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

On Fri, Feb 10, 2023 at 6:27 AM Kyotaro Horiguchi
<horikyota(dot)ntt(at)gmail(dot)com> wrote:
>
> At Thu, 9 Feb 2023 13:48:52 +0530, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote in
> > On Thu, Feb 9, 2023 at 10:45 AM Kyotaro Horiguchi
> > <horikyota(dot)ntt(at)gmail(dot)com> wrote:
> > >
> > > At Wed, 8 Feb 2023 09:03:03 +0000, "Hayato Kuroda (Fujitsu)" <kuroda(dot)hayato(at)fujitsu(dot)com> wrote in
> > > > Thank you for reviewing! PSA new version.
> > >
> > > + if (statusinterval_ms > 0 && diffms > statusinterval_ms)
> > >
> > > The next expected feedback time is measured from the last status
> > > report. Thus, it seems to me this may suppress feedbacks from being
> > > sent for an unexpectedly long time especially when min_apply_delay is
> > > shorter than wal_r_s_interval.
> > >
> >
> > I think the minimum time before we send any feedback during the wait
> > is wal_r_s_interval. Now, I think if there is no transaction for a
> > long time before we get a new transaction, there should be keep-alive
> > messages in between which would allow us to send feedback at regular
> > intervals (wal_receiver_status_interval). So, I think we should be
>
> Right.
>
> > able to send feedback in less than 2 * wal_receiver_status_interval
> > unless wal_sender/receiver timeout is very large and there is a very
> > low volume of transactions. Now, we can try to send the feedback
>
> We have suffered this kind of feedback silence many times. Thus I
> don't want to rely on luck here. I had in mind of exposing last_send
> itself or providing interval-calclation function to the logic.
>

I think we have last_send time in send_feedback(), so we can expose it
if we want but how would that solve the problem you are worried about?
The one simple idea as I shared in my last email was to send feedback
every wal_receiver_status_interval / 2. I think this should avoid any
timeout problem because we already recommend setting it to lesser than
wal_sender_timeout.

--
With Regards,
Amit Kapila.