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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: Amit Kapila <amit.kapila16@gmail.com>
Cc: "houzj.fnst@fujitsu.com" <houzj.fnst@fujitsu.com>, Peter Smith <smithpb2250@gmail.com>, "wangw.fnst@fujitsu.com" <wangw.fnst@fujitsu.com>, Dilip Kumar <dilipbalaut@gmail.com>, "shiy.fnst@fujitsu.com" <shiy.fnst@fujitsu.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2022-12-27T05:57:55Z
Lists: pgsql-hackers 
On Mon, Dec 26, 2022 at 10:29 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Mon, Dec 26, 2022 at 6:33 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> >
> > ---
> > +        if (!pa_can_start(xid))
> > +                return;
> > +
> > +        /* First time through, initialize parallel apply worker state
> > hashtable. */
> > +        if (!ParallelApplyTxnHash)
> > +        {
> > +                HASHCTL                ctl;
> > +
> > +                MemSet(&ctl, 0, sizeof(ctl));
> > +                ctl.keysize = sizeof(TransactionId);
> > +                ctl.entrysize = sizeof(ParallelApplyWorkerEntry);
> > +                ctl.hcxt = ApplyContext;
> > +
> > +                ParallelApplyTxnHash = hash_create("logical
> > replication parallel apply workershash",
> > +
> >              16, &ctl,
> > +
> >              HASH_ELEM |HASH_BLOBS | HASH_CONTEXT);
> > +        }
> > +
> > +        /*
> > +         * It's necessary to reread the subscription information
> > before assigning
> > +         * the transaction to a parallel apply worker. Otherwise, the
> > leader may
> > +         * not be able to reread the subscription information if streaming
> > +         * transactions keep coming and are handled by parallel apply workers.
> > +         */
> > +        maybe_reread_subscription();
> >
> > pa_can_start() checks if the skiplsn is an invalid xid or not, and
> > then maybe_reread_subscription() could update the skiplsn to a valid
> > value. As the comments in pa_can_start() says, it won't work. I think
> > we should call maybe_reread_subscription() in
> > apply_handle_stream_start() before calling pa_allocate_worker().
> >
>
> But I think a similar thing can happen when we start the worker and
> then before the transaction ends, we do maybe_reread_subscription().

Where do we do maybe_reread_subscription() in this case? IIUC if the
leader sends all changes to the worker, there is no chance for the
leader to do maybe_reread_subscription except for when waiting for the
input. On reflection, adding maybe_reread_subscription() to
apply_handle_stream_start() adds one extra call of it so it's not
good. Alternatively, we can do that in pa_can_start() before checking
the skiplsn. I think we do a similar thing in AllTablesyncsRead() --
update the information before the check if necessary.

> I think we should try to call maybe_reread_subscription() when we are
> reasonably sure that we are going to enter parallel mode, otherwise,
> anyway, it will be later called by the leader worker.

It isn't a big problem even if we update the skiplsn after launching a
worker since we will skip the transaction the next time. But it would
be more consistent with the current behavior. As I mentioned above,
doing it in pa_can_start() seems to be reasonable to me. What do you
think?

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.