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

On Wed, Feb 4, 2026 at 4:07 PM Yugo Nagata <nagata(at)sraoss(dot)co(dot)jp> 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?

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?

======
[1] https://www.postgresql.org/docs/current/runtime-config-replication.html#GUC-MAX-LOGICAL-REPLICATION-WORKERS

Kind Regards,
Peter Smith
Fujitsu Australia