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<title>Nine Nines: Sub protocols</title>
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<h1 class="lined-header"><span>Sub protocols</span></h1>
<div class="paragraph"><p>Sub protocols are used for creating new types of handlers that
provide extra functionality in a reusable way. Cowboy uses this
mechanism to provide its loop, REST and Websocket handlers.</p></div>
<div class="paragraph"><p>This chapter will explain how to create your own sub protocols
and handler types.</p></div>
<div class="sect1">
<h2 id="_usage">Usage</h2>
<div class="sectionbody">
<div class="paragraph"><p>To switch to a sub protocol, the <code>init/2</code> callback must return
the name of the sub protocol module. Everything past this point
is handled by the sub protocol.</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">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span>
{<span style="color: #FF6600">cowboy_websocket</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>The return value may also have a <code>Timeout</code> value and/or the
atom <code>hibernate</code>. These options are useful for long living
connections. When they are not provided, the timeout value
defaults to <code>infinity</code> and the hibernate value to <code>run</code>.</p></div>
<div class="paragraph"><p>The following snippet switches to the <code>my_protocol</code> sub
protocol, sets the timeout value to 5 seconds and enables
hibernation:</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">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span>
{<span style="color: #FF6600">my_protocol</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>, <span style="color: #993399">5000</span>, <span style="color: #FF6600">hibernate</span>}<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>If a sub protocol does not make use of these options, it should
crash if it receives anything other than the default values.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_upgrade">Upgrade</h2>
<div class="sectionbody">
<div class="paragraph"><p>After the <code>init/2</code> function returns, Cowboy will then call the
<code>upgrade/6</code> function. This is the only callback defined by the
<code>cowboy_sub_protocol</code> behavior.</p></div>
<div class="paragraph"><p>The function is named <code>upgrade</code> because it mimics the mechanism
of HTTP protocol upgrades. For some sub protocols, like Websocket,
an actual upgrade is performed. For others, like REST, this is
only an upgrade at Cowboy’s level and the client has nothing to
do about it.</p></div>
<div class="paragraph"><p>The upgrade callback receives the Req object, the middleware
environment, the handler and its options, and the aforementioned
timeout and hibernate values.</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">upgrade</span></span>(<span style="color: #009900">Req</span>, <span style="color: #009900">Env</span>, <span style="color: #009900">Handler</span>, <span style="color: #009900">HandlerOpts</span>, <span style="color: #009900">Timeout</span>, <span style="color: #009900">Hibernate</span>) <span style="color: #990000">-></span>
<span style="font-style: italic"><span style="color: #9A1900">%% Sub protocol code here.</span></span></tt></pre></div></div>
<div class="paragraph"><p>This callback is expected to behave like a middleware and to
return an updated environment and Req object.</p></div>
<div class="paragraph"><p>Sub protocols are expected to call the <code>cowboy_handler:terminate/4</code>
function when they terminate. This function will make sure that
the optional <code>terminate/3</code> callback is called, if present.</p></div>
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