Exceptions when running Erlang program on different terminal windows - erlang

Question

I am developing a simple framework in Erlang to handle 2-player turn-based games. The code is the following:
-module(game).
-export([start_server/0,generate_server/0,add_player/0,remove_player/0]).
generate_server() ->
Table_num = 0,
Player_num = 0,
io:format("Server generated...~n", []),
io:format("The current number of tables is ~w~n", [Table_num]),
io:format("The current number of players is ~w~n", [Player_num]),
receive
login ->
io:format("A new player has connected!~n", []),
New = Player_num + 1,
io:format("The current number of players is ~w~n", [New]);
logout ->
io:format("You have beeen succesfully disconnected~n", [])
end.
start_server() ->
io:format("Welcome player!~nInitializing game...~n", []),
io:format("Generating server...~n", []),
register(server,spawn(game, generate_server, [])).
add_player() ->
server ! login.
remove_player() ->
server ! logout.
There are two main problems when I run this code:
When I execute add_player() and then remove_player(), this second function crashes with an exception
If I launch the program on one terminal window and then execute add_player() on a second terminal windows, I get an error. What should I do to be able to run it on more than one terminal window?
Any help would be highly appreciated.

Answer 1

1/
There is no loop in your server. When you start it, after some print, it waits on the receive statement.
When it receives the login message, it executes the operations, and then the server code is finished; the process die and is unregistered. All variables disappear and the VM clean up the memory...
So, later on, any process sending a message to the server will crash because you are using a name which is no more registered.
To make it works you should keep a list of connected players (in list for example) and recall the server loop with this list as parameter.
generate_server(Tlist,Plist) ->
io:format("The current number of tables is ~w~n", [length(Tlist)]),
io:format("The current number of players is ~w~n", [length(Plist)]),
receive
{login,Name} ->
io:format("A new player ~p has connected!~n", [Name]),
generate_server(Tlist,[Name|Plist]);
{logout,Name} ->
io:format("You have beeen succesfully disconnected~n", []),
generate_server(Tlist,lists:delete(Name,Plist))
end.
and the call to generate_server is done by
register(server,spawn(game, generate_server, [[],[]]))
2/
in order to use erlang messages between 2 different nodes, you need to:
share the same erlang coockie
discover the nodes (using netadm for example)
get the server pid or use golbal registered name
see example at http://learnyousomeerlang.com/distribunomicon#alone-in-the-dark

Answer 2

You missed out a loop body for the server. Your program crashes because the server receives only one message and exits. Consider another version of the server below:
generate_server() ->
Table_num = 0,
Player_num = 0,
io:format("Server generated...~n", []),
io:format("The current number of tables is ~w~n", [Table_num]),
io:format("The current number of players is ~w~n", [Player_num]),
loop([]).
loop(Players)->
receive
{From,{login,PlayerId}} ->
io:format("A new player has connected!~n", []),
io:format("The current number of players is ~w~n", [New]),
NewPlayers = case lists:member(PlayerId,Players) of
true ->
From ! {login_failed,exists},
Players;
false ->
From ! {login_success,true},
[PlayerId|Players]
end,
loop(NewPlayers);
{From,{logout,PlayerId}} ->
NewPlayers = case lists:member(PlayerId,Players) of
true ->
From ! {logout,ok},
Players -- [PlayerId];
false ->
From ! {logout,failed},
Players
end, loop(NewPlayers);
_ -> loop(Players)
end.
There; that looks much better.