RE: Perform streaming logical transactions by background workers and parallel apply

On Friday, January 13, 2023 1:43 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> On Thu, Jan 12, 2023 at 9:34 PM houzj.fnst@fujitsu.com
> <houzj.fnst@fujitsu.com> wrote:
> >
> > On Thursday, January 12, 2023 7:08 PM Amit Kapila
> <amit.kapila16@gmail.com> wrote:
> > >
> > > On Thu, Jan 12, 2023 at 4:21 PM shveta malik <shveta.malik@gmail.com>
> wrote:
> > > >
> > > > On Thu, Jan 12, 2023 at 10:34 AM Amit Kapila
> > > > <amit.kapila16@gmail.com>
> > > wrote:
> > > > >
> > > > > On Thu, Jan 12, 2023 at 9:54 AM Peter Smith
> > > > > <smithpb2250@gmail.com>
> > > wrote:
> > > > > >
> > > > > >
> > > > > > doc/src/sgml/monitoring.sgml
> > > > > >
> > > > > > 5. pg_stat_subscription
> > > > > >
> > > > > > @@ -3198,11 +3198,22 @@ SELECT pid, wait_event_type,
> > > > > > wait_event FROM pg_stat_activity WHERE wait_event i
> > > > > >
> > > > > >       <row>
> > > > > >        <entry role="catalog_table_entry"><para
> > > > > > role="column_definition">
> > > > > > +       <structfield>apply_leader_pid</structfield>
> > > <type>integer</type>
> > > > > > +      </para>
> > > > > > +      <para>
> > > > > > +       Process ID of the leader apply worker, if this process is a
> apply
> > > > > > +       parallel worker. NULL if this process is a leader apply worker
> or a
> > > > > > +       synchronization worker.
> > > > > > +      </para></entry>
> > > > > > +     </row>
> > > > > > +
> > > > > > +     <row>
> > > > > > +      <entry role="catalog_table_entry"><para
> > > > > > + role="column_definition">
> > > > > >         <structfield>relid</structfield> <type>oid</type>
> > > > > >        </para>
> > > > > >        <para>
> > > > > >         OID of the relation that the worker is synchronizing; null for
> the
> > > > > > -       main apply worker
> > > > > > +       main apply worker and the parallel apply worker
> > > > > >        </para></entry>
> > > > > >       </row>
> > > > > >
> > > > > > 5a.
> > > > > >
> > > > > > (Same as general comment #1 about terminology)
> > > > > >
> > > > > > "apply_leader_pid" --> "leader_apply_pid"
> > > > > >
> > > > >
> > > > > How about naming this as just leader_pid? I think it could be
> > > > > helpful in the future if we decide to parallelize initial sync
> > > > > (aka parallel
> > > > > copy) because then we could use this for the leader PID of
> > > > > parallel sync workers as well.
> > > > >
> > > > > --
> > > >
> > > > I still prefer leader_apply_pid.
> > > > leader_pid does not tell which 'operation' it belongs to. 'apply'
> > > > gives the clarity that it is apply related process.
> > > >
> > >
> > > But then do you suggest that tomorrow if we allow parallel sync
> > > workers then we have a separate column leader_sync_pid? I think that
> > > doesn't sound like a good idea and moreover one can refer to docs for
> clarification.
> >
> > I agree that leader_pid would be better not only for future parallel
> > copy sync feature, but also it's more consistent with the leader_pid column in
> pg_stat_activity.
> >
> > And here is the version patch which addressed Peter's comments and
> > renamed all the related stuff to leader_pid.
> 
> Here are two comments on v79-0003 patch.

Thanks for the comments.

> 
> +        /* Force to serialize messages if stream_serialize_threshold
> is reached. */
> +        if (stream_serialize_threshold != -1 &&
> +                (stream_serialize_threshold == 0 ||
> +                 stream_serialize_threshold < parallel_stream_nchunks))
> +        {
> +                parallel_stream_nchunks = 0;
> +                return false;
> +        }
> 
> I think it would be better if we show the log message ""logical replication apply
> worker will serialize the remaining changes of remote transaction %u to a file"
> even in stream_serialize_threshold case.

Agreed and changed.

> 
> IIUC parallel_stream_nchunks won't be reset if pa_send_data() failed due to the
> timeout.

Changed.

Best Regards,
Hou zj