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

At Thu, 15 Dec 2022 09:18:55 +0530, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote in
> On Thu, Dec 15, 2022 at 7:22 AM Kyotaro Horiguchi
> <horikyota(dot)ntt(at)gmail(dot)com> wrote:
> >
> > At Wed, 14 Dec 2022 16:30:28 +0530, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote in
> > > On Wed, Dec 14, 2022 at 4:16 PM Hayato Kuroda (Fujitsu)
> > > <kuroda(dot)hayato(at)fujitsu(dot)com> wrote:
> > > > One idea to avoid that is to send the min_apply_delay subscriber option to publisher
> > > > and compare them, but it may be not sufficient. Because XXX_timout GUC parameters
> > > > could be modified later.
> > > >
> > >
> > > How about restarting the apply worker if min_apply_delay changes? Will
> > > that be sufficient?
> >
> > Mmm. If publisher knows that value, isn't it albe to delay *sending*
> > data in the first place? This will resolve many known issues including
> > walsender's un-terminatability, possible buffer-full and status packet
> > exchanging.
> >
>
> Yeah, but won't it change the meaning of this parameter? Say the

Internally changes, but does not change on its face. The difference is
only in where the choking point exists. If ".._apply_delay" should
work literally, we should go the way Kuroda-san proposed. Namely,
"apply worker has received the data, but will deilay applying it". If
we technically name it correctly for the current behavior, it would be
"min_receive_delay" or "min_choking_interval".

> subscriber was busy enough that it doesn't need to add an additional
> delay before applying a particular transaction(s) but adding a delay
> to such a transaction on the publisher will actually make it take much
> longer to reflect than expected. We probably need to name this

Isn't the name min_apply_delay implying the same behavior? Even though
the delay time will be a bit prolonged.

> parameter as min_send_delay if we want to do what you are saying and
> then I don't know if it serves the actual need and also it will be
> different from what we do in physical standby.

In the first place phisical and logical replication works differently
and the mechanism to delaying "apply" differs even in the current
state in terms of logrep delay choking stream.

I guess they cannot be different in terms of normal operation. But I'm
not sure.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center