Re: Conflict detection for update_deleted in logical replication - Mailing list pgsql-hackers

On Wed, Apr 16, 2025 at 10:30 AM shveta malik <shveta.malik@gmail.com> wrote:
>
> On Wed, Mar 26, 2025 at 4:17 PM Zhijie Hou (Fujitsu)
> <houzj.fnst@fujitsu.com> wrote:
> >
> > Here's a rebased version of the patch series.
> >
>

Few comments for patch004:
Config.sgml:
1)
+       <para>
+        Maximum duration (in milliseconds) for which conflict
+        information can be retained for conflict detection by the apply worker.
+        The default value is <literal>0</literal>, indicating that conflict
+        information is retained until it is no longer needed for detection
+        purposes.
+       </para>

IIUC, the above is not entirely accurate. Suppose the subscriber
manages to catch up and sets oldest_nonremovable_xid to 100, which is
then updated in slot. After this, the apply worker takes a nap and
begins a new xid update cycle. Now, let’s say the next candidate_xid
is 200, but this time the subscriber fails to keep up and exceeds
max_conflict_retention_duration. As a result, it sets
oldest_nonremovable_xid to InvalidTransactionId, and the launcher
skips updating the slot’s xmin. However, the previous xmin value (100)
is still there in the slot, causing its data to be retained beyond the
max_conflict_retention_duration. The xid 200 which actually honors
max_conflict_retention_duration was never marked for retention. If my
understanding is correct, then the documentation doesn’t fully capture
this scenario.


2)
+        The replication slot
+        <quote><literal>pg_conflict_detection</literal></quote> that used to
+        retain conflict information will be invalidated if all apply workers
+        associated with the subscription, where

Subscription --> subscriptions

3)
Name stop_conflict_retention in MyLogicalRepWorker is confusing. Shall
it be stop_conflict_info_retention?

thanks
Shveta