Re: Warn when creating or enabling a subscription with max_logical_replication_workers = 0 - Mailing list pgsql-hackers

On Mon, 13 Apr 2026 at 09:04, Yugo Nagata <nagata@sraoss.co.jp> wrote:

On Fri, 6 Feb 2026 18:28:06 +1100
Peter Smith <smithpb2250@gmail.com> wrote:

> On Fri, Feb 6, 2026 at 3:37 PM Yugo Nagata <nagata@sraoss.co.jp> wrote:
> >
> > On Fri, 6 Feb 2026 09:58:02 +1100
> > Peter Smith <smithpb2250@gmail.com> wrote:
> >
> > > On Thu, Feb 5, 2026 at 7:12 PM Zhijie Hou (Fujitsu)
> > > <houzj.fnst@fujitsu.com> wrote:
> > > >
> > > > On Thursday, February 5, 2026 3:47 PM Peter Smith <smithpb2250@gmail.com> wrote:
> > > > > On Thu, Feb 5, 2026 at 12:12 PM Yugo Nagata <nagata@sraoss.co.jp> wrote:
> > > > > >
> > > > > > On Wed, 4 Feb 2026 17:26:25 +1100
> > > > > > Peter Smith <smithpb2250@gmail.com> wrote:
> > > > > >
> > > ...
> > > > >
> > > > > Oh right, I mistook that you had run out of logical replication "workers", but in
> > > > > fact, because max_logical_replication_workers = 0 the main "logical
> > > > > replication launcher" process had failed to start, so logical replication was
> > > > > entirely disabled.
> > > > >
> > > > > See code: in backend/replication/logical/launcher.c
> > > > >
> > > > > ApplyLauncherRegister(void)
> > > > > {
> > > > > ...
> > > > >   if (max_logical_replication_workers == 0 || IsBinaryUpgrade)
> > > > >     return;
> > > > >
> > > > > ~~~
> > > > >
> > > > > Given this, I felt that instead of testing the GUC, what you really want to know
> > > > > is just whether that "logical replication launcher" is running or not.
> > > > >
> > > > > And that launcher pid is already tested when the Subscription commands send
> > > > > a "kill" to the launcher. e.g. see function ApplyLauncherWakeup.
> > > > >
> > > > > So, here is a diff patch, of what I tried:
> > > > >
> > > > > ------
> > > > > diff --git a/src/backend/replication/logical/launcher.c
> > > > > b/src/backend/replication/logical/launcher.c
> > > > > index 3ed86480be2..f880380ce4e 100644
> > > > > --- a/src/backend/replication/logical/launcher.c
> > > > > +++ b/src/backend/replication/logical/launcher.c
> > > > > @@ -1195,6 +1195,13 @@ ApplyLauncherWakeup(void)  {
> > > > >         if (LogicalRepCtx->launcher_pid != 0)
> > > > >                 kill(LogicalRepCtx->launcher_pid, SIGUSR1);
> > > > > +       else
> > > > > +       {
> > > > > +               if (max_logical_replication_workers == 0)
> > > > > +                       ereport(WARNING,
> > > > > +                               errmsg("Logical replication is
> > > > > currently disabled"),
> > > > > +                               errhint("\"%s\" is 0.",
> > > > > "max_logical_replication_workers"));
> > > > > +       }
> > > > >  }
> > > > > ------
> > > > >
> > > > > Thoughts?
> > > >
> > > > I think this is not the right place to check this issue. The launcher might fail
> > > > for some reasons and restart soon (pid will be set to 0), in which case this
> > > > warning wouldn't be appropriate.
> > >
> > > AFAIK, that's not possible. My warning is guarded by checking
> > > max_logical_replication_workers == 0. And in that case, the launcher
> > > cannot "fail" because it was never registered/started in the first
> > > place.
> >
> > I also initially considered emitting the warning in
> > ApplyLauncherWakeup() after checking　max_logical_replication_workers == 0.
> > However, I think checking pid == 0 is sufficient.
> >
> > Even when max_logical_replication_workers is non-zero and the launcher is
> > normally running, it could still be killed by user action or by the
> > OS, although such cases should be rare. In that situation, emitting the
> > same warning would not be appropriate.
>
> Sorry, I did not understand the previous paragraph. If "when
> max_logical_replication_workers is non-zero" then the warning cannot
> be emitted inappropriately because that warning is guarded by "if
> (max_logical_replication_workers == 0)"  (??).

You're right, I misunderstood that part. Sorry about that.

> > I placed the logic in subscriptioncmds.c so that the warning message can
> > better reflect the actual situation, while keeping consistency with
> > existing messages such as "subscription was created, but is not
> > connected".
>
>  In hindsgght your original patch is very similar to what I posted. I
> agree, your messages are better because they are more specific. But
> there are multiple calls to ApplyLauncherWakeup() -- not just those 2
> that you handled - so I was just unsure if there are some remaining
> holes.

I see your point.

There are other places where ApplyLauncherWakeupAtCommit() is called,
for example via AlterSubscription() when retain_dead_tuples is set or
when the owner is changed. However, these cases are not affected when
max_logical_replication_workers = 0 unless the subscription is enabled.

For now, I’ll keep the current approach.

> >
> > > >
> > > > Besides, I also think it would make more sense to issue a warning if the
> > > > subscription has no remaining workers to start instead of raising a
> > > > warning for 0 setting (the latter seems rare).
> > > >
> > >
> > > It might be rare, but by my understanding, the original post described
> > > this specific scenario, whereby the user had previously deliberately
> > > configured `max_logical_replication_workers` to 0. Then, some time
> > > later, when they attempted CREATE/ALTER SUBSCRIPTION, nothing
> > > happened, and there was only silence. If they'd forgotten about their
> > > `max_logical_replication_workers` setting, then it could be confusing
> > > why nothing was happening.
> > >
> > > OTOH, when max_logical_replication_workers > 0, then the logical
> > > replication launcher would be running, and in that case, there are
> > > already plenty of warning logs about not enough worker resources.
> >
> > Yes. In this case, warnings are emitted to the server log, but they do not
> > appear as a response to CREATE/ALTER SUBSCRIPTION. There is an option to
> > also emit such warnings during the CREATE/ALTER SUBSCRIPTION command, so
> > I will update the patch accordingly.
> >
> > Nevertheless, this seems to be a different situation from the case where
> > logical replication is disabled entirely, so I think the warning messages
> > should be handled separately.
>
> +1. I also think that running out of worker resources is a different
> scenario from the entire logical replication being disabled, so it
> should be handled separately

I've investigated whether we can emit a warning during CREATE/ALTER SUBSCRIPTION
when there are no available worker slots to start the subscription.

I find there is another more warning worthy scenario that should be covered too, what if everything is working normally and then someone changes max_logical_replication_workers to 0 in conf file and restarts, now logical replication fails to continue. In this case since no CREATE or ALTER subscription command was issued, the changes discussed upthread would not produce a warning. Shouldn't we cover this case also?

One issue is that, in the current implementation, this condition is only checked
when the launcher actually forks a worker. There is no infrastructure to perform
this check in advance from a backend process.

Adding such infrastructure would likely require acquiring an LWLock (at least in
shared mode) to inspect the current worker state. Moreover, even if we implement
such a mechanism, the situation may change between the time of the check and the
actual worker startup.

What about an alternate way to overcome this, following the same pattern as in apply_error_count for pg_stat_subscription_stats.

So, a rough sketch of the approach is like this.. Create a new field in PgStat_StatSubEntry say worker_failure_count and add a function to increment this field. Now, in logicalrep_worker_launch when it hits worker == NULL, call this function to increment the worker_failure_count. I have added a rough patch to explain the idea better.

Note that this won't be something as emitting a warning or notice but rather needs to be explicitly queried.

Given these limitations, I'm wondering whether introducing this kind of warning
is worthwhile.

--

Regards,
Rafia Sabih

CYBERTEC PostgreSQL International GmbH