Re: [PATCH] Reuse Workers and Replication Slots during Logical Replication - Mailing list pgsql-hackers

On Thu, Jul 20, 2023 at 8:02 AM Peter Smith <smithpb2250@gmail.com> wrote:
>
> On Tue, Jul 18, 2023 at 1:54 AM Melih Mutlu <m.melihmutlu@gmail.com> wrote:
> >
> > Hi,
> >
> > PFA updated patches. Rebased 0003 with minor changes. Addressed Peter's reviews for 0001 and 0002 with some small
commentsbelow. 
> >
> > Peter Smith <smithpb2250@gmail.com>, 10 Tem 2023 Pzt, 10:09 tarihinde şunu yazdı:
> >>
> >> 6. LogicalRepApplyLoop
> >>
> >> + /*
> >> + * apply_dispatch() may have gone into apply_handle_commit()
> >> + * which can call process_syncing_tables_for_sync.
> >> + *
> >> + * process_syncing_tables_for_sync decides whether the sync of
> >> + * the current table is completed. If it is completed,
> >> + * streaming must be already ended. So, we can break the loop.
> >> + */
> >> + if (MyLogicalRepWorker->is_sync_completed)
> >> + {
> >> + endofstream = true;
> >> + break;
> >> + }
> >> +
> >>
> >> and
> >>
> >> + /*
> >> + * If is_sync_completed is true, this means that the tablesync
> >> + * worker is done with synchronization. Streaming has already been
> >> + * ended by process_syncing_tables_for_sync. We should move to the
> >> + * next table if needed, or exit.
> >> + */
> >> + if (MyLogicalRepWorker->is_sync_completed)
> >> + endofstream = true;
> >>
> >> ~
> >>
> >> Instead of those code fragments above assigning 'endofstream' as a
> >> side-effect, would it be the same (but tidier) to just modify the
> >> other "breaking" condition below:
> >>
> >> BEFORE:
> >> /* Check if we need to exit the streaming loop. */
> >> if (endofstream)
> >> break;
> >>
> >> AFTER:
> >> /* Check if we need to exit the streaming loop. */
> >> if (endofstream || MyLogicalRepWorker->is_sync_completed)
> >> break;
> >
> >
> > First place you mentioned also breaks the infinite loop. Such an if statement is needed there with or without
endofstreamassignment. 
> >
> > I think if there is a flag to break a loop, using that flag to indicate that we should exit the loop seems more
appropriateto me. I see that it would be a bit tidier without endofstream = true lines, but I feel like it would also
beless readable. 
> >
> > I don't have a strong opinion though. I'm just keeping them as they are for now, but I can change them if you
disagree.
> >
>
> I felt it was slightly sneaky to re-use the existing variable as a
> convenient way to do what you want. But, I don’t feel strongly enough
> on this point to debate it -- maybe see later if others have an
> opinion about this.
>

I feel it is okay to use the existing variable 'endofstream' here but
shall we have an assertion that it is a tablesync worker?

> >>
> >>
> >> 10b.
> >> All the other tablesync-related fields of this struct are named as
> >> relXXX, so I wonder if is better for this to follow the same pattern.
> >> e.g. 'relsync_completed'
> >
> >
> > Aren't those start with rel because they're related to the relation that the tablesync worker is syncing?
is_sync_completedis not a relation specific field. I'm okay with changing the name but feel like relsync_completed
wouldbe misleading. 
>
> My reading of the code is slightly different: Only these fields have
> the prefix ‘rel’ and they are all grouped under the comment “/* Used
> for initial table synchronization. */” because AFAIK only these fields
> are TWS specific (not used for other kinds of workers).
>
> Since this new flag field is also TWS-specific, therefore IMO it
> should follow the same consistent name pattern. But, if you are
> unconvinced, maybe see later if others have an opinion about it.
>

+1 to use the prefix 'rel' here as the sync is specific to the
relation. Even during apply phase, we will apply the relation-specific
changes. See should_apply_changes_for_rel().

--
With Regards,
Amit Kapila.