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

Masahiko Sawada <sawada.mshk@gmail.com>

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From: Masahiko Sawada <sawada.mshk@gmail.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: Tomas Vondra <tomas.vondra@enterprisedb.com>, "houzj.fnst@fujitsu.com" <houzj.fnst@fujitsu.com>, "wangw.fnst@fujitsu.com" <wangw.fnst@fujitsu.com>, Peter Smith <smithpb2250@gmail.com>, "shiy.fnst@fujitsu.com" <shiy.fnst@fujitsu.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, Dilip Kumar <dilipbalaut@gmail.com>
Date: 2023-01-17T03:05:09Z
Lists: pgsql-hackers 
On Mon, Jan 16, 2023 at 3:19 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Sun, Jan 15, 2023 at 10:39 PM Tomas Vondra
> <tomas.vondra@enterprisedb.com> wrote:
> >
> > I think there's a bug in how get_transaction_apply_action() interacts
> > with handle_streamed_transaction() to decide whether the transaction is
> > streamed or not. Originally, the code was simply:
> >
> >     /* not in streaming mode */
> >     if (!in_streamed_transaction)
> >         return false;
> >
> > But now this decision was moved to get_transaction_apply_action(), which
> > does this:
> >
> >     if (am_parallel_apply_worker())
> >     {
> >         return TRANS_PARALLEL_APPLY;
> >     }
> >     else if (in_remote_transaction)
> >     {
> >         return TRANS_LEADER_APPLY;
> >     }
> >
> > and handle_streamed_transaction() then uses the result like this:
> >
> >     /* not in streaming mode */
> >     if (apply_action == TRANS_LEADER_APPLY)
> >         return false;
> >
> > Notice this is not equal to the original behavior, because the two flags
> > (in_remote_transaction and in_streamed_transaction) are not inverse.
> > That is,
> >
> >    in_remote_transaction=false
> >
> > does not imply we're processing streamed transaction. It's allowed both
> > flags are false, i.e. a change may be "non-transactional" and not
> > streamed, though the only example of such thing in the protocol are
> > logical messages. Which are however ignored in the apply worker, so I'm
> > not surprised no existing test failed on this.
> >
>
> Right, this is the reason we didn't catch it in our testing.
>
> > So I think get_transaction_apply_action() should do this:
> >
> >     if (am_parallel_apply_worker())
> >     {
> >         return TRANS_PARALLEL_APPLY;
> >     }
> >     else if (!in_streamed_transaction)
> >     {
> >         return TRANS_LEADER_APPLY;
> >     }
> >
>
> Yeah, something like this would work but some of the callers other
> than handle_streamed_transaction() also need to be changed. See
> attached.
>
> > FWIW I've noticed this after rebasing the sequence decoding patch, which
> > adds another type of protocol message with the transactional vs.
> > non-transactional behavior, similar to "logical messages" except that in
> > this case the worker does not ignore that.
> >
> > Also, I think get_transaction_apply_action() would deserve better
> > comments explaining how/why it makes the decisions.
> >
>
> Okay, I have added the comments in get_transaction_apply_action() and
> updated the comments to refer to the enum TransApplyAction where all
> the actions are explained.

Thank you for the patch.

@@ -1710,6 +1712,7 @@ apply_handle_stream_stop(StringInfo s)
        }

        in_streamed_transaction = false;
+       stream_xid = InvalidTransactionId;

We reset stream_xid also in stream_close_file() but probably it's no
longer necessary?

How about adding an assertion in apply_handle_stream_start() to make
sure the stream_xid is invalid?

---
It's not related to this issue but I realized that if the action
returned by get_transaction_apply_action() is not handled in the
switch statement, we do only Assert(false). Is it better to raise an
error like "unexpected apply action %d" just in case in order to
detect failure cases also in the production environment?


Regards,

--
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com

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.