On Mon, Apr 18, 2022 at 8:04 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Thu, Apr 14, 2022 at 9:09 AM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > On Thu, Apr 14, 2022 at 8:32 AM Masahiko Sawada <sawada.mshk@gmail.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.
Regards,
--
Masahiko Sawada
EDB: https://www.enterprisedb.com/
