Re: Exit walsender before confirming remote flush in logical replication

On Tue, Dec 27, 2022 at 1:44 PM Hayato Kuroda (Fujitsu)
<kuroda(dot)hayato(at)fujitsu(dot)com> wrote:
>
> Thanks for checking my proposal!
>
> > - * Note that if we determine that there's still more data to send, this
> > - * function will return control to the caller.
> > + * Note that if we determine that there's still more data to send or we are in
> > + * the physical replication more, this function will return control to the
> > + * caller.
> >
> > I think in this comment you meant to say
> >
> > 1. "or we are in physical replication mode and all WALs are not yet replicated"
> > 2. Typo /replication more/replication mode
>
> Firstly I considered 2, but I thought 1 seemed to be better.
> PSA the updated patch.
>

I think even for logical replication we should check whether there is
any pending WAL (via pq_is_send_pending()) to be sent. Otherwise, what
is the point to send the done message? Also, the caller of
WalSndDone() already has that check which is another reason why I
can't see why you didn't have the same check in function WalSndDone().

BTW, even after fixing this, I think logical replication will behave
differently when due to some reason (like time-delayed replication)
send buffer gets full and walsender is not able to send data. I think
this will be less of an issue with physical replication because there
is a separate walreceiver process to flush the WAL which doesn't wait
but the same is not true for logical replication. Do you have any
thoughts on this matter?

--
With Regards,
Amit Kapila.