Re: BUG #18897: Logical replication conflict after using pg_createsubscriber under heavy load

On Thu, Apr 17, 2025 at 4:58 AM PG Bug reporting form
<noreply@postgresql.org> wrote:
>
> The following bug has been logged on the website:
>
> Bug reference:      18897
> Logged by:          Zane Duffield
> Email address:      duffieldzane@gmail.com
> PostgreSQL version: 17.4
> Operating system:   RHEL9 Linux
> Description:
>
> Hi all,
>
> I'm in the process of converting our databases from pglogical logical
> replication to the native logical replication implementation on PostgreSQL
> 17. One of the bugs we encountered and had to work around with pglogical was
> the plugin dropping records while converting to a streaming replica to
> logical via pglogical_create_subscriber (reported
> https://github.com/2ndQuadrant/pglogical/issues/349). I was trying to
> confirm that the native logical replication implementation did not have this
> problem, and I've found that it might have a different problem.
>
> In rare cases, I'm finding that the logical replica can start with the
> logical decoder stuck on a conflicting primary key even though the replica
> received no connections, and only decoded changes from the one primary.
>
> I should say that I've been operating under the assumption that
> pg_createsubscriber is designed for use on a replica for a *live* primary
> database, if this isn't correct then someone please let me know.
>

As per my understanding, your assumption is correct, otherwise, we
should have given an ERROR at the start that we don't allow creating a
subscriber for the live primary.

--
With Regards,
Amit Kapila.