Re: Add 'worker_type' to pg_stat_subscription

On Mon, Sep 18, 2023 at 04:56:46PM +1000, Peter Smith wrote:
> On Mon, Sep 18, 2023 at 1:43 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>> One simple idea to reduce confusion could be to use (leader) in the
>> above two places. Do you see any other place which could be confusing
>> and what do you suggest to fix it?
>
> IIRC we first encountered this problem with the parallel apply workers
> were introduced -- "leader" was added wherever we needed to
> distinguish the main apply and the parallel apply worker. Perhaps at
> that time, we ought to have changed it *everywhere* instead of
> changing only the ambiguous places. Lately, I've been thinking it
> would have been easier to have *one* rule and always call the (main)
> apply worker the "leader apply" worker -- simply because 2 names
> ("leader apply" and "parallel apply") are easier to explain than 3
> names.
>
> A "leader apply" worker with no "parallel apply" workers is a bit like
> the "boss" of a company that has no employees -- IMO it's OK to still
> say that they are the "boss".

From the latest discussion, it sounds like you (Peter and Amit) are leaning
more towards something like the v7 patch [0]. I'm okay with that. Perhaps
it'd be worth starting a new thread after this one to make the terminology
consistent in the docs, error messages, views, etc. Fortunately, we have
some time to straighten this out for v17.

> Regardless, I think changing this in other docs and other code is
> outside the scope of this simple pg stats patch -- here we can just
> change the relevant config docs and the stats attribute value to
> "leader apply" and leave it at that.
>
> Changing every other place to consistently say "leader apply" is a
> bigger task for another thread because we will find lots more places
> to change. For example, there are messages like: "logical replication
> apply worker for subscription \"%s\" has started" that perhaps should
> say "logical replication leader apply worker for subscription \"%s\"
> has started". Such changes don't belong in this stats patch.

+1

[0] https://postgr.es/m/attachment/150345/v7-0001-Add-worker-type-to-pg_stat_subscription.patch

--
Nathan Bossart
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