On Mon, Jul 31, 2023 at 10:47 AM Peter Smith <smithpb2250@gmail.com> wrote:
>
> Hi hackers,
>
> BACKGROUND:
>
> The logical replication has different worker "types" (e.g. apply
> leader, apply parallel, tablesync).
>
> They all use a common structure called LogicalRepWorker, but at times
> it is necessary to know what "type" of worker a given LogicalRepWorker
> represents.
>
> Once, there were just apply workers and tablesync workers - these were
> easily distinguished because only tablesync workers had a valid
> 'relid' field. Next, parallel-apply workers were introduced - these
> are distinguishable from the apply leaders by the value of
> 'leader_pid' field.
>
>
> PROBLEM:
>
> IMO, deducing the worker's type by examining multiple different field
> values seems a dubious way to do it. This maybe was reasonable enough
> when there were only 2 types, but as more get added it becomes
> increasingly complicated.
>
> SOLUTION:
>
> AFAIK none of the complications is necessary anyway; the worker type
> is already known at launch time, and it never changes during the life
> of the process.
>
> The attached Patch 0001 introduces a new enum 'type' field, which is
> assigned during the launch.
>
> ~
>
> This change not only simplifies the code, but it also permits other
> code optimizations, because now we can switch on the worker enum type,
> instead of using cascading if/else. (see Patch 0002).
>
> Thoughts?
I can see the problem you stated and it's true that the worker's type
never changes during its lifetime. But I'm not sure we really need to
add a new variable to LogicalRepWorker since we already have enough
information there to distinguish the worker's type.
Currently, we deduce the worker's type by checking some fields that
never change such as relid and leader_piid. It's fine to me as long as
we encapcel the deduction of worker's type by macros (or inline
functions). Even with the proposed patch (0001 patch), we still need a
similar logic to determine the worker's type.
If we want to change both process_syncing_tables() and
should_apply_changes_for_rel() to use the switch statement instead of
using cascading if/else, I think we can have a function, say
LogicalRepWorkerType(), that returns LogicalRepWorkerType of the given
worker.
Regards,
--
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com
