Re: Conflict detection for update_deleted in logical replication

On Fri, May 16, 2025 at 2:40 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Fri, May 16, 2025 at 12:01 PM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
> >
> > On Fri, May 16, 2025 at 11:15 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > >
> > >
> > > BTW, another related point is that when we decide to stop retaining
> > > dead tuples (via should_stop_conflict_info_retention), should we also
> > > consider the case that the apply worker didn't even try to get the
> > > publisher status because previously it decided that
> > > oldest_nonremovable_xid cannot be advanced due to its
> > > OldestActiveTransactionId?
> > >
> >
> > Do you mean avoid stop-conflict-retention in such a case as apply
> > worker itself did not request status from the publisher? If I
> > understood your point correctly, then we can do that by advancing the
> > timer to a new value even if we did not update candidate-xid and did
> > not ask the status from the publisher.
> >
>
> But candidate_xid_time is also used in wait_for_local_flush() to check
> clock_skew between publisher and subscriber, so for that purpose, it
> is better to set it along with candidate_xid. However, can't we rely
> on the valid value of candidate_xid to ensure that apply worker didn't
> send any request? Note that we always reset candidate_xid once we have
> updated oldest_nonremovable_xid.
>

I think this is automatically taken care of because we call
should_stop_conflict_info_retention() only during 'wait' phase, which
should be done after candidate_xid is set. Having said that, we should
have assert for candidate_xid in should_stop_conflict_info_retention()
and also add in comments that it should be called only during the
'wait' phase. Additionally, we can also have an assert that
should_stop_conflict_info_retention() is called only during the 'wait'
phase.

--
With Regards,
Amit Kapila.