Re: Conflict detection for update_deleted in logical replication

On Fri, May 16, 2025 at 11:15 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Fri, Apr 25, 2025 at 4:06 PM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
> >
> > 2)
> > in wait_for_local_flush(), we have
> > should_stop_conflict_info_retention() before 'AllTablesyncsReady'
> > check. Should we give a discount for table-sync time and avoid doing
> > stop-conflict-retention when table-sync is going on? This is because
> > table-sync is one time operation (or done only on
> > subscription-refresh), so we shall not count time spent in table-sync
> > for 'max_conflict_retention_duration'. We can reset our timer if
> > table-sync is observed to be going on. Thoughts?
> >
>
> Sounds reasonable to me.
>
> >
> > 3)
> > In get_candidate_xid(), we first set candidate_xid_time and later
> > candidate_xid. And between these 2 there are chances that we return
> > without updating candidate_xid. See 'Return if the
> > oldest_nonremovable_xid cannot be advanced ' comment. That will leave
> > 'candidate_xid_time' set to new value while 'candidate_xid' is not
> > yet set.
> >
>
> Good point. I think we should set 'candidate_xid_time' along with
> candidate_xid (just after setting candidate_xid).
>
> > 4)
> > Do you think there should be some relation between
> > 'xid_advance_interval' and 'max_conflict_retention_duration'? Should
> > max of 'xid_advance_interval' be limited by
> > 'max_conflict_retention_duration'. Currently say
> > xid_advance_interval' is set to max 3 mins, what if
> > 'max_conflict_retention_duration' is set to 2 mins? In that case we
> > will not even check for new xids before 3 mins are over, while
> > 'max_conflict_retention_duration' sets a limit of 2 mins for dead
> > tuples retention.
> >
>
> Right, ideally, the 'xid_advance_interval' should be set to a value
> less than 'max_conflict_retention_duration' when no new_xid is found.
>
> 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. I think it is already happening
in get_candidate_xid(). It updates the timer but not the xid (my
concern #3 can be ignored then).

thanks
Shveta