Re: Proposal: GUC to control starting/stopping logical subscription workers

Hi,

On Tue, Aug 12, 2025 at 8:40 PM SATYANARAYANA NARLAPURAM
<satyanarlapuram(at)gmail(dot)com> wrote:
>
> I couldn't find a previous discussion on a new GUC to globally enable or disable logical subscription workers at the instance level. So starting a new thread on this.
>
> In multi-region or high-availability setups, a promoted standby often requires a controlled switchover before it should start applying logical replication changes from upstream. Without such control, a promoted standby may immediately attempt to connect to the publisher as a logical subscriber, which can cause it to unexpectedly take over replication slots, start pulling changes before the setup is ready, or even conflict with the original primary that is still using those slots. Disabling the subscription on the primary before promoting a standby is not possible in all cases, for example during PITR or data center outages.
>
> Providing a way to keep logical subscriptions globally disabled—via a GUC setting—prior to promotion ensures that no changes are accidentally pulled or applied before the system is fully prepared. This avoids race conditions and the risk of data divergence.
>
> I would like to propose adding a GUC with the following behavior:
>
> Default value for the GUC is ON, same behavior as now without the GUC
> When off, no new apply workers start and existing ones exit gracefully similar to when subscription disabled
> When turned on again, behavior will be the same as the current behavior
> This GUC shouldn't require a restart

If I understand correctly, the end effect is similar to disabling all
subscriptions. Why not just add ALTER SUBSCRIPTION ... DISABLE for all
subscriptions in the failover work flow? Migration of logical
replication slots docs says so -
https://www.postgresql.org/docs/18/logical-replication-upgrade.html.

--
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com