holding-area.asciidoc

Étude 11-3: A Simple Multi-User Chat _~~~~~~~~~~~ The purpose of this étude is to use a different OTP behavior to implement a multi-user chat. I spent a fair amount of time trying to shoehorn a chat into the mold of the gen_server behavior with multiple clients. That looks a bit like Multiple clients with gen_server.

Figure 1. Multiple clients with gen_server

This model is great if you have many clients all demanding weather reports, as in the previous études. None of the clients really needs to talk to the others; they grab information from the server and off they go to other things.

I could have used a client/server model, but I the clients would have to repeatedly ask "Do I have any messages yet?", and the internal state of the server got more and more complicated the more I thought about it.

Luckily, OTP has many different behaviors built into it, and the one that seemed best was the gen_event behavior. Processing flow in gen_event shows how it works.

Figure 2. Processing flow in gen_event

In addition to being able to do calls just as you did in gen_server, you can add event handlers to a gen_event module. When a client calls gen_event:notify/2, it sends the event manager some data in an Event. The event manager then sends the Event and the manager’s current state to all the Handler modules that it knows about.

In the Erlang documentation for the gen_event behavior, a typical use of this behavior would be to have two handlers added to the event manager. One would log an error to a terminal, and the other would log an error to a file. Thus, the client would notify the event manager that some error had occurred, and the manager would let the handlers take care of the rest.

Now, how does this match up with the chat room? My idea was to make each person in the room a "handler," and ignore the client altogether. This means that the handler gets the person’s input and notifies the event manager, which pushes the input to all the other handlers (and to the originator, who should ignore it!) The result looks like Using gen_event to create a chat room.

Warning

This is almost certainly not the way the creators of OTP intended when they developed gen_event. If a student of mine were to turn in this solution, I would take off 20% of the points for "abuse of OTP."

Figure 3. Using gen_event to create a chat room