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

Masahiko Sawada <sawada.mshk@gmail.com>

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From: Masahiko Sawada <sawada.mshk@gmail.com>
To: "houzj.fnst@fujitsu.com" <houzj.fnst@fujitsu.com>
Cc: Amit Kapila <amit.kapila16@gmail.com>, shveta malik <shveta.malik@gmail.com>, Peter Smith <smithpb2250@gmail.com>, "wangw.fnst@fujitsu.com" <wangw.fnst@fujitsu.com>, "shiy.fnst@fujitsu.com" <shiy.fnst@fujitsu.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, Dilip Kumar <dilipbalaut@gmail.com>
Date: 2023-01-13T05:43:21Z
Lists: pgsql-hackers 
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.

+        /* 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

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Fix invalid memory access during the shutdown of the parallel apply worker.

  2. Fix assertion failure in apply worker.

  3. Use elog to report unexpected action in handle_streamed_transaction().

  4. Use appropriate wait event when sending data in the apply worker.

  5. Allow the logical_replication_mode to be used on the subscriber.

  6. Rename GUC logical_decoding_mode to logical_replication_mode.

  7. Display the leader apply worker's PID for parallel apply workers.

  8. Improve the code to decide and process the apply action.

  9. Document the newly added wait events added by commit 216a784829.

  10. Perform apply of large transactions by parallel workers.

  11. Wake up a subscription's replication worker processes after DDL.

  12. Add copyright notices to meson files

  13. Better document logical replication parameters

  14. Add a common function to generate the origin name.

  15. Harmonize parameter names in storage and AM code.

  16. Avoid using list_length() to test for empty list.

  17. Improve two comments related to a boolean DefElem's value

  18. Fix partition table's REPLICA IDENTITY checking on the subscriber.

  19. Fix data inconsistency between publisher and subscriber.

  20. Fix cache look-up failures while applying changes in logical replication.