Re: logical decoding and replication of sequences, take 2

On Fri, Aug 18, 2023 at 4:28 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Fri, Aug 18, 2023 at 10:37 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > On Thu, Aug 17, 2023 at 7:13 PM Ashutosh Bapat
> > <ashutosh(dot)bapat(dot)oss(at)gmail(dot)com> wrote:
> > >
> > > On Wed, Aug 16, 2023 at 7:56 PM Tomas Vondra
> > > <tomas(dot)vondra(at)enterprisedb(dot)com> wrote:
> > > >
> > > > >
> > > > > But whether or not that's the case, downstream should not request (and
> > > > > hence receive) any changes that have been already applied (and
> > > > > committed) downstream as a principle. I think a way to achieve this is
> > > > > to update the replorigin_session_origin_lsn so that a sequence change
> > > > > applied once is not requested (and hence sent) again.
> > > > >
> > > >
> > > > I guess we could update the origin, per attached 0004. We don't have
> > > > timestamp to set replorigin_session_origin_timestamp, but it seems we
> > > > don't need that.
> > > >
> > > > The attached patch merges the earlier improvements, except for the part
> > > > that experimented with adding a "fake" transaction (which turned out to
> > > > have a number of difficult issues).
> > >
> > > 0004 looks good to me.
> >
> >
> > + {
> > CommitTransactionCommand();
> > +
> > + /*
> > + * Update origin state so we don't try applying this sequence
> > + * change in case of crash.
> > + *
> > + * XXX We don't have replorigin_session_origin_timestamp, but we
> > + * can just leave that set to 0.
> > + */
> > + replorigin_session_origin_lsn = seq.lsn;
> >
> > IIUC, your proposal is to update the replorigin_session_origin_lsn, so
> > that after restart, it doesn't use some prior origin LSN to start with
> > which can in turn lead the sequence to go backward. If so, it should
> > be updated before calling CommitTransactionCommand() as we are doing
> > in apply_handle_commit_internal(). If that is not the intention then
> > it is not clear to me how updating replorigin_session_origin_lsn after
> > commit is helpful.
> >
>
> typedef struct ReplicationState
> {
> ...
> /*
> * Location of the latest commit from the remote side.
> */
> XLogRecPtr remote_lsn;
>
> This is the variable that will be updated with the value of
> replorigin_session_origin_lsn. This means we will now track some
> arbitrary LSN location of the remote side in this variable. The above
> comment makes me wonder if there is anything we are missing or if it
> is just a matter of updating this comment because before the patch we
> always adhere to what is written in the comment.

I don't think we are missing anything. This value is used to track the
remote LSN upto which all the commits from upstream have been applied
locally. Since a non-transactional sequence change is like a single
WAL record transaction, it's LSN acts as the LSN of the mini-commit.
So it should be fine to update remote_lsn with sequence WAL record's
end LSN. That's what the patches do. I don't see any hazard. But you
are right, we need to update comments. Here and also at other places
like
replorigin_session_advance() which uses remote_commit as name of the
argument which gets assigned to ReplicationState::remote_lsn.

--
Best Wishes,
Ashutosh Bapat