[erlang-questions] Please help me relate a past project to Erlang, I'm trying to understand...

bill robertson <>
Fri Mar 13 00:26:20 CET 2009

So I've been thinking more about Erlang lately, and I still haven't
gotten over a conceptual hurdle.  Where do you keep state?

So I'd like to explain a previous project that I did, that would be a
natural fit for Erlang, and then discuss how one might approach it in
Erlang.  I'm not trying to advocate for the way the project worked in
the past, and I'm not trying to get free advice for working on it now.
 I no longer work at that company and haven't touch it in a long time.

The project was a controller for a conference calling server.  In the
setup there was a server running on another box that took simple
commands, e.g. dial a number (it used PRI), play a wav file to one or
more lines, answer, and bridge multiple lines together, disconnect
lines etc...  The whole thing was pretty small potatoes, so there was
no need to distribute the controller logic.  It was a lot of fun to
do.  I have to say that its a great experence when you hit "go" in the
debugger and your phone rings.  :)

The controller piece, which I implemented was in (please don't stop
reading) Java.  It would take commands from a web applicaiton, handle
events from the telephony server and report on and manage the state of
the conference calls.  When it was dealing with messages from the
telephony server it would first lock the conference call object and
then forward the message to an object that represented the participant
on the call.  The participant object would interpret the message,
update its internal state, and under some circumstances notify the
conference call object of a major state change (e.g. the line hung
up).  Upon update from a participant, the conference call object might
change its state e.g. the last person hung up, so call over.

Once the call stack unwound, the conference call object became
unlocked automatically (language mechanisms).  Overall it worked
pretty well.  The locking wasn't automatic, so it was possble to
forget to lock, which of course would cause havoc (it happened -- bad
decision on my part to leave it to fallible human memory).  The only
possible bottleneck it presented was when 30 or so people were trying
to jump on the call at the same time, although we never measured if
the bottleneck was in the controller or in the telephony server.

In the Erlang world, I can easily see how you would want a process for
the conference call, and possible a process for each participant.  I
also understand that you don't want to use a process local map for
storage.  So would you use an ets or a dets table?  Or would you track
it in a completely different fashion.

I would really appreciate your help.

Thank you.

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