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

Hi, Horiguchi-san


On Monday, February 13, 2023 10:26 AM Kyotaro Horiguchi  wrote:
> At Fri, 10 Feb 2023 10:34:49 +0530, Amit Kapila 
> wrote in
> > On Fri, Feb 10, 2023 at 10:11 AM Amit Kapila 
> wrote:
> > >
> > > On Fri, Feb 10, 2023 at 6:27 AM Kyotaro Horiguchi
> > >  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.
Thanks for your comments !

As described in your last sentence, in the latest patch v34 [1],
we use the last time set in send_feedback() and
based on it, we calculate and adjust the first timing of feedback message
in maybe_apply_delay() so that we can send the feedback message following
the interval of wal_receiver_status_interval. I wasn't sure if
the above concern is still valid for this implementation.

Could you please have a look at the latest patch and share your opinion ?


[1] - https://www.postgresql.org/message-id/TYCPR01MB83736C50C98CB2153728A7A8EDDE9%40TYCPR01MB8373.jpnprd01.prod.outlook.com


Best Regards,
	Takamichi Osumi