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

At Fri, 10 Feb 2023 10:34:49 +0530, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote in
> On Fri, Feb 10, 2023 at 10:11 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > On Fri, Feb 10, 2023 at 6:27 AM Kyotaro Horiguchi
> > <horikyota(dot)ntt(at)gmail(dot)com> wrote:
> > > 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?

Wal receiver can avoid a too-long sleep by knowing when to wake up for
the next feedback.

> I have an idea to use last_send time to avoid walsenders being
> timeout. Instead of directly using wal_receiver_status_interval as a
> minimum interval to send the feedback, we should check if it is
> greater than last_send time then we should send the feedback after
> (wal_receiver_status_interval - last_send). I think they can probably
> be different only on the very first time. Any better ideas?

If it means MyLogicalRepWorker->last_send_time, it is not the last
time when walreceiver sent a feedback but the last time when
wal*sender* sent a data. So I'm not sure that works.

We could use the variable that way, but AFAIU in turn when so many
changes have been spooled that the control doesn't return to
LogicalRepApplyLoop longer than wal_r_s_interval, maybe_apply_delay()
starts calling send_feedback() at every call after the first feedback
timing. Even in that case, send_feedback() won't send one actually
until the next feedback timing, I don't think that behavior is great.

The only packets walreceiver sends back is the feedback packets and
currently only send_feedback knows the last feedback time.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center