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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: "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: 2022-10-12T06:04:43Z
Lists: pgsql-hackers 
On Tue, Oct 11, 2022 at 5:52 AM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>
> On Fri, Oct 7, 2022 at 2:00 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > About your point that having different partition structures for
> > publisher and subscriber, I don't know how common it will be once we
> > have DDL replication. Also, the default value of
> > publish_via_partition_root is false which doesn't seem to indicate
> > that this is a quite common case.
>
> So how can we consider these concurrent issues that could happen only
> when streaming = 'parallel'? Can we restrict some use cases to avoid
> the problem or can we have a safeguard against these conflicts?
>

Yeah, right now the strategy is to disallow parallel apply for such
cases as you can see in *0003* patch.

> We
> could find a new problematic scenario in the future and if it happens,
> logical replication gets stuck, it cannot be resolved only by apply
> workers themselves.
>

I think users can change streaming option to on/off and internally the
parallel apply worker can detect and restart to allow replication to
proceed. Having said that, I think that would be a bug in the code and
we should try to fix it. We may need to disable parallel apply in the
problematic case.

The other ideas that occurred to me in this regard are (a) provide a
reloption (say parallel_apply) at table level and we can use that to
bypass various checks like different Unique Key between
publisher/subscriber, constraints/expressions having mutable
functions, Foreign Key (when enabled on subscriber), operations on
Partitioned Table. We can't detect whether those are safe or not
(primarily because of a different structure in publisher and
subscriber) so we prohibit parallel apply but if users use this
option, we can allow it even in those cases. (b) While enabling the
parallel option in the subscription, we can try to match all the
table(s) information of the publisher/subscriber. It will be tricky to
make this work because say even if match some trigger function name,
we won't be able to match the function body. The other thing is when
at a later point the table definition is changed on the subscriber, we
need to again validate the information between publisher and
subscriber which I think would be difficult as we would be already in
between processing some message and getting information from the
publisher at that stage won't be possible.

Thoughts?

-- 
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.