RE: Conflict detection and logging in logical replication

On Wednesday, August 14, 2024 10:15 PM Michail Nikolaev <michail(dot)nikolaev(at)gmail(dot)com> wrote:
> > This is as expected, and we have documented this in the code comments. We don't
> > need to report a conflict if the conflicting tuple has been removed or updated
> > due to concurrent transaction. The same is true if the transaction that
> > inserted the conflicting tuple is rolled back before CheckAndReportConflict().
> > We don't consider such cases as a conflict.
>
> That seems a little bit strange to me.
>
> From the perspective of a user, I expect that if a change from publisher is not
> applied - I need to know about it from the logs.

I think this is exactly the current behavior in the patch. In the race
condition we discussed, the insert will be applied if the conflicting tuple is
removed concurrently before CheckAndReportConflict().

> But in that case, I will not see any information about conflict in the logs
> in SOME cases. But in OTHER cases I will see it. However, in both cases the
> change from publisher was not applied. And these cases are just random and
> depend on the timing of race conditions. It is not something I am expecting
> from the database.

I think you might misunderstand the behavior of CheckAndReportConflict(), even
if it found a conflict, it still inserts the tuple into the index which means
the change is anyway applied.

Best Regards,
Hou zj