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 spawn
is used to create a new process:
spawn(Module, Exported_Function, List of Arguments)
.
Consider the following module:
-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 say_something
writes its first
argument the number of times specified by second argument. Now
look at the function start
. It starts two Erlang processes,
one which writes "hello" three times and one which writes
"goodbye" three times. Both of these processes use the function
say_something
. Note that a function used in this way by
spawn
to start a process must be exported from the module
(i.e. in the -export
at the start of the module).
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 start
is
spawn(tut14, say_something, [goodbye, 3]).
spawn
returns a process identifier, or
pid, which uniquely identifies the process. So <0.63.0>
is the pid of the spawn
function call above. We will see
how to use pids in the next example.
Note as well that we have used ~p instead of ~w in
io:format
. To quote the manual: "~p Writes the data with
standard syntax in the same way as ~w, but breaks terms whose
printed representation is longer than one line into many lines
and indents each line sensibly. It also tries to detect lists of
printable characters and to output these as strings".
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 start
first creates a process, let's call it
"pong":
Pong_PID = spawn(tut15, pong, [])
This process executes tut15:pong()
. Pong_PID
is
the process identity of the "pong" process. The function
start
now creates another process "ping".
spawn(tut15, ping, [3, Pong_PID]),
this process executes
tut15:ping(3, Pong_PID)
<0.36.0> is the return value from the start
function.
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
construct is used to allow processes to wait
for messages from other processes. It has the format:
receive pattern1 -> actions1; pattern2 -> actions2; .... patternN actionsN end.
Note: no ";" before the end
.
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 receive
, the first message in the queue
is matched against the first pattern in the receive
, if
this matches, the message is removed from the queue and
the actions corresponding to the the pattern are executed.
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 receive
.
Now back to the ping pong example.
"Pong" is waiting for messages. If the atom finished
is
received, "pong" writes "Pong finished" to the output and as it
has nothing more to do, terminates. If it receives a message with
the format:
{ping, Ping_PID}
it writes "Pong received ping" to the output and sends the atom
pong
to the process "ping":
Ping_PID ! pong
Note how the operator "!" is used to send messages. The syntax of "!" is:
Pid ! Message
I.e. Message
(any Erlang term) is sent to the process
with identity Pid
.
After sending the message pong
, to the process "ping",
"pong" calls the pong
function again, which causes it to
get back to the receive
again and wait for another message.
Now let's look at the process "ping". Recall that it was started
by executing:
tut15:ping(3, Pong_PID)
Looking at the function ping/2
we see that the second
clause of ping/2
is executed since the value of the first
argument is 3 (not 0) (first clause head is
ping(0,Pong_PID)
, second clause head is
ping(N,Pong_PID)
, so N
becomes 3).
The second clause sends a message to "pong":
Pong_PID ! {ping, self()},
self()
returns the pid of the process which executes
self()
, in this case the pid of "ping". (Recall the code
for "pong", this will land up in the variable Ping_PID
in
the receive
previously explained).
"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
function again.
ping(N - 1, Pong_PID)
N-1
causes the first argument to be decremented until it
becomes 0. When this occurs, the first clause of ping/2
will be executed:
ping(0, Pong_PID) -> Pong_PID ! finished, io:format("ping finished~n", []);
The atom finished
is sent to "pong" (causing it to
terminate as described above) and "ping finished" is written to
the output. "Ping" then itself terminates as it has nothing left
to do.
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
BIF:
register(some_atom, Pid)
We will now re-write the ping pong example using this and giving
the name pong
to the "pong" process:
-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 start/0
function,
register(pong, spawn(tut16, pong, [])),
both spawns the "pong" process and gives it the name pong
.
In the "ping" process we can now send messages to pong
by:
pong ! {ping, self()},
so that ping/2
now becomes ping/1
as we don't have
to use the argument Pong_PID
.
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
(*manual*). 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 .erlang.cookie
in your home
directory on all machines which on which you are going to run
Erlang systems communicating with each other (on Windows systems
the home directory is the directory where pointed to by the $HOME
environment variable - you may need to set this. On Linux or Unix
you can safely ignore this and simply create a file called
.erlang.cookie
in the directory you get to after executing
the command cd
without any argument).
The .erlang.cookie
file should contain on line with
the same atom. For example on Linux or Unix in the OS shell:
$ cd $ cat > .erlang.cookie this_is_very_secret $ chmod 400 .erlang.cookie
The chmod
above make the .erlang.cookie
file
accessible only by the owner of the file. This is a requirement.
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 (*manual*). 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: erl -sname
assumes that all nodes are in the same
IP domain and we can use only the first component of the IP
address, if we want to use nodes in different domains we use
-name
instead, but then all IP address must be given in
full (*manual*).
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 start_ping
function is
the node name of the Erlang system where "pong" is running):
(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 tut17
code we see that the pong
function itself is unchanged, the lines:
{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 {registered_name,node_name}
instead of
just the registered_name
.
In the previous example, we started "ping" and "pong" from
the shells of two separate Erlang nodes. spawn
can also be
used to start processes in other nodes. The next example is
the ping pong program, yet again, but this time we will start
"ping" in another node:
-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:
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.
%%% 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:
server_node()
function
messenger.beam
) to
the directory on each computer where you start Erlang.
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 server_transfer
function,
one with four arguments (server_transfer/4
) and one with
five (server_transfer/5
). These are regarded by Erlang as
two separate functions.
Note how we write the server
function so that it calls
itself, server(User_List)
and thus creates a loop.
The Erlang compiler is "clever" and optimizes the code so that
this really is a sort of loop and not a proper function call. But
this only works if there is no code after the call, otherwise
the compiler will expect the call to return and make a proper
function call. This would result in the process getting bigger
and bigger for every loop.
We use functions in the lists
module. This is a very
useful module and a study of the manual page is recommended
(erl -man lists
).
lists:keymember(Key,Position,Lists)
looks through a list
of tuples and looks at Position
in each tuple to see if it
is the same as Key
. The first element is position 1. If it
finds a tuple where the element at Position
is the same as
Key, it returns true
, otherwise false
.
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
lists:keydelete
works in the same way but deletes
the first tuple found (if any) and returns the remaining list:
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}]
lists:keysearch
is like lists:keymember
, but it
returns {value,Tuple_Found}
or the atom false
.
There are a lot more very useful functions in the lists
module.
An Erlang process will (conceptually) run until it does a
receive
and there is no message which it wants to receive
in the message queue. I say "conceptually" because the Erlang
system shares the CPU time between the active processes in
the system.
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 exit/1
. The argument to exit/1
has a
special meaning which we will look at later. In this example we
will do exit(normal)
which has the same effect as a
process running out of functions to call.
The BIF whereis(RegisteredName)
checks if a registered
process of name RegisteredName
exists and return the pid
of the process if it does exist or the atom undefined
if
it does not.
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
:
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 From
is in the User_List
:
lists:keysearch(From, 1, User_List)
If keysearch
returns the atom false
, some sort of
error has occurred and the server sends back the message:
From ! {messenger, stop, you_are_not_logged_on}
which is received by the client which in turn does
exit(normal)
and terminates. If keysearch
returns
{value,From,Name}
we know that the user is logged on and
is his name (peter) is in variable Name
. We now call:
server_transfer(From, peter, fred, "hello", User_List)
Note that as this is server_transfer/5
it is not the same
as the previous function server_transfer/4
. We do another
keysearch
on User_List
to find the pid of the client
corresponding to fred:
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 false
we know that
fred is not logged on and we send the message:
From ! {messenger, receiver_not_found};
which is received by the client, if keysearch
returns:
{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 await_result
function.