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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: Peter Smith <smithpb2250@gmail.com>
Cc: "houzj.fnst@fujitsu.com" <houzj.fnst@fujitsu.com>, "wangw.fnst@fujitsu.com" <wangw.fnst@fujitsu.com>, Dilip Kumar <dilipbalaut@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>, "shiy.fnst@fujitsu.com" <shiy.fnst@fujitsu.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2022-10-18T09:56:51Z
Lists: pgsql-hackers 
On Tue, Oct 18, 2022 at 8:06 AM Peter Smith <smithpb2250@gmail.com> wrote:
>
> Hi, here are my review comments for patch v38-0001.
>
> 3.
>
> + /* Ensure we are reading the data into our memory context. */
> + oldctx = MemoryContextSwitchTo(ApplyMessageContext);
>
> Doesn't something need to switch back to this 'oldctx' prior to
> breaking out of the for(;;) loop?
>
> ~~~
>
> 4.
>
> + apply_dispatch(&s);
> +
> + MemoryContextReset(ApplyMessageContext);
>
> Isn't this broken now? Since you've removed the
> MemoryContextSwitchTo(oldctx), so next iteration will switch to
> ApplyMessageContext again which will overwrite and lose knowledge of
> the original 'oldctx' (??)
>
> ~~
>
> 5.
>
> Maybe this is a silly idea, I'm not sure. Because this is an infinite
> loop, then instead of the multiple calls to
> MemoryContextReset(ApplyMessageContext) maybe there can be just a
> single call to it immediately before you switch to that context in the
> first place. The effect will be the same, won't it?
>

I think so but I think it will look a bit odd, especially for the
first time. If the purpose is to just do it once, won't it be better
to do it at the end of for loop?

>
> 9. apply_handle_stream_start
>
> + *
> + * XXX We can avoid sending pairs of the START/STOP messages to the parallel
> + * worker because unlike apply worker it will process only one transaction at a
> + * time. However, it is not clear whether that is worth the effort because it
> + * is sent after logical_decoding_work_mem changes.
>   */
>  static void
>  apply_handle_stream_start(StringInfo s)
>
> As previously mentioned ([1] #13b) it's not obvious to me what that
> last sentence means. e.g. "because it is sent"  - what is "it"?
>

Here, it refers to START/STOP messages, so I think we should say "...
because these messages are sent .." instead of "... because it is sent
...". Does that makes sense to you?

-- 
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.