Simple Client/Server Model in Erlang

gistfile1.erl

-module(server).
-export([factorial/2, factorial/1, client/2, server/1, start/0]).

%% BACKLOG is effectively the number of clients 
-define(BACKLOG, 1000).

factorial(1) -> 
    1;
factorial(X) -> 
    X * factorial(X-1).
factorial(Client, X) -> 
    Client ! factorial(X).

client(Data, ServerPID) ->
    ServerPID ! {fact, self(), Data},
    receive 
        Reply -> io:format("Client ~w got fact(~p)=~p.~n", 
                    [self(), Data, Reply])
    end.

server(0) -> 
    ok;
server(Backlog) ->
    receive
        {fact, Client, Data} -> 
            spawn(?MODULE, factorial, [Client, Data])
    end,
    server(Backlog-1).

start(0, _ServerPID) -> 
    ok;
start(N, ServerPID) -> 
    spawn(?MODULE, client, [random:uniform(10), ServerPID]),
    start(N-1, ServerPID).
start() ->
    start(?BACKLOG, spawn(?MODULE, server, [?BACKLOG])).