Re: Conflict detection for update_deleted in logical replication

On Thu, Jan 30, 2025 at 10:39 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Fri, Jan 31, 2025 at 4:10 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> >
> > I have one question about the 0004 patch; it implemented
> > max_conflict_retntion_duration as a subscription parameter. But the
> > launcher invalidates the pg_conflict_detection slot only if all
> > subscriptions with retain_conflict_info stopped retaining dead tuples
> > due to the max_conflict_retention_duration parameter. Therefore, even
> > if users set the parameter to a low value to avoid table bloats, it
> > would not make sense if other subscriptions set it to a larger value.
> > Is my understanding correct?
> >
>
> Yes, your understanding is correct. I think this could be helpful
> during resolution because the worker for which the duration has
> exceeded cannot detect conflicts reliably but others can. So, this
> info can be useful while performing resolutions. Do you have an
> opinion/suggestion on this matter?

I imagined a scenario like where two apply workers are running and
have different max_conflict_retention_duration values (say '5 min' and
'15 min'). Suppose both workers are roughly the same behind the
publisher(s), when both workers cannot advance the workers' xmin
values for 5 min or longer, one worker stops retaining dead tuples.
However, the pg_conflict_detection slot is not invalidated yet since
another worker is still using it, so both workers would continue to be
getting slower. The subscriber would end up retaining dead tuples
until both workers are behind for 15 min or longer, before
invalidating the slot. In this case, stopping dead tuple retention on
the first worker would help neither advance the slot's xmin nor
improve another worker's performance. I was not sure of the point of
making the max_conflict_retention_duration a per-subscription
parameter.

Regards,

--
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com