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

On Thur, Sep 22, 2022 at 18:12 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> Few comments on v33-0001
> =======================

Thanks for your comments.

> 1.
> + else if (data->streaming == SUBSTREAM_PARALLEL &&
> + data->protocol_version <
> LOGICALREP_PROTO_STREAM_PARALLEL_VERSION_NUM)
> + ereport(ERROR,
> + (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
> + errmsg("requested proto_version=%d does not support
> streaming=parallel mode, need %d or higher",
> + data->protocol_version,
> LOGICALREP_PROTO_STREAM_PARALLEL_VERSION_NUM)));
>
> I think we can improve this error message as: "requested
> proto_version=%d does not support parallel streaming mode, need %d or
> higher".

Improved.

> 2.
> --- a/doc/src/sgml/monitoring.sgml
> +++ b/doc/src/sgml/monitoring.sgml
> @@ -3184,7 +3184,7 @@ SELECT pid, wait_event_type, wait_event FROM
> pg_stat_activity WHERE wait_event i
> </para>
> <para>
> OID of the relation that the worker is synchronizing; null for the
> - main apply worker
> + main apply worker and the apply parallel worker
> </para></entry>
> </row>
>
> This and other changes in monitoring.sgml refers the workers as "apply
> parallel worker". Isn't it better to use parallel apply worker as we
> are using at other places in the patch? But, I have another question,
> do we really need to display entries for parallel apply workers in
> pg_stat_subscription if it doesn't have any meaningful information? I
> think we can easily avoid it in pg_stat_get_subscription by checking
> apply_leader_pid.

Make sense. Improved as suggested.
Do not display parallel apply worker related information in this view after
applying 0001 patch. But display entries for parallel apply worker after
applying 0005 patch.

> 3.
> ApplyWorkerMain()
> {
> ...
> ...
> +
> + if (server_version >= 160000 &&
> + MySubscription->stream == SUBSTREAM_PARALLEL)
> + options.proto.logical.streaming = pstrdup("parallel");
>
> After deciding here whether the parallel streaming mode is enabled or
> not, we recheck the same thing in apply_handle_stream_abort() and
> parallel_apply_can_start(). In parallel_apply_can_start(), we do it
> via two different checks. How about storing this information say in
> structure MyLogicalRepWorker in ApplyWorkerMain() and then use it at
> other places?

Improved as suggested.
Added a new flag "in_parallel_apply" to structure MyLogicalRepWorker.

Because the patch set could not be applied cleanly, I rebased and shared them
for review.
I have not addressed the comment you posted in [1]. I will share the new patch
set when I finish them.

The new patches were attached in [2].

[1] - https://www.postgresql.org/message-id/CAA4eK1KjGNA8T8O77rRhkv6bRT6OsdQaEy--2hNrJFCc80bN0A%40mail.gmail.com
[2] - https://www.postgresql.org/message-id/OS3PR01MB6275F4A7CA186412E2FF2ED29E529%40OS3PR01MB6275.jpnprd01.prod.outlook.com

Regards,
Wang wei