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

On Thu, Dec 15, 2022 at 10:11 AM Kyotaro Horiguchi
<horikyota(dot)ntt(at)gmail(dot)com> wrote:
>
> 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.
>

Sorry, I don't understand what you intend to say in this point. In
above, I mean that the currently proposed patch won't have such a
problem but if we apply delay on publisher the problem can happen.

> > 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 think the first preference is to make it work in a similar way (as
much as possible) to how this parameter works in physical standby and
if that is not at all possible then we may consider other approaches.

--
With Regards,
Amit Kapila.