+++
date = "2012-11-14T00:00:00+01:00"
title = "Build an FTP Server with Ranch in 30 Minutes"

+++

Last week I was speaking at the
http://www.erlang-factory.com/conference/London2012/speakers/LoicHoguin[London Erlang Factory Lite]
where I presented a live demonstration of building an FTP server using
http://ninenines.eu/docs/en/ranch/HEAD/README[Ranch].
As there was no slide, you should use this article as a reference instead.

The goal of this article is to showcase how to use Ranch for writing
a network protocol implementation, how Ranch gets out of the way to let
you write the code that matters, and the common techniques used when
writing servers.

Let's start by creating an empty project. Create a new directory and
then open a terminal into that directory. The first step is to add Ranch
as a dependency. Create the `rebar.config` file and add the
following 3 lines.

[source,erlang]
----
{deps, [
    {ranch, ".*", {git, "git://github.com/extend/ranch.git", "master"}}
]}.
----

This makes your application depend on the last Ranch version available
on the _master_ branch. This is fine for development, however when
you start pushing your application to production you will want to revisit
this file to hardcode the exact version you are using, to make sure you
run the same version of dependencies in production.

You can now fetch the dependencies.

[source,bash]
----
$ rebar get-deps
==> ranch_ftp (get-deps)
Pulling ranch from {git,"git://github.com/extend/ranch.git","master"}
Cloning into 'ranch'...
==> ranch (get-deps)
----

This will create a 'deps/' folder containing Ranch.

We don't actually need anything else to write the protocol code.
We could make an application for it, but this isn't the purpose of this
article so let's just move on to writing the protocol itself. Create
the file 'ranch_ftp_protocol.erl' and open it in your favorite
editor.

[source,bash]
$ vim ranch_ftp_protocol.erl

Let's start with a blank protocol module.

[source,erlang]
----
-module(ranch_ftp_protocol).
-export([start_link/4, init/3]).

start_link(ListenerPid, Socket, Transport, Opts) ->
    Pid = spawn_link(?MODULE, init, [ListenerPid, Socket, Transport]),
    {ok, Pid}.

init(ListenerPid, Socket, Transport) ->
    io:format("Got a connection!~n"),
    ok.
----

When Ranch receives a connection, it will call the <code>start_link/4</code>
function with the listener's pid, socket, transport module to be used,
and the options we define when starting the listener. We don't need options
for the purpose of this article, so we don't pass them to the process we are
creating.

Let's open a shell and start a Ranch listener to begin accepting
connections. We only need to call one function. You should probably open
it in another terminal and keep it open for convenience. If you quit
the shell you will have to repeat the commands to proceed.

Also note that you need to type `c(ranch_ftp_protocol).`
to recompile and reload the code for the protocol. You do not need to
restart any process however.

[source,bash]
----
$ erl -pa ebin deps/*/ebin
Erlang R15B02 (erts-5.9.2) [source] [64-bit] [smp:4:4] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.9.2  (abort with ^G)
----

[source,erlang]
----
1> application:start(ranch).
ok
2> ranch:start_listener(my_ftp, 10,
       ranch_tcp, [{port, 2121}],
       ranch_ftp_protocol, []).
{ok,<0.40.0>}
----

This starts a listener named `my_ftp` that runs your very own
`ranch_ftp_protocol` over TCP, listening on port `2121`.
The last argument is the options given to the protocol that we ignored
earlier.

To try your code, you can use the following command. It should be able
to connect, the server will print a message in the console, and then
the client will print an error.

[source,bash]
$ ftp localhost 2121

Let's move on to actually writing the protocol.

Once you have created the new process and returned the pid, Ranch will
give ownership of the socket to you. This requires a synchronization
step though.

[source,erlang]
----
init(ListenerPid, Socket, Transport) ->
    ok = ranch:accept_ack(ListenerPid),
    ok.
----

Now that you acknowledged the new connection, you can use it safely.

When an FTP server accepts a connection, it starts by sending a
welcome message which can be one or more lines starting with the
code `200`. Then the server will wait for the client
to authenticate the user, and if the authentication went successfully,
which it will always do for the purpose of this article, it will reply
with a `230` code.

[source,erlang]
----
init(ListenerPid, Socket, Transport) ->
    ok = ranch:accept_ack(ListenerPid),
    Transport:send(Socket, <<"200 My cool FTP server welcomes you!\r\n">>),
    {ok, Data} = Transport:recv(Socket, 0, 30000),
    auth(Socket, Transport, Data).

auth(Socket, Transport, <<"USER ", Rest/bits>>) ->
    io:format("User authenticated! ~p~n", [Rest]),
    Transport:send(Socket, <<"230 Auth OK\r\n">>),
    ok.
----

As you can see we don't need complex parsing code. We can simply
match on the binary in the argument!

Next we need to loop receiving data commands and optionally
execute them, if we want our server to become useful.

We will replace the <code>ok.</code> line with the call to
the following function. The new function is recursive, each call
receiving data from the socket and sending a response. For now
we will send an error response for all commands the client sends.

[source,erlang]
----
loop(Socket, Transport) ->
    case Transport:recv(Socket, 0, 30000) of
        {ok, Data} ->
            handle(Socket, Transport, Data),
            loop(Socket, Transport);
        {error, _} ->
            io:format("The client disconnected~n")
    end.

handle(Socket, Transport, Data) ->
    io:format("Command received ~p~n", [Data]),
    Transport:send(Socket, <<"500 Bad command\r\n">>).
----

With this we are almost ready to start implementing commands.
But with code like this we might have errors if the client doesn't
send just one command per packet, or if the packets arrive too fast,
or if a command is split over multiple packets.

To solve this, we need to use a buffer. Each time we receive data,
we will append to the buffer, and then check if we have received a
command fully before running it. The code could look similar to the
following.

[source,erlang]
----
loop(Socket, Transport, Buffer) ->
    case Transport:recv(Socket, 0, 30000) of
        {ok, Data} ->
			Buffer2 = << Buffer/binary, Data/binary >>,
			{Commands, Rest} = split(Buffer2),
			[handle(Socket, Transport, C) || C <- Commands],
            loop(Socket, Transport, Rest);
        {error, _} ->
            io:format("The client disconnected~n")
    end.
----

The implementation of `split/1` is left as an exercice
to the reader. You will also probably want to handle the `QUIT`
command, which must stop any processing and close the connection.

The attentive reader will also take note that in the case of text-
based protocols where commands are separated by line breaks, you can
set an option using `Transport:setopts/2` and have all the
buffering done for you for free by Erlang itself.

As you can surely notice by now, Ranch allows us to build network
applications by getting out of our way entirely past the initial setup.
It lets you use the power of binary pattern matching to write text and
binary protocol implementations in just a few lines of code.

* http://www.erlang-factory.com/conference/London2012/speakers/LoicHoguin[Watch the talk]