Re: Skipping logical replication transactions on subscriber side

On Thu, Dec 9, 2021 at 6:16 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Thu, Dec 9, 2021 at 2:24 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> >
> > On Thu, Dec 9, 2021 at 11:47 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > >
> > > I am thinking that we can start a transaction, update the catalog,
> > > commit that transaction. Then start a new one to update
> > > origin_lsn/timestamp, finishprepared, and commit it. Now, if it
> > > crashes after the first transaction, only commit prepared will be
> > > resent again and this time we don't need to update the catalog as that
> > > entry would be already cleared.
> >
> > Sounds good. In the crash case, it should be fine since we will just
> > commit an empty transaction. The same is true for the case where
> > skip_xid has been changed after skipping and preparing the transaction
> > and before handling commit_prepared.
> >
> > Regarding the case where the user specifies XID of the transaction
> > after it is prepared on the subscriber (i.g., the transaction is not
> > empty), we won’t skip committing the prepared transaction. But I think
> > that we don't need to support skipping already-prepared transaction
> > since such transaction doesn't conflict with anything regardless of
> > having changed or not.
> >
>
> Yeah, this makes sense to me.
>

I've attached an updated patch. The new syntax is like "ALTER
SUBSCRIPTION testsub SKIP (xid = '123')".

I’ve been thinking we can do something safeguard for the case where
the user specified the wrong xid. For example, can we somewhat use the
stats in pg_stat_subscription_workers? An idea is that logical
replication worker fetches the xid from the stats when reading the
subscription and skips the transaction if the xid matches to
subskipxid. That is, the worker checks the error reported by the
worker previously working on the same subscription. The error could
not be a conflict error (e.g., connection error etc.) or might have
been cleared by the reset function, But given the worker is in an
error loop, the worker can eventually get xid in question. We can
prevent an unrelated transaction from being skipped unexpectedly. It
seems not a stable solution though. Or it might be enough to warn
users when they specified an XID that doesn’t match to last_error_xid.
Anyway, I think it’s better to have more discussion on this. Any
ideas?

Regards,

--
Masahiko Sawada
EDB: https://www.enterprisedb.com/