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<title>Nine Nines: Protocols</title>
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<h1 class="lined-header"><span>Protocols</span></h1>
<div class="paragraph"><p>A protocol handler starts a connection process and defines the
protocol logic executed in this process.</p></div>
<div class="sect1">
<h2 id="_writing_a_protocol_handler">Writing a protocol handler</h2>
<div class="sectionbody">
<div class="paragraph"><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/6</code>. This callback must
return <code>{ok, Pid}</code>, with <code>Pid</code> the pid of the new process.</p></div>
<div class="paragraph"><p>The newly started process can then freely initialize itself. However,
it must call <code>ranch:accept_ack/1</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>
<div class="listingblock">
<div class="title">Acknowledge accepting the socket</div>
<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: #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">Ref</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>If your protocol code requires specific socket options, you should
set them while initializing your connection process, after
calling <code>ranch:accept_ack/1</code>. You can use <code>Transport:setopts/2</code>
for that purpose.</p></div>
<div class="paragraph"><p>Following is the complete protocol code for the example found
in <code>examples/tcp_echo/</code>.</p></div>
<div class="listingblock">
<div class="title">Protocol module that echoes everything it receives</div>
<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">echo_protocol</span>)<span style="color: #990000">.</span>
<span style="font-weight: bold"><span style="color: #000080">-behaviour</span></span>(<span style="color: #FF6600">ranch_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="color: #990000">.</span>
<span style="font-weight: bold"><span style="color: #000080">-export</span></span>([<span style="font-weight: bold"><span style="color: #000000">init</span></span><span style="color: #990000">/</span><span style="color: #993399">4</span>])<span style="color: #990000">.</span>
<span style="font-weight: bold"><span style="color: #000000">start_link</span></span>(<span style="color: #009900">Ref</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">Ref</span>, <span style="color: #009900">Socket</span>, <span style="color: #009900">Transport</span>, <span style="color: #009900">Opts</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">Ref</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: #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">Ref</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: #990000">.</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: #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">5000</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">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: #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: #990000">_</span> <span style="color: #990000">-></span>
<span style="color: #0000FF">ok</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">close</span></span>(<span style="color: #009900">Socket</span>)
<span style="font-weight: bold"><span style="color: #0000FF">end</span></span><span style="color: #990000">.</span></tt></pre></div></div>
</div>
</div>
<div class="sect1">
<h2 id="_using_gen_server">Using gen_server</h2>
<div class="sectionbody">
<div class="paragraph"><p>Special processes like the ones that use the <code>gen_server</code> or <code>gen_fsm</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:accept_ack/1</code> from the <code>init</code> callback
as this would cause a deadlock to happen.</p></div>
<div class="paragraph"><p>There are two ways of solving this problem.</p></div>
<div class="paragraph"><p>The first, and probably the most elegant one, is to make use of the
<code>gen_server:enter_loop/3</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_server</code> execution loop.</p></div>
<div class="listingblock">
<div class="title">Use a gen_server for protocol handling</div>
<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">my_protocol</span>)<span style="color: #990000">.</span>
<span style="font-weight: bold"><span style="color: #000080">-behaviour</span></span>(<span style="color: #FF6600">gen_server</span>)<span style="color: #990000">.</span>
<span style="font-weight: bold"><span style="color: #000080">-behaviour</span></span>(<span style="color: #FF6600">ranch_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="color: #990000">.</span>
<span style="font-weight: bold"><span style="color: #000080">-export</span></span>([<span style="font-weight: bold"><span style="color: #000000">init</span></span><span style="color: #990000">/</span><span style="color: #993399">4</span>])<span style="color: #990000">.</span>
<span style="font-style: italic"><span style="color: #9A1900">%% Exports of other gen_server callbacks here.</span></span>
<span style="font-weight: bold"><span style="color: #000000">start_link</span></span>(<span style="color: #009900">Ref</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="font-weight: bold"><span style="color: #000000">proc_lib:start_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">Ref</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="font-weight: bold"><span style="color: #000000">init</span></span>(<span style="color: #009900">Ref</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: #990000">-></span>
<span style="color: #0000FF">ok</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">proc_lib:init_ack</span></span>({<span style="color: #FF6600">ok</span>, <span style="font-weight: bold"><span style="color: #000080">self</span></span>()}),
<span style="font-style: italic"><span style="color: #9A1900">%% Perform any required state initialization here.</span></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">Ref</span>),
<span style="color: #0000FF">ok</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">setopts</span></span>(<span style="color: #009900">Socket</span>, [{<span style="color: #FF6600">active</span>, <span style="color: #FF6600">once</span>}]),
<span style="font-weight: bold"><span style="color: #000000">gen_server:enter_loop</span></span>(<span style="font-weight: bold"><span style="color: #000080">?MODULE</span></span>, [], {<span style="color: #FF6600">state</span>, <span style="color: #009900">Socket</span>, <span style="color: #009900">Transport</span>})<span style="color: #990000">.</span>
<span style="font-style: italic"><span style="color: #9A1900">%% Other gen_server callbacks here.</span></span></tt></pre></div></div>
<div class="paragraph"><p>The second method involves triggering a timeout just after <code>gen_server:init</code>
ends. If you return a timeout value of <code>0</code> then the <code>gen_server</code> will call
<code>handle_info(timeout, _, _)</code> right away.</p></div>
<div class="listingblock">
<div class="title">Use a gen_server for protocol handling, method 2</div>
<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">my_protocol</span>)<span style="color: #990000">.</span>
<span style="font-weight: bold"><span style="color: #000080">-behaviour</span></span>(<span style="color: #FF6600">gen_server</span>)<span style="color: #990000">.</span>
<span style="font-weight: bold"><span style="color: #000080">-behaviour</span></span>(<span style="color: #FF6600">ranch_protocol</span>)<span style="color: #990000">.</span>
<span style="font-style: italic"><span style="color: #9A1900">%% Exports go here.</span></span>
<span style="font-weight: bold"><span style="color: #000000">init</span></span>([<span style="color: #009900">Ref</span>, <span style="color: #009900">Socket</span>, <span style="color: #009900">Transport</span>]) <span style="color: #990000">-></span>
{<span style="color: #FF6600">ok</span>, {<span style="color: #FF6600">state</span>, <span style="color: #009900">Ref</span>, <span style="color: #009900">Socket</span>, <span style="color: #009900">Transport</span>}, <span style="color: #993399">0</span>}<span style="color: #990000">.</span>
<span style="font-weight: bold"><span style="color: #000000">handle_info</span></span>(<span style="color: #FF6600">timeout</span>, <span style="color: #009900">State</span><span style="color: #990000">=</span>{<span style="color: #FF6600">state</span>, <span style="color: #009900">Ref</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">Ref</span>),
<span style="color: #0000FF">ok</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">setopts</span></span>(<span style="color: #009900">Socket</span>, [{<span style="color: #FF6600">active</span>, <span style="color: #FF6600">once</span>}]),
{<span style="color: #FF6600">noreply</span>, <span style="color: #009900">State</span>};
<span style="font-style: italic"><span style="color: #9A1900">%% ...</span></span></tt></pre></div></div>
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