Several things are wrong with the messenger example in
Or what happens if the server goes down in the middle of sending a
message, leaving the sending client hanging forever in
the
Before improving the messenger program, let us look at some
general principles, using the ping pong program as an example.
Recall that when "ping" finishes, it tells "pong" that it has
done so by sending the atom
-module(tut19).
-export([start_ping/1, start_pong/0, ping/2, pong/0]).
ping(0, Pong_Node) ->
io:format("ping finished~n", []);
ping(N, Pong_Node) ->
{pong, Pong_Node} ! {ping, self()},
receive
pong ->
io:format("Ping received pong~n", [])
end,
ping(N - 1, Pong_Node).
pong() ->
receive
{ping, Ping_PID} ->
io:format("Pong received ping~n", []),
Ping_PID ! pong,
pong()
after 5000 ->
io:format("Pong timed out~n", [])
end.
start_pong() ->
register(pong, spawn(tut19, pong, [])).
start_ping(Pong_Node) ->
spawn(tut19, ping, [3, Pong_Node]).
After this is compiled and the file
(pong@kosken)1> tut19:start_pong(). true Pong received ping Pong received ping Pong received ping Pong timed out
And the following is seen on (ping@gollum):
(ping@gollum)1> tut19:start_ping(pong@kosken). <0.36.0> Ping received pong Ping received pong Ping received pong ping finished
The time-out is set in:
pong() ->
receive
{ping, Ping_PID} ->
io:format("Pong received ping~n", []),
Ping_PID ! pong,
pong()
after 5000 ->
io:format("Pong timed out~n", [])
end.
The time-out (
after pong_timeout() ->
In general, there are better ways than using time-outs to supervise parts of a distributed Erlang system. Time-outs are usually appropriate to supervise external events, for example, if you have expected a message from some external system within a specified time. For example, a time-out can be used to log a user out of the messenger system if they have not accessed it for, say, ten minutes.
Before going into details of the supervision and error handling in an Erlang system, let us see how Erlang processes terminate, or in Erlang terminology, exit.
A process which executes
A process which encounters a runtime error (for example, divide by zero,
bad match, trying to call a function that does not exist and so on)
exits with an error, that is, has an abnormal exit. A
process which executes
An Erlang process can set up links to other Erlang processes. If
a process calls
The signal carries information about the pid it was sent from and the exit reason.
The default behaviour of a process that receives a normal exit is to ignore the signal.
The default behaviour in the two other cases (that is, abnormal exit) above is to:
In this way you can connect all processes in a
transaction together using links. If one of the processes
exits abnormally, all the processes in the transaction are
killed. As it is often wanted to create a process and link to it at
the same time, there is a special BIF,
Now an example of the ping pong example using links to terminate "pong":
-module(tut20).
-export([start/1, ping/2, pong/0]).
ping(N, Pong_Pid) ->
link(Pong_Pid),
ping1(N, Pong_Pid).
ping1(0, _) ->
exit(ping);
ping1(N, Pong_Pid) ->
Pong_Pid ! {ping, self()},
receive
pong ->
io:format("Ping received pong~n", [])
end,
ping1(N - 1, Pong_Pid).
pong() ->
receive
{ping, Ping_PID} ->
io:format("Pong received ping~n", []),
Ping_PID ! pong,
pong()
end.
start(Ping_Node) ->
PongPID = spawn(tut20, pong, []),
spawn(Ping_Node, tut20, ping, [3, PongPID]).
(s1@bill)3> tut20:start(s2@kosken). Pong received ping <3820.41.0> Ping received pong Pong received ping Ping received pong Pong received ping Ping received pong
This is a slight modification of the ping pong program where both
processes are spawned from the same
It is possible to modify the default behaviour of a process so
that it does not get killed when it receives abnormal exit
signals. Instead, all signals are turned into normal messages on
the format
process_flag(trap_exit, true)
There are several other process flags, see
-module(tut21).
-export([start/1, ping/2, pong/0]).
ping(N, Pong_Pid) ->
link(Pong_Pid),
ping1(N, Pong_Pid).
ping1(0, _) ->
exit(ping);
ping1(N, Pong_Pid) ->
Pong_Pid ! {ping, self()},
receive
pong ->
io:format("Ping received pong~n", [])
end,
ping1(N - 1, Pong_Pid).
pong() ->
process_flag(trap_exit, true),
pong1().
pong1() ->
receive
{ping, Ping_PID} ->
io:format("Pong received ping~n", []),
Ping_PID ! pong,
pong1();
{'EXIT', From, Reason} ->
io:format("pong exiting, got ~p~n", [{'EXIT', From, Reason}])
end.
start(Ping_Node) ->
PongPID = spawn(tut21, pong, []),
spawn(Ping_Node, tut21, ping, [3, PongPID]).
(s1@bill)1> tut21:start(s2@gollum).
<3820.39.0>
Pong received ping
Ping received pong
Pong received ping
Ping received pong
Pong received ping
Ping received pong
pong exiting, got {'EXIT',<3820.39.0>,ping}
Let us return to the messenger program and add changes to make it more robust:
%%% Message passing utility.
%%% User interface:
%%% login(Name)
%%% One user at a time can log in from each Erlang node in the
%%% system messenger: and choose a suitable Name. If the Name
%%% is already logged in at another node or if someone else is
%%% already logged in at the same node, login will be rejected
%%% with a suitable error message.
%%% logoff()
%%% Logs off anybody at that node
%%% message(ToName, Message)
%%% sends Message to ToName. Error messages if the user of this
%%% function is not logged on or if ToName is not logged on at
%%% any node.
%%%
%%% One node in the network of Erlang nodes runs a server which maintains
%%% data about the logged on users. The server is registered as "messenger"
%%% Each node where there is a user logged on runs a client process registered
%%% as "mess_client"
%%%
%%% Protocol between the client processes and the server
%%% ----------------------------------------------------
%%%
%%% To server: {ClientPid, logon, UserName}
%%% Reply {messenger, stop, user_exists_at_other_node} stops the client
%%% Reply {messenger, logged_on} logon was successful
%%%
%%% When the client terminates for some reason
%%% To server: {'EXIT', ClientPid, Reason}
%%%
%%% To server: {ClientPid, message_to, ToName, Message} send a message
%%% Reply: {messenger, stop, you_are_not_logged_on} stops the client
%%% Reply: {messenger, receiver_not_found} no user with this name logged on
%%% Reply: {messenger, sent} Message has been sent (but no guarantee)
%%%
%%% To client: {message_from, Name, Message},
%%%
%%% Protocol between the "commands" and the client
%%% ----------------------------------------------
%%%
%%% Started: messenger:client(Server_Node, Name)
%%% To client: logoff
%%% To client: {message_to, ToName, Message}
%%%
%%% Configuration: change the server_node() function to return the
%%% name of the node where the messenger server runs
-module(messenger).
-export([start_server/0, server/0,
logon/1, logoff/0, message/2, client/2]).
%%% Change the function below to return the name of the node where the
%%% messenger server runs
server_node() ->
messenger@super.
%%% This is the server process for the "messenger"
%%% the user list has the format [{ClientPid1, Name1},{ClientPid22, Name2},...]
server() ->
process_flag(trap_exit, true),
server([]).
server(User_List) ->
receive
{From, logon, Name} ->
New_User_List = server_logon(From, Name, User_List),
server(New_User_List);
{'EXIT', From, _} ->
New_User_List = server_logoff(From, User_List),
server(New_User_List);
{From, message_to, To, Message} ->
server_transfer(From, To, Message, User_List),
io:format("list is now: ~p~n", [User_List]),
server(User_List)
end.
%%% Start the server
start_server() ->
register(messenger, spawn(messenger, server, [])).
%%% Server adds a new user to the user list
server_logon(From, Name, User_List) ->
%% check if logged on anywhere else
case lists:keymember(Name, 2, User_List) of
true ->
From ! {messenger, stop, user_exists_at_other_node}, %reject logon
User_List;
false ->
From ! {messenger, logged_on},
link(From),
[{From, Name} | User_List] %add user to the list
end.
%%% Server deletes a user from the user list
server_logoff(From, User_List) ->
lists:keydelete(From, 1, User_List).
%%% Server transfers a message between user
server_transfer(From, To, Message, User_List) ->
%% check that the user is logged on and who he is
case lists:keysearch(From, 1, User_List) of
false ->
From ! {messenger, stop, you_are_not_logged_on};
{value, {_, Name}} ->
server_transfer(From, Name, To, Message, User_List)
end.
%%% If the user exists, send the message
server_transfer(From, Name, To, Message, User_List) ->
%% Find the receiver and send the message
case lists:keysearch(To, 2, User_List) of
false ->
From ! {messenger, receiver_not_found};
{value, {ToPid, To}} ->
ToPid ! {message_from, Name, Message},
From ! {messenger, sent}
end.
%%% User Commands
logon(Name) ->
case whereis(mess_client) of
undefined ->
register(mess_client,
spawn(messenger, client, [server_node(), Name]));
_ -> already_logged_on
end.
logoff() ->
mess_client ! logoff.
message(ToName, Message) ->
case whereis(mess_client) of % Test if the client is running
undefined ->
not_logged_on;
_ -> mess_client ! {message_to, ToName, Message},
ok
end.
%%% The client process which runs on each user node
client(Server_Node, Name) ->
{messenger, Server_Node} ! {self(), logon, Name},
await_result(),
client(Server_Node).
client(Server_Node) ->
receive
logoff ->
exit(normal);
{message_to, ToName, Message} ->
{messenger, Server_Node} ! {self(), message_to, ToName, Message},
await_result();
{message_from, FromName, Message} ->
io:format("Message from ~p: ~p~n", [FromName, Message])
end,
client(Server_Node).
%%% wait for a response from the server
await_result() ->
receive
{messenger, stop, Why} -> % Stop the client
io:format("~p~n", [Why]),
exit(normal);
{messenger, What} -> % Normal response
io:format("~p~n", [What])
after 5000 ->
io:format("No response from server~n", []),
exit(timeout)
end.
The following changes are added:
The messenger server traps exits. If it receives an exit signal,
If an exit signal is received as above, the tuple
Also, a time-out of five seconds has been introduced in
the
An interesting case is if the client terminates before
the server links to it. This is taken care of because linking to a
non-existent process causes an exit signal,