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

Peter Smith <smithpb2250@gmail.com>

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From: Peter Smith <smithpb2250@gmail.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: "wangw.fnst@fujitsu.com" <wangw.fnst@fujitsu.com>, "houzj.fnst@fujitsu.com" <houzj.fnst@fujitsu.com>, Dilip Kumar <dilipbalaut@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>, "shiy.fnst@fujitsu.com" <shiy.fnst@fujitsu.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2022-08-23T04:01:52Z
Lists: pgsql-hackers 
On Mon, Aug 22, 2022 at 7:01 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Mon, Aug 22, 2022 at 4:42 AM Peter Smith <smithpb2250@gmail.com> wrote:
> >
> > On Fri, Aug 19, 2022 at 7:55 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> > >
> > > On Fri, Aug 19, 2022 at 3:05 PM Peter Smith <smithpb2250@gmail.com> wrote:
> > > >
> > > > On Fri, Aug 19, 2022 at 7:10 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> > > > >
> > > > > On Fri, Aug 19, 2022 at 2:36 PM Peter Smith <smithpb2250@gmail.com> wrote:
> > > > > >
> > > > > > Here are my review comments for the v23-0005 patch:
> > > > > >
> > > > > > ======
> > > > > >
> > > > > > Commit Message says:
> > > > > > main_worker_pid is Process ID of the main apply worker, if this process is a
> > > > > > apply background worker. NULL if this process is a main apply worker or a
> > > > > > synchronization worker.
> > > > > > The new column can make it easier to distinguish main apply worker and apply
> > > > > > background worker.
> > > > > >
> > > > > > --
> > > > > >
> > > > > > Having a column called ‘main_worker_pid’ which is defined to be NULL
> > > > > > if the process *is* the main apply worker does not make any sense to
> > > > > > me.
> > > > > >
> > > > >
> > > > > I haven't read this part of a patch but it seems to me we have
> > > > > something similar for parallel query workers. Refer 'leader_pid'
> > > > > column in pg_stat_activity.
> > > > >
> > > >
> > > > IIUC (from the patch 0005 commit message) the intention is to be able
> > > > to easily distinguish the worker types.
> > > >
> > >
> > > I think it is only to distinguish between leader apply worker and
> > > background apply workers. The tablesync worker can be distinguished
> > > based on relid field.
> > >
> >
> > Right. But that's the reason for my question in the first place - why
> > implement the patch so that the user still has to jump through hoops
> > just to know the worker type information?
> >
>
> I think it is not only to judge worker type but also to know the pid
> of each of the workers during parallel apply. Isn't it better to have
> both main apply worker pid and parallel apply worker pid as we have
> for the parallel query system?
>

OK, thanks for pointing me to that other view. Now that I see the
existing pg_stat_activity already has 'pid' and 'leader_pid' [1], it
suddenly seems more reasonable to do similar for this
pg_stat_subscription.

This background information needs to be conveyed better in the patch
0005 commit message. The current commit message said nothing about
trying to be consistent with the existing stats views; it only says
this field was added to distinguish more easily between the types of
apply workers.

------
[1] https://www.postgresql.org/docs/devel/monitoring-stats.html

Kind Regards,
Peter Smith.
Fujitsu Australia

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.