Re: Skipping logical replication transactions on subscriber side

On Fri, Jan 7, 2022 at 6:35 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> On Wed, Jan 5, 2022 at 12:31 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > On Mon, Dec 27, 2021 at 9:54 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> > >
> > > On Thu, Dec 16, 2021 at 2:42 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> > > >
> > > > On Thu, Dec 16, 2021 at 2:21 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > > > >
> > > > > On Thu, Dec 16, 2021 at 10:37 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> > > > > >
> > > > > > On Thu, Dec 16, 2021 at 11:43 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > > > > > >
> > > > > > > I thought we just want to lock before clearing the skip_xid something
> > > > > > > like take the lock, check if the skip_xid in the catalog is the same
> > > > > > > as we have skipped, if it is the same then clear it, otherwise, leave
> > > > > > > it as it is. How will that disallow users to change skip_xid when we
> > > > > > > are skipping changes?
> > > > > >
> > > > > > Oh I thought we wanted to keep holding the lock while skipping changes
> > > > > > (changing skip_xid requires acquiring the lock).
> > > > > >
> > > > > > So if skip_xid is already changed, the apply worker would do
> > > > > > replorigin_advance() with WAL logging, instead of committing the
> > > > > > catalog change?
> > > > > >
> > > > >
> > > > > Right. BTW, how are you planning to advance the origin? Normally, a
> > > > > commit transaction would do it but when we are skipping all changes,
> > > > > the commit might not do it as there won't be any transaction id
> > > > > assigned.
> > > >
> > > > I've not tested it yet but replorigin_advance() with wal_log = true
> > > > seems to work for this case.
> > >
> > > I've tested it and realized that we cannot use replorigin_advance()
> > > for this purpose without changes. That is, the current
> > > replorigin_advance() doesn't allow to advance the origin by the owner:
> > >
> > > /* Make sure it's not used by somebody else */
> > > if (replication_state->acquired_by != 0)
> > > {
> > > ereport(ERROR,
> > > (errcode(ERRCODE_OBJECT_IN_USE),
> > > errmsg("replication origin with OID %d is already
> > > active for PID %d",
> > > replication_state->roident,
> > > replication_state->acquired_by)));
> > > }
> > >
> > > So we need to change it so that the origin owner can advance its
> > > origin, which makes sense to me.
> > >
> > > Also, when we have to update the origin instead of committing the
> > > catalog change while updating the origin, we cannot record the origin
> > > timestamp.
> > >
> >
> > Is it because we currently update the origin timestamp with commit record?
>
> Yes.
>
> >
> > > This behavior makes sense to me because we skipped the
> > > transaction. But ISTM it’s not good if we emit the origin timestamp
> > > only when directly updating the origin. So probably we need to always
> > > omit origin timestamp.
> > >
> >
> > Do you mean to say that you want to omit it even when we are
> > committing the changes?
>
> Yes, it would be better to record only origin lsn in terms of consistency.
>

I am not so sure about this point because then what purpose origin
timestamp will serve in the code.

> >
> > > Apart from that, I'm vaguely concerned that the logic seems to be
> > > getting complex. Probably it comes from the fact that we store
> > > skip_xid in the catalog and update the catalog to clear/set the
> > > skip_xid. It might be worth revisiting the idea of storing skip_xid on
> > > shmem (e.g., ReplicationState)?
> > >
> >
> > IIRC, the problem with that idea was that we won't remember skip_xid
> > information after server restart and the user won't even know that it
> > has to set it again.
>
> Right, I agree that it’s not convenient when the server restarts or
> crashes, but these problems could not be critical in the situation
> where users have to use this feature; the subscriber already entered
> an error loop so they can know xid again and it’s an uncommon case
> that they need to restart during skipping changes.
>
> Anyway, I'll submit an updated patch soon so we can discuss complexity
> vs. convenience.
>

Okay, that sounds reasonable.

--
With Regards,
Amit Kapila.