[erlang-questions] Associating contextual info with monitors

[Rick Pettit]

On Sun, Feb 17, 2008 at 07:52:56PM -0700, Dave Smith wrote:
> Greetings,
> 
> On several occasions, I've found myself wanting to be able to do some
> cleanup (e.g. remove an entry in ets) when a process dies. Usually I
> solve this by maintaining an ets table keyed by pid, that stores the
> necessary contextual info needed for cleanup, and then setup a monitor
> on the pid of interest. This works ok, but it's a little annoying to
> need to maintain yet another ets table for this context info that is
> only used when the process dies. It seems like what I _really_ want is
> a way to associate a term with a monitor -- that way when I get the
> DOWN message, I would also get the associated term and have everything
> I need to do the cleanup.
> 
> Is there a "Right Way" to accomplish this? Maintaining ets tables for
> this sort of contextual info seems a bit overkill -- is there a
> simpler approach? Has anyone else ever encountered this sort of issue?
> I tried searching through the archives for this problem, but couldn't
> find the right set of search terms. :)

The way I typically solve this problem is by putting the cleanup code in
a gen_server:terminate/2 callback (this way when the gen_server dies it 
cleans up after itself).

For instances where the processes being spawned aren't gen_* servers, I would
most likely spawn a wrapper process which in turn spawn_links the real child
process, then blocks forever waiting to receive the EXIT for that process (the
cleanup code is triggered upon receipt of the EXIT, then the wrapper process
itself terminates).

-Rick