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

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: Masahiko Sawada <sawada.mshk@gmail.com>
Cc: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, Alexander Lakhin <exclusion@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2023-05-08T06:33:48Z
Lists: pgsql-hackers 
On Mon, May 8, 2023 at 11:08 AM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>
> On Mon, May 8, 2023 at 12:52 PM Zhijie Hou (Fujitsu)
> <houzj.fnst@fujitsu.com> wrote:
> >
> > On Monday, May 8, 2023 11:08 AM Masahiko Sawada <sawada.mshk@gmail.com>
> >
> > Hi,
> >
> > >
> > > On Tue, May 2, 2023 at 12:22 PM Amit Kapila <amit.kapila16@gmail.com>
> > > wrote:
> > > >
> > > > On Fri, Apr 28, 2023 at 11:48 AM Masahiko Sawada
> > > <sawada.mshk@gmail.com> wrote:
> > > > >
> > > > > While investigating this issue, I've reviewed the code around
> > > > > callbacks and worker termination etc and I found a problem.
> > > > >
> > > > > A parallel apply worker calls the before_shmem_exit callbacks in the
> > > > > following order:
> > > > >
> > > > > 1. ShutdownPostgres()
> > > > > 2. logicalrep_worker_onexit()
> > > > > 3. pa_shutdown()
> > > > >
> > > > > Since the worker is detached during logicalrep_worker_onexit(),
> > > > > MyLogicalReplication->leader_pid is an invalid when we call
> > > > > pa_shutdown():
> > > > >
> > > > > static void
> > > > > pa_shutdown(int code, Datum arg)
> > > > > {
> > > > >     Assert(MyLogicalRepWorker->leader_pid != InvalidPid);
> > > > >     SendProcSignal(MyLogicalRepWorker->leader_pid,
> > > > >                    PROCSIG_PARALLEL_APPLY_MESSAGE,
> > > > >                    InvalidBackendId);
> > > > >
> > > > > Also, if the parallel apply worker fails shm_toc_lookup() during the
> > > > > initialization, it raises an error (because of noError = false) but
> > > > > ends up a SEGV as MyLogicalRepWorker is still NULL.
> > > > >
> > > > > I think that we should not use MyLogicalRepWorker->leader_pid in
> > > > > pa_shutdown() but instead store the leader's pid to a static variable
> > > > > before registering pa_shutdown() callback.
> > > > >
> > > >
> > > > Why not simply move the registration of pa_shutdown() to someplace
> > > > after logicalrep_worker_attach()?
> > >
> > > If we do that, the worker won't call dsm_detach() if it raises an
> > > ERROR in logicalrep_worker_attach(), is that okay? It seems that it's
> > > no practically problem since we call dsm_backend_shutdown() in
> > > shmem_exit(), but if so why do we need to call it in pa_shutdown()?
> >
> > I think the dsm_detach in pa_shutdown was intended to fire on_dsm_detach
> > callbacks to give callback a chance to report stat before the stat system is
> > shutdown, following what we do in ParallelWorkerShutdown() (e.g.
> > sharedfileset.c callbacks cause fd.c to do ReportTemporaryFileUsage(), so we
> > need to fire that earlier).
> >
> > But for parallel apply, we currently only have one on_dsm_detach
> > callback(shm_mq_detach_callback) which doesn't report extra stats. So the
> > dsm_detach in pa_shutdown is only used to make it a bit future-proof in case
> > we add some other on_dsm_detach callbacks in the future which need to report
> > stats.
>
> Make sense . Given that it's possible that we add other callbacks that
> report stats in the future, I think it's better not to move the
> position to register pa_shutdown() callback.
>

Hmm, what kind of stats do we expect to be collected before we
register pa_shutdown? I think if required we can register such a
callback after pa_shutdown. I feel without reordering the callbacks,
the fix would be a bit complicated as explained in my previous email,
so I don't think it is worth complicating this code unless really
required.

-- 
With Regards,
Amit Kapila.

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.