Re: Conflict detection for update_deleted in logical replication

On Tue, Sep 24, 2024 at 2:35 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> >
> > As each apply worker needs a separate slot to retain deleted rows, the
> > requirement for slots will increase. The other possibility is to
> > maintain one slot by launcher or some other central process that
> > traverses all subscriptions, remember the ones marked with
> > retain_dead_rows (let's call this list as retain_sub_list). Then using
> > running_transactions get the oldest running_xact, and then get the
> > remote flush location from the other node (publisher node) and store
> > those as candidate values (candidate_xmin and
> > candidate_remote_wal_lsn) in slot. We can probably reuse existing
> > candidate variables of the slot. Next, we can check the remote_flush
> > locations from all the origins corresponding in retain_sub_list and if
> > all are ahead of candidate_remote_wal_lsn, we can update the slot's
> > xmin to candidate_xmin.
>
> Does it mean that we use one candiate_remote_wal_lsn in a slot for all
> subscriptions (in retain_sub_list)? IIUC candiate_remote_wal_lsn is a
> LSN of one of publishers, so other publishers could have completely
> different LSNs. How do we compare the candidate_remote_wal_lsn to
> remote_flush locations from all the origins?
>

This should be an array/list with one element per publisher. We can
copy candidate_xmin to actual xmin only when the
candiate_remote_wal_lsn's corresponding to all publishers have been
applied aka their remote_flush locations (present in origins) are
ahead. The advantages I see with this are (a) reduces the number of
slots required to achieve the retention of deleted rows for conflict
detection, (b) in some cases we can avoid sending messages to the
publisher because with this we only need to send message to a
particular publisher once rather than by all the apply workers
corresponding to same publisher node.

--
With Regards,
Amit Kapila.