Re: Column Filtering in Logical Replication

On Tue, Apr 19, 2022 at 6:58 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> On Mon, Apr 18, 2022 at 8:04 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > On Thu, Apr 14, 2022 at 9:09 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > >
> > > On Thu, Apr 14, 2022 at 8:32 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> > > > >
> > > > > The other part of the puzzle is the below check in the code:
> > > > > /*
> > > > > * If we reached the sync worker limit per subscription, just exit
> > > > > * silently as we might get here because of an otherwise harmless race
> > > > > * condition.
> > > > > */
> > > > > if (nsyncworkers >= max_sync_workers_per_subscription)
> > > > >
> > > > > It is not clear to me why this check is there, if this wouldn't be
> > > > > there, the user would have got either a WARNING to increase the
> > > > > max_logical_replication_workers or the apply worker would have been
> > > > > restarted. Do you have any idea about this?
> > > >
> > > > Yeah, I'm also puzzled with this check. It seems that this function
> > > > doesn't work well when the apply worker is not running and some
> > > > tablesync workers are running. I initially thought that the apply
> > > > worker calls to this function as many as tables that needs to be
> > > > synced, but it checks the max_sync_workers_per_subscription limit
> > > > before calling to logicalrep_worker_launch(). So I'm not really sure
> > > > we need this check.
> > > >
> > >
> > > I just hope that the original author Petr J. responds to this point. I
> > > have added him to this email. This will help us to find the best
> > > solution for this problem.
> > >
> >
> > I did some more investigation for this code. It is added by commit [1]
> > and the patch that led to this commit is first time posted on -hackers
> > in email [2]. Now, neither the commit message nor the patch (comments)
> > gives much idea as to why this part of code is added but I think there
> > is some hint in the email [2]. In particular, read the paragraph in
> > the email [2] that has the lines: ".... and limiting sync workers per
> > subscription theoretically wasn't either (although I don't think it
> > could happen in practice).".
> >
> > It seems that this check has been added to theoretically limit the
> > sync workers even though that can't happen because apply worker
> > ensures that before trying to launch the sync worker. Does this theory
> > make sense to me? If so, I think we can change the check as: "if
> > (OidIsValid(relid) && nsyncworkers >=
> > max_sync_workers_per_subscription)" in launcher.c. This will serve the
> > purpose of the original code and will solve the issue being discussed
> > here. I think we can even backpatch this. What do you think?
>
> +1. I also think it's a bug so back-patching makes sense to me.
>

Pushed. Thanks Tomas and Sawada-San.

--
With Regards,
Amit Kapila.