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<h1 class="lined-header"><span>Protocols</span></h1>
<p>A protocol handler starts a connection process and defines the protocol logic executed in this process.</p>
<h2 id="_writing_a_protocol_handler">Writing a protocol handler</h2>
<p>All protocol handlers must implement the <code>ranch_protocol</code> behavior which defines a single callback, <code>start_link/4</code>. This callback is responsible for spawning a new process for handling the connection. It receives four arguments: the name of the listener, the socket, the transport handler being used and the protocol options defined in the call to <code>ranch:start_listener/5</code>. This callback must return <code>{ok, Pid}</code>, with <code>Pid</code> the pid of the new process.</p>
<p>The newly started process can then freely initialize itself. However, it must call <code>ranch:handshake/1,2</code> before doing any socket operation. This will ensure the connection process is the owner of the socket. It expects the listener's name as argument.</p>
<div class="listingblock"><div class="title">Perform the socket handshake</div>
<div class="content"><!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt>{<font color="#FF6600">ok</font>, <font color="#009900">Socket</font>} <font color="#990000">=</font> <b><font color="#000000">ranch:handshake</font></b>(<font color="#009900">Ref</font>)<font color="#990000">.</font></tt></pre>
</div></div>
<p>If your protocol code requires specific socket options, you should set them while initializing your connection process, after calling <code>ranch:handshake/1,2</code>. You can use <code>Transport:setopts/2</code> for that purpose.</p>
<p>Following is the complete protocol code for the example found in <code>examples/tcp_echo/</code>.</p>
<div class="listingblock"><div class="title">Protocol module that echoes everything it receives</div>
<div class="content"><!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><b><font color="#000080">-module</font></b>(<font color="#FF6600">echo_protocol</font>)<font color="#990000">.</font>
<b><font color="#000080">-behaviour</font></b>(<font color="#FF6600">ranch_protocol</font>)<font color="#990000">.</font>
<b><font color="#000080">-export</font></b>([<b><font color="#000000">start_link</font></b><font color="#990000">/</font><font color="#993399">4</font>])<font color="#990000">.</font>
<b><font color="#000080">-export</font></b>([<b><font color="#000000">init</font></b><font color="#990000">/</font><font color="#993399">3</font>])<font color="#990000">.</font>
<b><font color="#000000">start_link</font></b>(<font color="#009900">Ref</font>, <font color="#009900">_Socket</font>, <font color="#009900">Transport</font>, <font color="#009900">Opts</font>) <font color="#990000">-></font>
<font color="#009900">Pid</font> <font color="#990000">=</font> <b><font color="#000080">spawn_link</font></b>(<b><font color="#000080">?MODULE</font></b>, <font color="#FF6600">init</font>, [<font color="#009900">Ref</font>, <font color="#009900">Transport</font>, <font color="#009900">Opts</font>]),
{<font color="#FF6600">ok</font>, <font color="#009900">Pid</font>}<font color="#990000">.</font>
<b><font color="#000000">init</font></b>(<font color="#009900">Ref</font>, <font color="#009900">Transport</font>, <font color="#009900">_Opts</font> <font color="#990000">=</font> []) <font color="#990000">-></font>
{<font color="#FF6600">ok</font>, <font color="#009900">Socket</font>} <font color="#990000">=</font> <b><font color="#000000">ranch:handshake</font></b>(<font color="#009900">Ref</font>),
<b><font color="#000000">loop</font></b>(<font color="#009900">Socket</font>, <font color="#009900">Transport</font>)<font color="#990000">.</font>
<b><font color="#000000">loop</font></b>(<font color="#009900">Socket</font>, <font color="#009900">Transport</font>) <font color="#990000">-></font>
<b><font color="#0000FF">case</font></b> <font color="#009900">Transport</font><font color="#990000">:</font><b><font color="#000000">recv</font></b>(<font color="#009900">Socket</font>, <font color="#993399">0</font>, <font color="#993399">5000</font>) <b><font color="#0000FF">of</font></b>
{<font color="#FF6600">ok</font>, <font color="#009900">Data</font>} <font color="#990000">-></font>
<font color="#009900">Transport</font><font color="#990000">:</font><b><font color="#000000">send</font></b>(<font color="#009900">Socket</font>, <font color="#009900">Data</font>),
<b><font color="#000000">loop</font></b>(<font color="#009900">Socket</font>, <font color="#009900">Transport</font>);
<font color="#990000">_</font> <font color="#990000">-></font>
<font color="#0000FF">ok</font> <font color="#990000">=</font> <font color="#009900">Transport</font><font color="#990000">:</font><b><font color="#000000">close</font></b>(<font color="#009900">Socket</font>)
<b><font color="#0000FF">end</font></b><font color="#990000">.</font></tt></pre>
</div></div>
<h2 id="_using_gen_statem">Using gen_statem</h2>
<p>Special processes like the ones that use the <code>gen_statem</code> or <code>gen_server</code> behaviours have the particularity of having their <code>start_link</code> call not return until the <code>init</code> function returns. This is problematic, because you won't be able to call <code>ranch:handshake/1,2</code> from the <code>init</code> callback as this would cause a deadlock to happen.</p>
<p>Use the <code>gen_statem:enter_loop/4</code> function. It allows you to start your process normally (although it must be started with <code>proc_lib</code> like all special processes), then perform any needed operations before falling back into the normal <code>gen_statem</code> execution loop.</p>
<div class="listingblock"><div class="title">Use a gen_statem for protocol handling</div>
<div class="content"><!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><b><font color="#000080">-module</font></b>(<font color="#FF6600">my_protocol</font>)<font color="#990000">.</font>
<b><font color="#000080">-behaviour</font></b>(<font color="#FF6600">gen_statem</font>)<font color="#990000">.</font>
<b><font color="#000080">-behaviour</font></b>(<font color="#FF6600">ranch_protocol</font>)<font color="#990000">.</font>
<b><font color="#000080">-export</font></b>([<b><font color="#000000">start_link</font></b><font color="#990000">/</font><font color="#993399">4</font>])<font color="#990000">.</font>
<b><font color="#000080">-export</font></b>([<b><font color="#000000">init</font></b><font color="#990000">/</font><font color="#993399">1</font>])<font color="#990000">.</font>
<i><font color="#9A1900">%% Exports of other gen_statem callbacks here.</font></i>
<b><font color="#000000">start_link</font></b>(<font color="#009900">Ref</font>, <font color="#009900">_Socket</font>, <font color="#009900">Transport</font>, <font color="#009900">Opts</font>) <font color="#990000">-></font>
{<font color="#FF6600">ok</font>, <b><font color="#000000">proc_lib:spawn_link</font></b>(<b><font color="#000080">?MODULE</font></b>, <font color="#FF6600">init</font>, [{<font color="#009900">Ref</font>, <font color="#009900">Transport</font>, <font color="#009900">Opts</font>}])}<font color="#990000">.</font>
<b><font color="#000000">init</font></b>({<font color="#009900">Ref</font>, <font color="#009900">Transport</font>, <font color="#009900">_Opts</font> <font color="#990000">=</font> []}) <font color="#990000">-></font>
<i><font color="#9A1900">%% Perform any required state initialization here.</font></i>
{<font color="#FF6600">ok</font>, <font color="#009900">Socket</font>} <font color="#990000">=</font> <b><font color="#000000">ranch:handshake</font></b>(<font color="#009900">Ref</font>),
<font color="#0000FF">ok</font> <font color="#990000">=</font> <font color="#009900">Transport</font><font color="#990000">:</font><b><font color="#000000">setopts</font></b>(<font color="#009900">Socket</font>, [{<font color="#FF6600">active</font>, <font color="#FF6600">once</font>}]),
<b><font color="#000000">gen_statem:enter_loop</font></b>(<b><font color="#000080">?MODULE</font></b>, [], <font color="#FF6600">state_name</font>, {<font color="#FF6600">state_data</font>, <font color="#009900">Socket</font>, <font color="#009900">Transport</font>})<font color="#990000">.</font>
<i><font color="#9A1900">%% Other gen_statem callbacks here.</font></i></tt></pre>
</div></div>
<p>Check the <code>tcp_reverse</code> example for a complete example.</p>
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