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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>, 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: 2022-12-26T13:29:44Z
Lists: pgsql-hackers 
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(). 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.

-- 
With Regards,
Amit Kapila.

Commits

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  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.