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<title>Nine Nines: Middlewares</title>
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<h1 class="lined-header"><span>Middlewares</span></h1>
<div class="paragraph"><p>Cowboy delegates the request processing to middleware components.
By default, two middlewares are defined, for the routing and handling
of the request, as is detailed in most of this guide.</p></div>
<div class="paragraph"><p>Middlewares give you complete control over how requests are to be
processed. You can add your own middlewares to the mix or completely
change the chain of middlewares as needed.</p></div>
<div class="paragraph"><p>Cowboy will execute all middlewares in the given order, unless one
of them decides to stop processing.</p></div>
<div class="sect1">
<h2 id="_usage">Usage</h2>
<div class="sectionbody">
<div class="paragraph"><p>Middlewares only need to implement a single callback: <code>execute/2</code>.
It is defined in the <code>cowboy_middleware</code> behavior.</p></div>
<div class="paragraph"><p>This callback has two arguments. The first is the <code>Req</code> object.
The second is the environment.</p></div>
<div class="paragraph"><p>Middlewares can return one of three different values:</p></div>
<div class="ulist"><ul>
<li>
<p>
<code>{ok, Req, Env}</code> to continue the request processing
</p>
</li>
<li>
<p>
<code>{suspend, Module, Function, Args}</code> to hibernate
</p>
</li>
<li>
<p>
<code>{stop, Req}</code> to stop processing and move on to the next request
</p>
</li>
</ul></div>
<div class="paragraph"><p>Of note is that when hibernating, processing will resume on the given
MFA, discarding all previous stacktrace. Make sure you keep the <code>Req</code>
and <code>Env</code> in the arguments of this MFA for later use.</p></div>
<div class="paragraph"><p>If an error happens during middleware processing, Cowboy will not try
to send an error back to the socket, the process will just crash. It
is up to the middleware to make sure that a reply is sent if something
goes wrong.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_configuration">Configuration</h2>
<div class="sectionbody">
<div class="paragraph"><p>The middleware environment is defined as the <code>env</code> protocol option.
In the previous chapters we saw it briefly when we needed to pass
the routing information. It is a list of tuples with the first
element being an atom and the second any Erlang term.</p></div>
<div class="paragraph"><p>Two values in the environment are reserved:</p></div>
<div class="ulist"><ul>
<li>
<p>
<code>listener</code> contains the name of the listener
</p>
</li>
<li>
<p>
<code>result</code> contains the result of the processing
</p>
</li>
</ul></div>
<div class="paragraph"><p>The <code>listener</code> value is always defined. The <code>result</code> value can be
set by any middleware. If set to anything other than <code>ok</code>, Cowboy
will not process any subsequent requests on this connection.</p></div>
<div class="paragraph"><p>The middlewares that come with Cowboy may define or require other
environment values to perform.</p></div>
<div class="paragraph"><p>You can update the environment by calling the <code>cowboy:set_env/3</code>
convenience function, adding or replacing a value in the environment.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_routing_middleware">Routing middleware</h2>
<div class="sectionbody">
<div class="paragraph"><p>The routing middleware requires the <code>dispatch</code> value. If routing
succeeds, it will put the handler name and options in the <code>handler</code>
and <code>handler_opts</code> values of the environment, respectively.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_handler_middleware">Handler middleware</h2>
<div class="sectionbody">
<div class="paragraph"><p>The handler middleware requires the <code>handler</code> and <code>handler_opts</code>
values. It puts the result of the request handling into <code>result</code>.</p></div>
</div>
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Streams
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<a style="float:right" href="https://ninenines.eu/docs/en/cowboy/2.4/guide/migrating_from_2.3/">
Migrating from Cowboy 2.3 to 2.4
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<h3>
Cowboy
2.4
User Guide
</h3>
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<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/guide">2.4</a></li>
<li><a href="/docs/en/cowboy/2.3/guide">2.3</a></li>
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