Horton
Menu

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

Masahiko Sawada <sawada.mshk@gmail.com>

View thread
From: Masahiko Sawada <sawada.mshk@gmail.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: "houzj.fnst@fujitsu.com" <houzj.fnst@fujitsu.com>, "wangw.fnst@fujitsu.com" <wangw.fnst@fujitsu.com>, Peter Smith <smithpb2250@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>, "shiy.fnst@fujitsu.com" <shiy.fnst@fujitsu.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2022-12-07T04:39:24Z
Lists: pgsql-hackers 
On Wed, Dec 7, 2022 at 1:29 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Wed, Dec 7, 2022 at 9:00 AM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> >
> > On Thu, Dec 1, 2022 at 7:17 PM houzj.fnst@fujitsu.com
> > <houzj.fnst@fujitsu.com> wrote:
> > >
> > > > ---
> > > > if (am_parallel_apply_worker() && on_subinfo_change) {
> > > >     /*
> > > >      * If a parallel apply worker exits due to the subscription
> > > >      * information change, we notify the leader apply worker so that the
> > > >      * leader can report more meaningful message in time and restart the
> > > >      * logical replication.
> > > >      */
> > > >     pq_putmessage('X', NULL, 0);
> > > > }
> > > >
> > > > and
> > > >
> > > > ereport(ERROR,
> > > >                 (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
> > > >                  errmsg("logical replication parallel apply worker exited
> > > > because of subscription information change")));
> > > >
> > > > Do we really need an additional message in case of 'X'? When we call
> > > > apply_worker_clean_exit with on_subinfo_change = true, we have reported the
> > > > error message such as:
> > > >
> > > > ereport(LOG,
> > > >         (errmsg("logical replication parallel apply worker for subscription
> > > > \"%s\" will stop because of a parameter change",
> > > >                 MySubscription->name)));
> > > >
> > > > I think that reporting a similar message from the leader might not be
> > > > meaningful for users.
> > >
> > > The intention is to let leader report more meaningful message if a worker
> > > exited due to subinfo change. Otherwise, the leader is likely to report an
> > > error like " lost connection ... to parallel apply worker" when trying to send
> > > data via shared memory if the worker exited. What do you think ?
> >
> > Agreed. But do we need to have the leader exit with an error in spite
> > of the fact that the worker cleanly exits? If the leader exits with an
> > error, the subscription will be disabled if disable_on_error is true,
> > right?
> >
>
> Right, but the leader will anyway exit at some point either due to an
> ERROR like "lost connection ... to parallel worker" or with a LOG
> like: "... will restart because of a parameter change" but I see your
> point. So, will it be better if we have a LOG message here and then
> proc_exit()? Do you have something else in mind for this?

No, I was thinking that too. It's better to write a LOG message and do
proc_exit().

Regarding the error "lost connection ... to parallel worker", it could
still happen depending on the timing even if the parallel worker
cleanly exits due to parameter changes, right? If so, I'm concerned
that it could lead to disable the subscription unexpectedly if
disable_on_error is enabled.

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.