Cowboy 2.4.0

author: Loïc Hoguin <[email protected]> 2018-05-02 11:35:26 +0200
committer: Loïc Hoguin <[email protected]> 2018-05-02 11:35:26 +0200
commit: fec98300ec9af6c8b5f3120a60f217983c451076 (patch)
tree: 9772977293124e8b9b5b253e44d8c2695cddfb44 /docs/en/cowboy/2.4/guide/middlewares/index.html
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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>
+</div>
+
+
+
+	
+		
+		
+		
+		
+		
+
+		<nav style="margin:1em 0">
+			
+				<a style="float:left" href="https://ninenines.eu/docs/en/cowboy/2.4/guide/streams/">
+					Streams
+				</a>
+			
+
+			
+				<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
+				</a>
+			
+		</nav>
+	
+
+
+
+</div>
+
+<div class="span3 sidecol">
+
+
+<h3>
+	Cowboy
+	2.4
+	
+	User Guide
+</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>
+	
+	
+</ul>
+
+<h4 id="docs-nav">Navigation</h4>
+
+<h4>Version select</h4>
+<ul>
+	
+	
+	
+		<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>
+	
+		<li><a href="/docs/en/cowboy/2.2/guide">2.2</a></li>
+	
+		<li><a href="/docs/en/cowboy/2.1/guide">2.1</a></li>
+	
+		<li><a href="/docs/en/cowboy/2.0/guide">2.0</a></li>
+	
+		<li><a href="/docs/en/cowboy/1.0/guide">1.0</a></li>
+	
+</ul>
+
+</div>
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author	Loïc Hoguin <[email protected]>	2018-05-02 11:35:26 +0200
committer	Loïc Hoguin <[email protected]>	2018-05-02 11:35:26 +0200
commit	fec98300ec9af6c8b5f3120a60f217983c451076 (patch)
tree	9772977293124e8b9b5b253e44d8c2695cddfb44 /docs/en/cowboy/2.4/guide/middlewares/index.html
parent	bfd2b6e01cbb36b0be0b3c860f5baf8d5b49fe7a (diff)
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