One of the main reasons for using Erlang instead of other functional languages is Erlang's ability to handle concurrency and distributed programming. By concurrency we mean programs which can handle several threads of execution at the same time. For example, modern operating systems would allow you to use a word processor, a spreadsheet, a mail client and a print job all running at the same time. Of course each processor (CPU) in the system is probably only handling one thread (or job) at a time, but it swaps between the jobs a such a rate that it gives the illusion of running them all at the same time. It is easy to create parallel threads of execution in an Erlang program and it is easy to allow these threads to communicate with each other. In Erlang we call each thread of execution a process.
(Aside: the term "process" is usually used when the threads of execution share no data with each other and the term "thread" when they share data in some way. Threads of execution in Erlang share no data, that's why we call them processes).
The Erlang BIF
-module(tut14).
-export([start/0, say_something/2]).
say_something(What, 0) ->
done;
say_something(What, Times) ->
io:format("~p~n", [What]),
say_something(What, Times - 1).
start() ->
spawn(tut14, say_something, [hello, 3]),
spawn(tut14, say_something, [goodbye, 3]).
5> c(tut14). {ok,tut14} 6> tut14:say_something(hello, 3). hello hello hello done
We can see that function
9> tut14:start(). hello goodbye <0.63.0> hello goodbye hello goodbye
Notice that it didn't write "hello" three times and then
"goodbye" three times, but the first process wrote a "hello",
the second a "goodbye", the first another "hello" and so forth.
But where did the <0.63.0> come from? The return value of a
function is of course the return value of the last "thing" in
the function. The last thing in the function
spawn(tut14, say_something, [goodbye, 3]).
Note as well that we have used ~p instead of ~w in
In the following example we create two processes which send messages to each other a number of times.
-module(tut15).
-export([start/0, ping/2, pong/0]).
ping(0, Pong_PID) ->
Pong_PID ! finished,
io:format("ping finished~n", []);
ping(N, Pong_PID) ->
Pong_PID ! {ping, self()},
receive
pong ->
io:format("Ping received pong~n", [])
end,
ping(N - 1, Pong_PID).
pong() ->
receive
finished ->
io:format("Pong finished~n", []);
{ping, Ping_PID} ->
io:format("Pong received ping~n", []),
Ping_PID ! pong,
pong()
end.
start() ->
Pong_PID = spawn(tut15, pong, []),
spawn(tut15, ping, [3, Pong_PID]).
1> c(tut15). {ok,tut15} 2> tut15: start(). <0.36.0> Pong received ping Ping received pong Pong received ping Ping received pong Pong received ping Ping received pong ping finished Pong finished
The function
Pong_PID = spawn(tut15, pong, [])
This process executes
spawn(tut15, ping, [3, Pong_PID]),
This process executes:
tut15:ping(3, Pong_PID)
<0.36.0> is the return value from the
The process "pong" now does:
receive
finished ->
io:format("Pong finished~n", []);
{ping, Ping_PID} ->
io:format("Pong received ping~n", []),
Ping_PID ! pong,
pong()
end.
The
receive
pattern1 ->
actions1;
pattern2 ->
actions2;
....
patternN
actionsN
end.
Note: no ";" before the
Messages between Erlang processes are simply valid Erlang terms. I.e. they can be lists, tuples, integers, atoms, pids etc.
Each process has its own input queue for messages it receives.
New messages received are put at the end of the queue. When a
process executes a
However, if the first pattern does not match, the second pattern is tested, if this matches the message is removed from the queue and the actions corresponding to the second pattern are executed. If the second pattern does not match the third is tried and so on until there are no more pattern to test. If there are no more patterns to test, the first message is kept in the queue and we try the second message instead. If this matches any pattern, the appropriate actions are executed and the second message is removed from the queue (keeping the first message and any other messages in the queue). If the second message does not match we try the third message and so on until we reach the end of the queue. If we reach the end of the queue, the process blocks (stops execution) and waits until a new message is received and this procedure is repeated.
Of course the Erlang implementation is "clever" and minimizes
the number of times each message is tested against the patterns
in each
Now back to the ping pong example.
"Pong" is waiting for messages. If the atom
{ping, Ping_PID}
it writes "Pong received ping" to the output and sends the atom
Ping_PID ! pong
Note how the operator "!" is used to send messages. The syntax of "!" is:
Pid ! Message
I.e.
After sending the message
tut15:ping(3, Pong_PID)
Looking at the function
The second clause sends a message to "pong":
Pong_PID ! {ping, self()},
"Ping" now waits for a reply from "pong":
receive
pong ->
io:format("Ping received pong~n", [])
end,
and writes "Ping received pong" when this reply arrives, after
which "ping" calls the
ping(N - 1, Pong_PID)
ping(0, Pong_PID) ->
Pong_PID ! finished,
io:format("ping finished~n", []);
The atom
In the above example, we first created "pong" so as to be able
to give the identity of "pong" when we started "ping". I.e. in
some way "ping" must be able to know the identity of "pong" in
order to be able to send a message to it. Sometimes processes
which need to know each others identities are started completely
independently of each other. Erlang thus provides a mechanism for
processes to be given names so that these names can be used as
identities instead of pids. This is done by using
the
register(some_atom, Pid)
We will now re-write the ping pong example using this and giving
the name
-module(tut16).
-export([start/0, ping/1, pong/0]).
ping(0) ->
pong ! finished,
io:format("ping finished~n", []);
ping(N) ->
pong ! {ping, self()},
receive
pong ->
io:format("Ping received pong~n", [])
end,
ping(N - 1).
pong() ->
receive
finished ->
io:format("Pong finished~n", []);
{ping, Ping_PID} ->
io:format("Pong received ping~n", []),
Ping_PID ! pong,
pong()
end.
start() ->
register(pong, spawn(tut16, pong, [])),
spawn(tut16, ping, [3]).
2> c(tut16). {ok, tut16} 3> tut16:start(). <0.38.0> Pong received ping Ping received pong Pong received ping Ping received pong Pong received ping Ping received pong ping finished Pong finished
In the
register(pong, spawn(tut16, pong, [])),
both spawns the "pong" process and gives it the name
pong ! {ping, self()},
so that
Now let's re-write the ping pong program with "ping" and "pong"
on different computers. Before we do this, there are a few things
we need to set up to get this to work. The distributed Erlang
implementation provides a basic security mechanism to prevent
unauthorized access to an Erlang system on another computer.
Erlang systems which talk to each other must have
the same magic cookie. The easiest way to achieve this
is by having a file called
$ cd $ cat > .erlang.cookie this_is_very_secret $ chmod 400 .erlang.cookie
The
When you start an Erlang system which is going to talk to other Erlang systems, you must give it a name, eg:
$ erl -sname my_name
We will see more details of this later. If you want to experiment with distributed Erlang, but you only have one computer to work on, you can start two separate Erlang systems on the same computer but give them different names. Each Erlang system running on a computer is called an Erlang node.
(Note:
Here is the ping pong example modified to run on two separate nodes:
-module(tut17).
-export([start_ping/1, start_pong/0, ping/2, pong/0]).
ping(0, Pong_Node) ->
{pong, Pong_Node} ! finished,
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
finished ->
io:format("Pong finished~n", []);
{ping, Ping_PID} ->
io:format("Pong received ping~n", []),
Ping_PID ! pong,
pong()
end.
start_pong() ->
register(pong, spawn(tut17, pong, [])).
start_ping(Pong_Node) ->
spawn(tut17, ping, [3, Pong_Node]).
Let us assume we have two computers called gollum and kosken. We will start a node on kosken called ping and then a node on gollum called pong.
On kosken (on a Linux/Unix system):
kosken> erl -sname ping Erlang (BEAM) emulator version 5.2.3.7 [hipe] [threads:0] Eshell V5.2.3.7 (abort with ^G) (ping@kosken)1>
On gollum:
gollum> erl -sname pong Erlang (BEAM) emulator version 5.2.3.7 [hipe] [threads:0] Eshell V5.2.3.7 (abort with ^G) (pong@gollum)1>
Now we start the "pong" process on gollum:
(pong@gollum)1> tut17:start_pong(). true
and start the "ping" process on kosken (from the code above you
will see that a parameter of the
(ping@kosken)1> tut17:start_ping(pong@gollum). <0.37.0> Ping received pong Ping received pong Ping received pong ping finished
Here we see that the ping pong program has run, on the "pong" side we see:
(pong@gollum)2> Pong received ping Pong received ping Pong received ping Pong finished (pong@gollum)2>
Looking at the
{ping, Ping_PID} ->
io:format("Pong received ping~n", []),
Ping_PID ! pong,
work in the same way irrespective of on which node the "ping" process is executing. Thus Erlang pids contain information about where the process executes so if you know the pid of a process, the "!" operator can be used to send it a message if the process is on the same node or on a different node.
A difference is how we send messages to a registered process on another node:
{pong, Pong_Node} ! {ping, self()},
We use a tuple
In the previous example, we started "ping" and "pong" from
the shells of two separate Erlang nodes.
-module(tut18).
-export([start/1, ping/2, pong/0]).
ping(0, Pong_Node) ->
{pong, Pong_Node} ! finished,
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
finished ->
io:format("Pong finished~n", []);
{ping, Ping_PID} ->
io:format("Pong received ping~n", []),
Ping_PID ! pong,
pong()
end.
start(Ping_Node) ->
register(pong, spawn(tut18, pong, [])),
spawn(Ping_Node, tut18, ping, [3, node()]).
Assuming an Erlang system called ping (but not the "ping" process) has already been started on kosken, then on gollum we do:
(pong@gollum)1> tut18:start(ping@kosken). <3934.39.0> Pong received ping Ping received pong Pong received ping Ping received pong Pong received ping Ping received pong Pong finished ping finished
Notice we get all the output on gollum. This is because the io system finds out where the process is spawned from and sends all output there.
Now for a larger example. We will make an extremely simple "messenger". The messenger is a program which allows users to log in on different nodes and send simple messages to each other.
Before we start, let's note the following:
This example will just show the message passing logic- no attempt at all has been made to provide a nice graphical user interface. This can, of course, also be done in Erlang - but that's another tutorial.
This sort of problem can be solved more easily if you use the facilities in OTP, which will also provide methods for updating code on the fly etc. But again, that's another tutorial.
The first program we write will contain some inadequacies regarding the handling of nodes which disappear. We will correct these in a later version of the program.
We will set up the messenger by allowing "clients" to connect to a central server and say who and where they are. I.e. a user won't need to know the name of the Erlang node where another user is located to send a message.
File
%%% Message passing utility.
%%% User interface:
%%% logon(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 at 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
%%%
%%% To server: {ClientPid, logoff}
%%% Reply: {messenger, logged_off}
%%%
%%% To server: {ClientPid, logoff}
%%% Reply: no reply
%%%
%%% 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/1, 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@bill.
%%% This is the server process for the "messenger"
%%% the user list has the format [{ClientPid1, Name1},{ClientPid22, Name2},...]
server(User_List) ->
receive
{From, logon, Name} ->
New_User_List = server_logon(From, Name, User_List),
server(New_User_List);
{From, logoff} ->
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},
[{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, {From, 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 server node
client(Server_Node, Name) ->
{messenger, Server_Node} ! {self(), logon, Name},
await_result(),
client(Server_Node).
client(Server_Node) ->
receive
logoff ->
{messenger, Server_Node} ! {self(), 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])
end.
To use this program you need to:
In the following example of use of this program I have started nodes on four different computers, but if you don't have that many machines available on your network you could start up several nodes on the same machine.
We start up four Erlang nodes: messenger@super, c1@bilbo, c2@kosken, c3@gollum.
First we start up a the server at messenger@super:
(messenger@super)1> messenger:start_server(). true
Now Peter logs on at c1@bilbo:
(c1@bilbo)1> messenger:logon(peter). true logged_on
James logs on at c2@kosken:
(c2@kosken)1> messenger:logon(james). true logged_on
and Fred logs on at c3@gollum:
(c3@gollum)1> messenger:logon(fred). true logged_on
Now Peter sends Fred a message:
(c1@bilbo)2> messenger:message(fred, "hello"). ok sent
And Fred receives the message and sends a message to Peter and logs off:
Message from peter: "hello" (c3@gollum)2> messenger:message(peter, "go away, I'm busy"). ok sent (c3@gollum)3> messenger:logoff(). logoff
James now tries to send a message to Fred:
(c2@kosken)2> messenger:message(fred, "peter doesn't like you"). ok receiver_not_found
But this fails as Fred has already logged off.
First let's look at some of the new concepts we have introduced.
There are two versions of the
Note how we write the
We use functions from the
3> lists:keymember(a, 2, [{x,y,z},{b,b,b},{b,a,c},{q,r,s}]). true 4> lists:keymember(p, 2, [{x,y,z},{b,b,b},{b,a,c},{q,r,s}]). false
5> lists:keydelete(a, 2, [{x,y,z},{b,b,b},{b,a,c},{q,r,s}]). [{x,y,z},{b,b,b},{q,r,s}]
There are a lot more very useful functions in the
An Erlang process will (conceptually) run until it does a
A process terminates when there is nothing more for it to do,
i.e. the last function it calls simply returns and doesn't call
another function. Another way for a process to terminate is for
it to call
The BIF
You should by now be able to understand most of the code above so I'll just go through one case: a message is sent from one user to another.
The first user "sends" the message in the example above by:
messenger:message(fred, "hello")
After testing that the client process exists:
whereis(mess_client)
and a message is sent to
mess_client ! {message_to, fred, "hello"}
The client sends the message to the server by:
{messenger, messenger@super} ! {self(), message_to, fred, "hello"},
and waits for a reply from the server.
The server receives this message and calls:
server_transfer(From, fred, "hello", User_List),
which checks that the pid
lists:keysearch(From, 1, User_List)
If
From ! {messenger, stop, you_are_not_logged_on}
which is received by the client which in turn does
server_transfer(From, peter, fred, "hello", User_List)
Note that as this is
lists:keysearch(fred, 2, User_List)
This time we use argument 2 which is the second element in
the tuple. If this returns the atom
From ! {messenger, receiver_not_found};
which is received by the client, if
{value, {ToPid, fred}}
we send the message:
ToPid ! {message_from, peter, "hello"},
to fred's client and the message:
From ! {messenger, sent}
to peter's client.
Fred's client receives the message and prints it:
{message_from, peter, "hello"} ->
io:format("Message from ~p: ~p~n", [peter, "hello"])
and peter's client receives the message in
the