What is the best, most efficient, Client pool technique with Erlang

Question

I\'m a real Erlang newbie (started 1 week ago), and I\'m trying to learn this language by creating a small but efficient chat server. (When I say efficient I mean I have 5 serve

Answer 1

Now, Messaging systems are what everyone wants to do when they come to Erlang because the two naturally blend. However, there are a number of things to look into before one continues. Messaging basically involves the following things: User Registration, User Authentication, Sessions Management,Logging, Message Switching/routing e.t.c.

Now, to do all or most of these, one needs to have a Database, certainly IN-MEMORY, thats leads me to either Mnesia or ETS Tables. Since you are new to Erlang, i suppose you have not yet really mastered working with these. At one moment, you will need to maintain Who is communicating with who, Who is available for Chat e.t.c. Hence you might need to look up things and write things some where.

Another thing is you have not told us the Client. Is it going to be a Web Client (HTTP), is it an entirely new protocol you are implementing over raw Sockets ? Which ever way, you will need to master something called: Concurrency in Erlang. If a user connects and is assigned an ID, if your design is A process Per User, then you will have to save the Pids of these Processes or register them against some criteria, yet again monitor them if they die e.t.c. Which brings me to OTP and Supervision trees. There is quite alot, however, tell us more about the Client and Server interaction, the Network Communication you need e.t.c. Or is it just a simple Erlang RPC project you are doing for your own revision ?

EDIT

Use ETS Tables, or use Mnesia RAM tables. Do not think of registering these Pids or Storing them in a list, Array or set. Look at this solution which was given to this question

Answer 2

I'm doing something similar to your chat program using gproc as a pubsub (similar to the demo on that page). Each client registers as it's id. To find a particular client, you do a lookup on that client id. To subscribe to a client, you add a property to that process of the client id being subscribed to. To publish, you call gproc:send(ClientId,Message). This covers your use case, the more general room based chat as well, and can handle distributed masterless registry of processes.

I haven't tested to see if it scales to millions, but it uses ets to do the storage and gproc is rock solid code by Ulf Wiger. I wouldn't count on being able to write a better implementation.

Answer 3

I'm also kind of new to Erlang (a couple of months), so I hope this can put you in the correct path :)

First of all, since you're a "newbie", you should know about these sites:

• Erlang Official Documentation: Most common modules are in the stdlib application, so start from there.
• Alternative Documentation: There's a real time search engine, so it is really good when searching for specific modules.
• Erlang Programming Rules: For you to enter in the mindset of erlang programming.
• Learn You Some Erlang Book: A must read for everyone starting with Erlang. It's really comprehensive and fun to read!
• Trapexit.org: Forum and cookbooks, to search for common problems faced by programmers.

Well, thinking about a non persistent database, I would suggest the sets or gb_sets modules (documentation here).

If you want persistence, you should try dets (see documentation above), but I can't state anything about efficiency, so you should research this topic a bit further.

In the book Learn You Some Erlang there is a chapter on data structures that says that sets are better for read intensive systems, while gb_sets is more appropriate for a balanced usage.