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

Wei Wang (Fujitsu) <wangw.fnst@fujitsu.com>

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From: "wangw.fnst@fujitsu.com" <wangw.fnst@fujitsu.com>
To: "houzj.fnst@fujitsu.com" <houzj.fnst@fujitsu.com>, Amit Kapila <amit.kapila16@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>
Cc: Peter Smith <smithpb2250@gmail.com>, "shiy.fnst@fujitsu.com" <shiy.fnst@fujitsu.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2022-05-25T02:24:59Z
Lists: pgsql-hackers

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On Fri, May 13, 2022 4:53 PM houzj.fnst@fujitsu.com wrote:
> On Wednesday, May 11, 2022 1:10 PM Amit Kapila <amit.kapila16@gmail.com>
> wrote:
> >
> > On Wed, May 11, 2022 at 9:35 AM Masahiko Sawada
> > <sawada.mshk@gmail.com> wrote:
> > >
> > > On Tue, May 10, 2022 at 5:59 PM Amit Kapila <amit.kapila16@gmail.com>
> > wrote:
> > > >
> > > > On Tue, May 10, 2022 at 10:39 AM Masahiko Sawada
> > <sawada.mshk@gmail.com> wrote:
> > > > >
> > > > > Having it optional seems a good idea. BTW can the user configure
> > > > > how many apply bgworkers can be used per subscription or in the
> > > > > whole system? Like max_sync_workers_per_subscription, is it better
> > > > > to have a configuration parameter or a subscription option for
> > > > > that? If so, setting it to 0 probably means to disable the parallel apply
> > feature.
> > > > >
> > > >
> > > > Yeah, that might be useful but we are already giving an option while
> > > > creating a subscription whether to allow parallelism, so will it be
> > > > useful to give one more way to disable this feature? OTOH, having
> > > > something like max_parallel_apply_workers/max_bg_apply_workers at
> > > > the system level can give better control for how much parallelism
> > > > the user wishes to allow for apply work.
> > >
> > > Or we can have something like
> > > max_parallel_apply_workers_per_subscription that controls how many
> > > parallel apply workers can launch per subscription. That also gives
> > > better control for the number of parallel apply workers.
> > >
> >
> > I think we can go either way in this matter as both have their pros and cons. I
> > feel limiting the parallel workers per subscription gives better control but
> > OTOH, it may not allow max usage of parallelism because some quota from
> > other subscriptions might remain unused. Let us see what Hou-San or others
> > think on this matter?
> 
> Thanks for Amit and Sawada-san's comments !
> I will think over these approaches and reply soon.
After reading the thread, I wrote two patches for these comments.

The first patch (see v6-0003):
Improve the feature as suggested in [1].
For the issue mentioned by Amit-san (there is a block problem in the case
mentioned by Sawada-san), after investigating, I think this issue is caused by
unique index. So I added a check to make sure the unique columns are the same
between publisher and subscriber.
For other cases, I added the check that if there is any non-immutable function
present in expression in subscriber's relation. Check from the following 3
items:
    a. The function in triggers;
    b. Column default value expressions and domain constraints;
    c. Constraint expressions.
BTW, I do not add partitioned table related code. I think this part needs other
additional modifications. I will add this later when these modifications are
finished.

The second patch (see v6-0004):
Improve the feature as suggested in [2].
Add a GUC "max_apply_bgworkers_per_subscription" to control parallelism. This
GUC controls how many apply background workers can be launched per
subscription. I set its default value to 3 and do not change the default value
of other GUCs.

[1] - https://www.postgresql.org/message-id/CAA4eK1JwahU_WuP3S%2B7POqta%3DPhm_3gxZeVmJuuoUq1NV%3DkrXA%40mail.gmail.com
[2] - https://www.postgresql.org/message-id/CAA4eK1%2B7D4qAQUQEE8zzQ0fGCqeBWd3rzTaY5N0jVs-VXFc_Xw%40mail.gmail.com

Attach the patches. (Did not change v6-0001 and v6-0002.)

Regards,
Wang wei

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.