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

On Wed, Dec 14, 2022 at 4:16 PM Hayato Kuroda (Fujitsu)
<kuroda(dot)hayato(at)fujitsu(dot)com> wrote:
>
> Dear Horiguchi-san, Amit,
>
> > > On Tue, Dec 13, 2022 at 7:35 AM Kyotaro Horiguchi
> > > <horikyota(dot)ntt(at)gmail(dot)com> wrote:
> > > >
> > > > At Mon, 12 Dec 2022 18:10:00 +0530, Amit Kapila
> > <amit(dot)kapila16(at)gmail(dot)com> wrote in
> > > Yeah, I think ideally it will timeout but if we have a solution like
> > > during delay, we keep sending ping messages time-to-time, it should
> > > work fine. However, that needs to be verified. Do you see any reasons
> > > why that won't work?
>
> I have implemented and tested that workers wake up per wal_receiver_timeout/2
> and send keepalive. Basically it works well, but I found two problems.
> Do you have any good suggestions about them?
>
> 1)
>
> With this PoC at present, workers calculate sending intervals based on its
> wal_receiver_timeout, and it is suppressed when the parameter is set to zero.
>
> This means that there is a possibility that walsender is timeout when wal_sender_timeout
> in publisher and wal_receiver_timeout in subscriber is different.
> Supposing that wal_sender_timeout is 2min, wal_receiver_tiemout is 5min,
> and min_apply_delay is 10min. The worker on subscriber will wake up per 2.5min and
> send keepalives, but walsender exits before the message arrives to publisher.
>
> 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?

--
With Regards,
Amit Kapila.