Subject: RE: Warn when creating or enabling a subscription with max_logical_replication_workers = 0

From: "Zhijie Hou (Fujitsu)" <[email-masked]>
Date: Thu, 5 Feb 2026 08:12:33 +0000

On Thursday, February 5, 2026 3:47 PM Peter Smith <[email-masked]> wrote:
> On Thu, Feb 5, 2026 at 12:12 PM Yugo Nagata <[email-masked]> wrote:
> >
> > On Wed, 4 Feb 2026 17:26:25 +1100
> > Peter Smith <[email-masked]> wrote:
> >
> > > On Wed, Feb 4, 2026 at 4:07 PM Yugo Nagata <[email-masked]>
> wrote:
> > > >
> > > > Hi,
> > > >
> > > > I would like to propose emitting a warning when creating or
> > > > enabling a subscription while max_logical_replication_workers is
> > > > set to 0. In this case, the CREATE/ALTER SUBSCRIPTION command
> > > > completes successfully without any warning, making it difficult to
> > > > notice that logical replication cannot start.
> > > >
> > > > Of course, users can confirm whether logical replication is
> > > > working by checking system views such as pg_stat_replication or
> pg_stat_subscription.
> > > > However, emitting warnings explicitly in these cases would make
> > > > this situation more visible. We have seen user reports where this
> > > > behavior caused confusion, with users wondering why replication did not
> start.
> > > >
> > >
> > > Hi Nagata-San.
> > >
> > > AFAIK the default for `max_logical_replication_workers` is 4. So how
> > > does the maximum get to be 0 unless the user had explicitly
> > > configured it that way?
> >
> > That's correct, but the goal here is simply to make it easier for
> > users to be aware of this condition, since the current behavior
> > provides no indication that replication will not start.
> >
> > > Also subscriptions require multiple workers in order to work
> > > properly [1] so why check only 0? Why not check 1 or 2 or 3....
> > > those low numbers are also likely to cause similar problems aren't they?
> > >
> > > And what about when the  `max_logical_replication_workers` is 100,
> > > but those 100 are already being used. IOW, would it be more useful
> > > to warn when you do not have enough *available* workers for the
> > > Subscription to function properly, rather than checking what the
> > > maximum value is set to?
> >
> > When max_logical_replication_workers is zero, the logical replication
> > launcher will never start. Otherwise, it does start and emits the
> > following warning to the server log when workers cannot be obtained:
> >
> > WARNING: out of logical replication worker slots
> > HINT: You might need to increase "max_logical_replication_workers"
> >
> > Given this, I think it is sufficient to warn only when
> > max_logical_replication_workers is zero.
> >
> 
> Hi Nagata-San,
> 
> 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.

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).

Best Regards,
Hou zj

---

From: Peter Smith <[email-masked]>
Date: Fri, 6 Feb 2026 09:58:02 +1100

...
> 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.

> 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.

e.g. when max_logical_replication_workers = 1

----------
test_sub=# create subscription sub1 connection 'dbname=test_pub'
publication pub1;
NOTICE:  created replication slot "sub1" on publisher
CREATE SUBSCRIPTION
test_sub=# 2026-02-06 08:50:35.306 AEDT [629942] LOG:  logical
replication apply worker for subscription "sub1" has started
2026-02-06 08:50:35.348 AEDT [629942] WARNING:  out of logical
replication worker slots
2026-02-06 08:50:35.348 AEDT [629942] HINT:  You might need to
increase "max_logical_replication_workers".
2026-02-06 08:50:40.356 AEDT [629942] WARNING:  out of logical
replication worker slots
2026-02-06 08:50:40.356 AEDT [629942] HINT:  You might need to
increase "max_logical_replication_workers".
2026-02-06 08:50:45.365 AEDT [629942] WARNING:  out of logical
replication worker slots
2026-02-06 08:50:45.365 AEDT [629942] HINT:  You might need to
increase "max_logical_replication_workers".
2026-02-06 08:50:50.426 AEDT [629942] WARNING:  out of logical
replication worker slots
2026-02-06 08:50:50.426 AEDT [629942] HINT:  You might need to
increase "max_logical_replication_workers".
2026-02-06 08:50:55.531 AEDT [629942] WARNING:  out of logical
replication worker slots
2026-02-06 08:50:55.531 AEDT [629942] HINT:  You might need to
increase "max_logical_replication_workers".
...
----------

======
Kind Regards,
Peter Smith.
Fujitsu Australia

---

From: Yugo Nagata <[email-masked]>
Date: Fri, 6 Feb 2026 13:37:35 +0900

On Fri, 6 Feb 2026 09:58:02 +1100

> cannot "fail" because it was never registered/started in the first

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.

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".

> 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.

Regards,
Yugo Nagata

-- 
Yugo Nagata <[email-masked]>

---

From: Peter Smith <[email-masked]>
Date: Fri, 6 Feb 2026 18:28:06 +1100

> 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)"  (??).

> existing messages such as "subscription was created, but is not

 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.

> 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

======
Kind Regards,
Peter Smith.
Fujitsu Australia

---

From: Yugo Nagata <[email-masked]>
Date: Mon, 13 Apr 2026 16:04:05 +0900

On Fri, 6 Feb 2026 18:28:06 +1100

> (max_logical_replication_workers == 0)"  (??).

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

> that you handled - so I was just unsure if there are some remaining

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.

> 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.

Regards,
Yugo Nagata