Re: Logical replication keepalive flood

Hi,

On Wed, Jun 9, 2021 at 2:30 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:

> On Wed, Jun 9, 2021 at 1:47 PM Kyotaro Horiguchi
> <horikyota(dot)ntt(at)gmail(dot)com> wrote:
> >
> > At Wed, 9 Jun 2021 11:21:55 +0900, Kyotaro Horiguchi <
> horikyota(dot)ntt(at)gmail(dot)com> wrote in
> > > The issue - if actually it is - we send a keep-alive packet before a
> > > quite short sleep.
> > >
> > > We really want to send it if the sleep gets long but we cannot predict
> > > that before entering a sleep.
> > >
> > > Let me think a little more on this..
> >
> > After some investigation, I find out that the keepalives are sent
> > almost always after XLogSendLogical requests for the *next* record.
> >
>
> Does these keepalive messages are sent at the same frequency even for
> subscribers?

Yes, I have tested it with one publisher and one subscriber.
The moment I start pgbench session I can see keepalive messages sent and
replied by the subscriber with same frequency.

> Basically, I wanted to check if we have logical
> replication set up between 2 nodes then do we send these keep-alive
> messages flood?

Yes we do.

> If not, then why is it different in the case of
> pg_recvlogical?

Nothing, the WAL sender behaviour is same in both cases.

> Is it possible that the write/flush location is not
> updated at the pace at which we expect?

Well, it is async replication. The receiver can choose to update LSNs at
its own will, say after 10 mins interval.
It should only impact the size of WAL retained by the server.

Please see commit 41d5f8ad73
> which seems to be talking about a similar problem.
>

That commit does not address this problem.

>
> --
> With Regards,
> Amit Kapila.
>

--
--
*Abbas*
Senior Architect

Ph: 92.334.5100153
Skype ID: gabbasb
edbpostgres.com

*Follow us on Twitter*
@EnterpriseDB