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<header>
<h1 class="lined-header"><span>Build an FTP Server with Ranch in 30 Minutes</span></h1>
<p class="date">
<span class="day">14</span>
<span class="month">Nov</span>
</p>
</header>
<div class="paragraph"><p>Last week I was speaking at the
<a href="http://www.erlang-factory.com/conference/London2012/speakers/LoicHoguin">London Erlang Factory Lite</a>
where I presented a live demonstration of building an FTP server using
<a href="http://ninenines.eu/docs/en/ranch/HEAD/README">Ranch</a>.
As there was no slide, you should use this article as a reference instead.</p></div>
<div class="paragraph"><p>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.</p></div>
<div class="paragraph"><p>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 <code>rebar.config</code> file and add the
following 3 lines.</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt>{<span style="color: #FF6600">deps</span>, [
{<span style="color: #FF6600">ranch</span>, <span style="color: #FF0000">".*"</span>, {<span style="color: #FF6600">git</span>, <span style="color: #FF0000">"git://github.com/extend/ranch.git"</span>, <span style="color: #FF0000">"master"</span>}}
]}<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>This makes your application depend on the last Ranch version available
on the <em>master</em> 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.</p></div>
<div class="paragraph"><p>You can now fetch the dependencies.</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt>$ rebar get-deps
<span style="color: #990000">==></span> ranch_ftp <span style="color: #990000">(</span>get-deps<span style="color: #990000">)</span>
Pulling ranch from {git<span style="color: #990000">,</span><span style="color: #FF0000">"git://github.com/extend/ranch.git"</span><span style="color: #990000">,</span><span style="color: #FF0000">"master"</span>}
Cloning into <span style="color: #FF0000">'ranch'</span><span style="color: #990000">...</span>
<span style="color: #990000">==></span> ranch <span style="color: #990000">(</span>get-deps<span style="color: #990000">)</span></tt></pre></div></div>
<div class="paragraph"><p>This will create a <em>deps/</em> folder containing Ranch.</p></div>
<div class="paragraph"><p>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 <em>ranch_ftp_protocol.erl</em> and open it in your favorite
editor.</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt>$ vim ranch_ftp_protocol<span style="color: #990000">.</span>erl</tt></pre></div></div>
<div class="paragraph"><p>Let’s start with a blank protocol module.</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="font-weight: bold"><span style="color: #000080">-module</span></span>(<span style="color: #FF6600">ranch_ftp_protocol</span>)<span style="color: #990000">.</span>
<span style="font-weight: bold"><span style="color: #000080">-export</span></span>([<span style="font-weight: bold"><span style="color: #000000">start_link</span></span><span style="color: #990000">/</span><span style="color: #993399">4</span>, <span style="font-weight: bold"><span style="color: #000000">init</span></span><span style="color: #990000">/</span><span style="color: #993399">3</span>])<span style="color: #990000">.</span>
<span style="font-weight: bold"><span style="color: #000000">start_link</span></span>(<span style="color: #009900">ListenerPid</span>, <span style="color: #009900">Socket</span>, <span style="color: #009900">Transport</span>, <span style="color: #009900">Opts</span>) <span style="color: #990000">-></span>
<span style="color: #009900">Pid</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000080">spawn_link</span></span>(<span style="font-weight: bold"><span style="color: #000080">?MODULE</span></span>, <span style="color: #FF6600">init</span>, [<span style="color: #009900">ListenerPid</span>, <span style="color: #009900">Socket</span>, <span style="color: #009900">Transport</span>]),
{<span style="color: #FF6600">ok</span>, <span style="color: #009900">Pid</span>}<span style="color: #990000">.</span>
<span style="font-weight: bold"><span style="color: #000000">init</span></span>(<span style="color: #009900">ListenerPid</span>, <span style="color: #009900">Socket</span>, <span style="color: #009900">Transport</span>) <span style="color: #990000">-></span>
<span style="font-weight: bold"><span style="color: #000000">io:format</span></span>(<span style="color: #FF0000">"Got a connection!~n"</span>),
<span style="color: #FF6600">ok</span><span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>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.</p></div>
<div class="paragraph"><p>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.</p></div>
<div class="paragraph"><p>Also note that you need to type <code>c(ranch_ftp_protocol).</code>
to recompile and reload the code for the protocol. You do not need to
restart any process however.</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt>$ erl -pa ebin deps<span style="color: #990000">/*</span>/ebin
Erlang R15B02 <span style="color: #990000">(</span>erts-<span style="color: #993399">5.9</span><span style="color: #990000">.</span><span style="color: #993399">2</span><span style="color: #990000">)</span> <span style="color: #990000">[</span><span style="font-weight: bold"><span style="color: #0000FF">source</span></span><span style="color: #990000">]</span> <span style="color: #990000">[</span><span style="color: #993399">64</span>-bit<span style="color: #990000">]</span> <span style="color: #990000">[</span>smp<span style="color: #990000">:</span><span style="color: #993399">4</span><span style="color: #990000">:</span><span style="color: #993399">4</span><span style="color: #990000">]</span> <span style="color: #990000">[</span>async-threads<span style="color: #990000">:</span><span style="color: #993399">0</span><span style="color: #990000">]</span> <span style="color: #990000">[</span>hipe<span style="color: #990000">]</span> <span style="color: #990000">[</span>kernel-poll<span style="color: #990000">:</span><span style="font-weight: bold"><span style="color: #0000FF">false</span></span><span style="color: #990000">]</span>
Eshell V5<span style="color: #990000">.</span><span style="color: #993399">9.2</span> <span style="color: #990000">(</span>abort with <span style="color: #990000">^</span>G<span style="color: #990000">)</span></tt></pre></div></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="color: #993399">1</span><span style="color: #990000">></span> <span style="font-weight: bold"><span style="color: #000000">application:start</span></span>(<span style="color: #FF6600">ranch</span>)<span style="color: #990000">.</span>
<span style="color: #FF6600">ok</span>
<span style="color: #993399">2</span><span style="color: #990000">></span> <span style="font-weight: bold"><span style="color: #000000">ranch:start_listener</span></span>(<span style="color: #FF6600">my_ftp</span>, <span style="color: #993399">10</span>,
<span style="color: #FF6600">ranch_tcp</span>, [{<span style="color: #FF6600">port</span>, <span style="color: #993399">2121</span>}],
<span style="color: #FF6600">ranch_ftp_protocol</span>, [])<span style="color: #990000">.</span>
{<span style="color: #FF6600">ok</span>,<span style="color: #990000"><</span><span style="color: #993399">0.40</span><span style="color: #990000">.</span><span style="color: #993399">0</span><span style="color: #990000">></span>}</tt></pre></div></div>
<div class="paragraph"><p>This starts a listener named <code>my_ftp</code> that runs your very own
<code>ranch_ftp_protocol</code> over TCP, listening on port <code>2121</code>.
The last argument is the options given to the protocol that we ignored
earlier.</p></div>
<div class="paragraph"><p>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.</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt>$ ftp localhost <span style="color: #993399">2121</span></tt></pre></div></div>
<div class="paragraph"><p>Let’s move on to actually writing the protocol.</p></div>
<div class="paragraph"><p>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.</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="font-weight: bold"><span style="color: #000000">init</span></span>(<span style="color: #009900">ListenerPid</span>, <span style="color: #009900">Socket</span>, <span style="color: #009900">Transport</span>) <span style="color: #990000">-></span>
<span style="color: #0000FF">ok</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">ranch:accept_ack</span></span>(<span style="color: #009900">ListenerPid</span>),
<span style="color: #FF6600">ok</span><span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>Now that you acknowledged the new connection, you can use it safely.</p></div>
<div class="paragraph"><p>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 <code>200</code>. 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 <code>230</code> code.</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="font-weight: bold"><span style="color: #000000">init</span></span>(<span style="color: #009900">ListenerPid</span>, <span style="color: #009900">Socket</span>, <span style="color: #009900">Transport</span>) <span style="color: #990000">-></span>
<span style="color: #0000FF">ok</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">ranch:accept_ack</span></span>(<span style="color: #009900">ListenerPid</span>),
<span style="color: #009900">Transport</span><span style="color: #990000">:</span><span style="font-weight: bold"><span style="color: #000000">send</span></span>(<span style="color: #009900">Socket</span>, <span style="color: #990000"><<</span><span style="color: #FF0000">"200 My cool FTP server welcomes you!\r\n"</span><span style="color: #990000">>></span>),
{<span style="color: #FF6600">ok</span>, <span style="color: #009900">Data</span>} <span style="color: #990000">=</span> <span style="color: #009900">Transport</span><span style="color: #990000">:</span><span style="font-weight: bold"><span style="color: #000000">recv</span></span>(<span style="color: #009900">Socket</span>, <span style="color: #993399">0</span>, <span style="color: #993399">30000</span>),
<span style="font-weight: bold"><span style="color: #000000">auth</span></span>(<span style="color: #009900">Socket</span>, <span style="color: #009900">Transport</span>, <span style="color: #009900">Data</span>)<span style="color: #990000">.</span>
<span style="font-weight: bold"><span style="color: #000000">auth</span></span>(<span style="color: #009900">Socket</span>, <span style="color: #009900">Transport</span>, <span style="color: #990000"><<</span><span style="color: #FF0000">"USER "</span>, <span style="color: #009900">Rest</span><span style="color: #990000">/</span><span style="color: #FF6600">bits</span><span style="color: #990000">>></span>) <span style="color: #990000">-></span>
<span style="font-weight: bold"><span style="color: #000000">io:format</span></span>(<span style="color: #FF0000">"User authenticated! ~p~n"</span>, [<span style="color: #009900">Rest</span>]),
<span style="color: #009900">Transport</span><span style="color: #990000">:</span><span style="font-weight: bold"><span style="color: #000000">send</span></span>(<span style="color: #009900">Socket</span>, <span style="color: #990000"><<</span><span style="color: #FF0000">"230 Auth OK\r\n"</span><span style="color: #990000">>></span>),
<span style="color: #FF6600">ok</span><span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>As you can see we don’t need complex parsing code. We can simply
match on the binary in the argument!</p></div>
<div class="paragraph"><p>Next we need to loop receiving data commands and optionally
execute them, if we want our server to become useful.</p></div>
<div class="paragraph"><p>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.</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="font-weight: bold"><span style="color: #000000">loop</span></span>(<span style="color: #009900">Socket</span>, <span style="color: #009900">Transport</span>) <span style="color: #990000">-></span>
<span style="font-weight: bold"><span style="color: #0000FF">case</span></span> <span style="color: #009900">Transport</span><span style="color: #990000">:</span><span style="font-weight: bold"><span style="color: #000000">recv</span></span>(<span style="color: #009900">Socket</span>, <span style="color: #993399">0</span>, <span style="color: #993399">30000</span>) <span style="font-weight: bold"><span style="color: #0000FF">of</span></span>
{<span style="color: #FF6600">ok</span>, <span style="color: #009900">Data</span>} <span style="color: #990000">-></span>
<span style="font-weight: bold"><span style="color: #000000">handle</span></span>(<span style="color: #009900">Socket</span>, <span style="color: #009900">Transport</span>, <span style="color: #009900">Data</span>),
<span style="font-weight: bold"><span style="color: #000000">loop</span></span>(<span style="color: #009900">Socket</span>, <span style="color: #009900">Transport</span>);
{<span style="color: #FF6600">error</span>, <span style="color: #990000">_</span>} <span style="color: #990000">-></span>
<span style="font-weight: bold"><span style="color: #000000">io:format</span></span>(<span style="color: #FF0000">"The client disconnected~n"</span>)
<span style="font-weight: bold"><span style="color: #0000FF">end</span></span><span style="color: #990000">.</span>
<span style="font-weight: bold"><span style="color: #000000">handle</span></span>(<span style="color: #009900">Socket</span>, <span style="color: #009900">Transport</span>, <span style="color: #009900">Data</span>) <span style="color: #990000">-></span>
<span style="font-weight: bold"><span style="color: #000000">io:format</span></span>(<span style="color: #FF0000">"Command received ~p~n"</span>, [<span style="color: #009900">Data</span>]),
<span style="color: #009900">Transport</span><span style="color: #990000">:</span><span style="font-weight: bold"><span style="color: #000000">send</span></span>(<span style="color: #009900">Socket</span>, <span style="color: #990000"><<</span><span style="color: #FF0000">"500 Bad command\r\n"</span><span style="color: #990000">>></span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>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.</p></div>
<div class="paragraph"><p>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.</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="font-weight: bold"><span style="color: #000000">loop</span></span>(<span style="color: #009900">Socket</span>, <span style="color: #009900">Transport</span>, <span style="color: #009900">Buffer</span>) <span style="color: #990000">-></span>
<span style="font-weight: bold"><span style="color: #0000FF">case</span></span> <span style="color: #009900">Transport</span><span style="color: #990000">:</span><span style="font-weight: bold"><span style="color: #000000">recv</span></span>(<span style="color: #009900">Socket</span>, <span style="color: #993399">0</span>, <span style="color: #993399">30000</span>) <span style="font-weight: bold"><span style="color: #0000FF">of</span></span>
{<span style="color: #FF6600">ok</span>, <span style="color: #009900">Data</span>} <span style="color: #990000">-></span>
<span style="color: #009900">Buffer2</span> <span style="color: #990000">=</span> <span style="color: #990000"><<</span> <span style="color: #009900">Buffer</span><span style="color: #990000">/</span><span style="font-weight: bold"><span style="color: #000080">binary</span></span>, <span style="color: #009900">Data</span><span style="color: #990000">/</span><span style="font-weight: bold"><span style="color: #000080">binary</span></span> <span style="color: #990000">>></span>,
{<span style="color: #009900">Commands</span>, <span style="color: #009900">Rest</span>} <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">split</span></span>(<span style="color: #009900">Buffer2</span>),
[<span style="font-weight: bold"><span style="color: #000000">handle</span></span>(<span style="color: #009900">Socket</span>, <span style="color: #009900">Transport</span>, <span style="color: #009900">C</span>) || <span style="color: #009900">C</span> <span style="color: #990000"><-</span> <span style="color: #009900">Commands</span>],
<span style="font-weight: bold"><span style="color: #000000">loop</span></span>(<span style="color: #009900">Socket</span>, <span style="color: #009900">Transport</span>);
{<span style="color: #FF6600">error</span>, <span style="color: #990000">_</span>} <span style="color: #990000">-></span>
<span style="font-weight: bold"><span style="color: #000000">io:format</span></span>(<span style="color: #FF0000">"The client disconnected~n"</span>)
<span style="font-weight: bold"><span style="color: #0000FF">end</span></span><span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>The implementation of <code>split/1</code> is left as an exercice
to the reader. You will also probably want to handle the <code>QUIT</code>
command, which must stop any processing and close the connection.</p></div>
<div class="paragraph"><p>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 <code>Transport:setopts/2</code> and have all the
buffering done for you for free by Erlang itself.</p></div>
<div class="paragraph"><p>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.</p></div>
<div class="ulist"><ul>
<li>
<p>
<a href="http://www.erlang-factory.com/conference/London2012/speakers/LoicHoguin">Watch the talk</a>
</p>
</li>
</ul></div>
</article>
</div>
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<h3>Feedback</h3>
<p>Feel free to <a href="mailto:[email protected]">email us</a>
if you found any mistake or need clarification on any of the
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