Re: Dropping publication breaks logical replication

On Fri, Aug 1, 2025 at 11:14 AM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
>
> On Fri, Aug 1, 2025 at 10:55 AM Ashutosh Bapat
> <ashutosh(dot)bapat(dot)oss(at)gmail(dot)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] "

Thanks for noticing it. Here it is

[1] https://www.postgresql.org/message-id/flat/CALDaNm0-n8FGAorM%2BbTxkzn%2BAOUyx5%3DL_XmnvOP6T24%2B-NcBKg%40mail.gmail.com

--
Best Wishes,
Ashutosh Bapat