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

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.

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.

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