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<title>Nine Nines: cowboy_handler(3)</title>
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<h1 class="lined-header"><span>cowboy_handler(3)</span></h1>
<div class="sect1">
<h2 id="_name">Name</h2>
<div class="sectionbody">
<div class="paragraph"><p>cowboy_handler - Plain HTTP handlers</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_description">Description</h2>
<div class="sectionbody">
<div class="paragraph"><p>The <code>cowboy_handler</code> middleware executes the handler selected
by the router or any other preceding middleware.</p></div>
<div class="paragraph"><p>This middleware takes the handler module and initial state
from the <code>handler</code> and <code>handler_opts</code> environment values,
respectively. On completion, it adds a <code>result</code> value to
the middleware environment, containing the return value
of the <code>terminate/3</code> callback (if defined) and <code>ok</code> otherwise.</p></div>
<div class="paragraph"><p>This module also defines a callback interface for handling
HTTP requests.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_callbacks">Callbacks</h2>
<div class="sectionbody">
<div class="paragraph"><p>Plain HTTP handlers implement the following interface:</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight
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">ok</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}
<span style="font-weight: bold"><span style="color: #000000">terminate</span></span>(<span style="color: #009900">Reason</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></span> <span style="color: #FF6600">ok</span> <span style="font-style: italic"><span style="color: #9A1900">%% optional</span></span>
<span style="color: #009900">Req</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:req</span></span>()
<span style="color: #009900">State</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000000">any</span></span>()
<span style="color: #009900">Reason</span> <span style="color: #990000">::</span> <span style="font-weight: bold"><span style="color: #000080">normal</span></span>
| {<span style="color: #FF6600">crash</span>, <span style="color: #FF6600">error</span> | <span style="font-weight: bold"><span style="color: #000080">exit</span></span> | <span style="font-weight: bold"><span style="color: #000080">throw</span></span>, <span style="font-weight: bold"><span style="color: #000000">any</span></span>()}</tt></pre></div></div>
<div class="paragraph"><p>These two callbacks are common to all handlers.</p></div>
<div class="paragraph"><p>Plain HTTP handlers do all their work in the <code>init/2</code>
callback. Returning <code>ok</code> terminates the handler. If no
response is sent, Cowboy will send a <code>204 No Content</code>.</p></div>
<div class="paragraph"><p>The optional <code>terminate/3</code> callback will ultimately be called
with the reason for the termination of the handler.
Cowboy will terminate the process right after this. There
is no need to perform any cleanup in this callback.</p></div>
<div class="paragraph"><p>The following terminate reasons are defined for plain HTTP
handlers:</p></div>
<div class="dlist"><dl>
<dt class="hdlist1">
normal
</dt>
<dd>
<p>
The connection was closed normally.
</p>
</dd>
<dt class="hdlist1">
{crash, Class, Reason}
</dt>
<dd>
<p>
A crash occurred in the handler. <code>Class</code> and <code>Reason</code> can be
used to obtain more information about the crash. The function
<code>erlang:get_stacktrace/0</code> can also be called to obtain the
stacktrace of the process when the crash occurred.
</p>
</dd>
</dl></div>
</div>
</div>
<div class="sect1">
<h2 id="_exports">Exports</h2>
<div class="sectionbody">
<div class="paragraph"><p>The following function should be called by modules implementing
custom handlers to execute the optional terminate callback:</p></div>
<div class="ulist"><ul>
<li>
<p>
<a href="../cowboy_handler.terminate">cowboy_handler:terminate(3)</a> - Terminate the handler
</p>
</li>
</ul></div>
</div>
</div>
<div class="sect1">
<h2 id="_see_also">See also</h2>
<div class="sectionbody">
<div class="paragraph"><p><a href="..">cowboy(7)</a></p></div>
</div>
</div>
</div>
<div class="span3 sidecol">
<h3>
Cowboy
2.4
Function Reference
</h3>
<ul>
<li><a href="/docs/en/cowboy/2.4/guide">User Guide</a></li>
<li><a href="/docs/en/cowboy/2.4/manual">Function Reference</a></li>
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<h4>Version select</h4>
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<li><a href="/docs/en/cowboy/2.4/manual">2.4</a></li>
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<li><a href="/docs/en/cowboy/2.1/manual">2.1</a></li>
<li><a href="/docs/en/cowboy/2.0/manual">2.0</a></li>
<li><a href="/docs/en/cowboy/1.0/manual">1.0</a></li>
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