Re: Dropping publication breaks logical replication

On Fri, Aug 1, 2025 at 10:55 AM Ashutosh Bapat
<ashutosh.bapat.oss@gmail.com> wrote:
>
> Hi Vignesh, Amit,
> We encountered a situation where a customer dropped a publication
> accidentally and that broke logical replication in an irrecoverable
> manner. This is PG 15.3 but the team confirmed that the behaviour is
> reproducible with PG 17 as well.
>
> When a WAL sender processes a WAL record recording a change in
> publication, it ends up calling LoadPublication() which throws an
> error if a publication mentioned in START_REPLICATION command is not
> found. The downstream tries to reconnect but the WAL sender again
> repeats the same process going in an error loop. Creating the
> publication does not help since WAL sender will always encounter the
> WAL record dropping the publication first.
>
> There are ways to come out of this situation, but not very clean always
> 1. Remove publication from subscription, run logical replication till
> it passes the point where publication was added, add the publication
> back and continue. It's not always possible to know when the
> publication was added back and thus it becomes tedious or next to
> impossible to apply these steps.
> 2. Reseeding the replication slot which involves copying all the data
> again and not feasible in case of large databases.
> 3. Skipping the transaction which dropped the publication. This will
> work if drop publication was the only thing in that transaction but
> not otherwise. Confirming that is tricky and requires some expert
> help.
>
> In PG 18 onwards, this behaviour is fixed by throwing a WARNING
> instead of an error. In the relevant thread [1] where the fix to PG 18
> was discussed, backpatching was also discussed. Back then it was
> deferred because of lack of field reports. But we are seeing this
> situation now. So maybe it's time to backpatch the fix. Further PG 15
> documentation mentions that
> https://www.postgresql.org/docs/15/sql-createsubscription.html. So the
> users will expect that their logical replication will not be affected
> (except for the data published by the publication) if a publication is
> dropped or does not exist. So, backpatching the change would make the
> behaviour compatible with the documentation.
>
> The backport seems to be straight forward. Please let me know if you
> need my help in doing so, if we decide to backport the fix.

I think you missed to add the link to the "relevant thread [1] "

--
Regards,
Dilip Kumar
Google