pid lookup via global

[Karl Velicka]

Hi,

Firstly - I think that your use case is not quite what global is made for.

In my company's codebase we have a behaviour called tracked_supervisor
which does almost exactly what you're describing - it has a
simple_one_for_one supervisor for supervising some children where each
child has a unique id (which is just a term) and there's also a tracker
module (a gen_server) which is responsible for starting the children and
keeping the pid->name mapping in an ETS table. Our behaviour does not do
the reverse lookup (pid -> name) but it would be trivial to add such a
feature.

There's also a library called Director that seems to do something similar -
https://github.com/pouriya/director, however it round-trips a process for
every lookup which may be unacceptable performance-wise (lookups in both
global and aforementioned tracked_supervisor are direct ETS reads).

Last but not least, I'd recommend that you look into the global module
itself (lib/kernel/src/global.erl in the OTP repo). It's not complicated
and you can see what the shape of the data in its ETS table is. With that
knowledge you could use ets:match to get the name of your pid directly from
global's ETS table. However, this is NOT something I would recommend unless
you're _certain_ you know what you're doing - I'm sure that the OTP team
makes no guarantee that internal data structures retain their shape so code
relying on such details may break one day with no warning whatsoever! I
only mention this to encourage people to not be afraid to look under the
covers of the OTP platform because a number of often used modules are
implemented in plain Erlang so it's not at all difficult to understand how
they work.

Hope this helps!

Karl

On Fri, 8 May 2020 at 09:14, Nato <nato@REDACTED> wrote:

> Hi list
>
> I have a gen_statem where I am creating simple_one_for_one
> children and using the global+term strategy. So each child's
> key, as it were, is a term. I have read the manuals for
> global, supervisor, sys, and can't seem to find a way to
> reverse lookup a child's id, via its known Pid. I suppose
> I was expecting global to have a way to do this... something
> I'm missing, no doubt.
>
> Right now, via a collect on `supervisor:which_children(foo_sup)`
> I can access the child Pids, and in an unsavory way, for
> each, matching on a whopper term via `sys:get_status/1`
> I can extract the Id in a `header' tuple. Is there
> another way?
>
>
>
> --
> Nato
>
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