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

On Thu, Jan 12, 2023 at 9:34 PM houzj(dot)fnst(at)fujitsu(dot)com
<houzj(dot)fnst(at)fujitsu(dot)com> wrote:
>
> On Thursday, January 12, 2023 7:08 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > On Thu, Jan 12, 2023 at 4:21 PM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
> > >
> > > On Thu, Jan 12, 2023 at 10:34 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
> > wrote:
> > > >
> > > > On Thu, Jan 12, 2023 at 9:54 AM Peter Smith <smithpb2250(at)gmail(dot)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.

+ /* 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.

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

Regards,

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