Re: Perform streaming logical transactions by background workers and parallel apply

On Wed, Dec 7, 2022 at 1:29 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Wed, Dec 7, 2022 at 9:00 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> >
> > On Thu, Dec 1, 2022 at 7:17 PM houzj(dot)fnst(at)fujitsu(dot)com
> > <houzj(dot)fnst(at)fujitsu(dot)com> wrote:
> > >
> > > > ---
> > > > if (am_parallel_apply_worker() && on_subinfo_change) {
> > > > /*
> > > > * If a parallel apply worker exits due to the subscription
> > > > * information change, we notify the leader apply worker so that the
> > > > * leader can report more meaningful message in time and restart the
> > > > * logical replication.
> > > > */
> > > > pq_putmessage('X', NULL, 0);
> > > > }
> > > >
> > > > and
> > > >
> > > > ereport(ERROR,
> > > > (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
> > > > errmsg("logical replication parallel apply worker exited
> > > > because of subscription information change")));
> > > >
> > > > Do we really need an additional message in case of 'X'? When we call
> > > > apply_worker_clean_exit with on_subinfo_change = true, we have reported the
> > > > error message such as:
> > > >
> > > > ereport(LOG,
> > > > (errmsg("logical replication parallel apply worker for subscription
> > > > \"%s\" will stop because of a parameter change",
> > > > MySubscription->name)));
> > > >
> > > > I think that reporting a similar message from the leader might not be
> > > > meaningful for users.
> > >
> > > The intention is to let leader report more meaningful message if a worker
> > > exited due to subinfo change. Otherwise, the leader is likely to report an
> > > error like " lost connection ... to parallel apply worker" when trying to send
> > > data via shared memory if the worker exited. What do you think ?
> >
> > Agreed. But do we need to have the leader exit with an error in spite
> > of the fact that the worker cleanly exits? If the leader exits with an
> > error, the subscription will be disabled if disable_on_error is true,
> > right?
> >
>
> Right, but the leader will anyway exit at some point either due to an
> ERROR like "lost connection ... to parallel worker" or with a LOG
> like: "... will restart because of a parameter change" but I see your
> point. So, will it be better if we have a LOG message here and then
> proc_exit()? Do you have something else in mind for this?

No, I was thinking that too. It's better to write a LOG message and do
proc_exit().

Regarding the error "lost connection ... to parallel worker", it could
still happen depending on the timing even if the parallel worker
cleanly exits due to parameter changes, right? If so, I'm concerned
that it could lead to disable the subscription unexpectedly if
disable_on_error is enabled.

Regards,

--
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com