 7:21 PM vignesh C <vignesh21(at)gmail(dot)com> wrote:
> >
> > Few comments:
> > 1) I feel if the user has specified a long delay there is a chance
> > that replication may not continue if the replication slot falls behind
> > the current LSN by more than max_slot_wal_keep_size. I feel we should
> > add this reference in the documentation of min_apply_delay as the
> > replication will not continue in this case.
> >
>
> This makes sense to me.
>
> > 2) I also noticed that if we have to shut down the publisher server
> > with a long min_apply_delay configuration, the publisher server cannot
> > be stopped as the walsender waits for the data to be replicated. Is
> > this behavior ok for the server to wait in this case? If this behavior
> > is ok, we could add a log message as it is not very evident from the
> > log files why the server could not be shut down.
> >
>
> I think for this case, the behavior should be the same as for physical
> replication. Can you please check what is behavior for the case you
> are worried about in physical replication? Note, we already have a
> similar parameter for recovery_min_apply_delay for physical
> replication.

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

Regards,
Vignesh