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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>, "shiy.fnst@fujitsu.com" <shiy.fnst@fujitsu.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2022-05-02T09:09:36Z
Lists: pgsql-hackers 
On Mon, May 2, 2022 at 11:47 AM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>
> On Fri, Apr 8, 2022 at 6:14 PM houzj.fnst@fujitsu.com
> <houzj.fnst@fujitsu.com> wrote:
> >
> > On Wednesday, April 6, 2022 1:20 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > > In this email, I would like to discuss allowing streaming logical
> > > transactions (large in-progress transactions) by background workers
> > > and parallel apply in general. The goal of this work is to improve the
> > > performance of the apply work in logical replication.
> > >
> > > Currently, for large transactions, the publisher sends the data in
> > > multiple streams (changes divided into chunks depending upon
> > > logical_decoding_work_mem), and then on the subscriber-side, the apply
> > > worker writes the changes into temporary files and once it receives
> > > the commit, it read from the file and apply the entire transaction. To
> > > improve the performance of such transactions, we can instead allow
> > > them to be applied via background workers. There could be multiple
> > > ways to achieve this:
> > >
> > > Approach-1: Assign a new bgworker (if available) as soon as the xact's
> > > first stream came and the main apply worker will send changes to this
> > > new worker via shared memory. We keep this worker assigned till the
> > > transaction commit came and also wait for the worker to finish at
> > > commit. This preserves commit ordering and avoid writing to and
> > > reading from file in most cases. We still need to spill if there is no
> > > worker available. We also need to allow stream_stop to complete by the
> > > background worker to finish it to avoid deadlocks because T-1's
> > > current stream of changes can update rows in conflicting order with
> > > T-2's next stream of changes.
> > >
> >
> > Attach the POC patch for the Approach-1 of "Perform streaming logical
> > transactions by background workers". The patch is still a WIP patch as
> > there are serval TODO items left, including:
> >
> > * error handling for bgworker
> > * support for SKIP the transaction in bgworker
> > * handle the case when there is no more worker available
> >   (might need spill the data to the temp file in this case)
> > * some potential bugs
>
> Are you planning to support "Transaction dependency" Amit mentioned in
> his first mail in this patch? IIUC since the background apply worker
> applies the streamed changes as soon as receiving them from the main
> apply worker, a conflict that doesn't happen in the current streaming
> logical replication could happen.
>

This patch seems to be waiting for stream_stop to finish, so I don't
see how the issues related to "Transaction dependency" can arise? What
type of conflict/issues you have in mind?


-- 
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.