Cowboy 2.3.0

author: Loïc Hoguin <[email protected]> 2018-04-04 13:13:37 +0200
committer: Loïc Hoguin <[email protected]> 2018-04-04 13:13:37 +0200
commit: d2a3f2cedd7c00d0933222aed9c06b3149aa4db4 (patch)
tree: 4f1499bac639c782b250af0fbf0dec2064315813 /docs/en/cowboy/2.3/guide/handlers/index.html
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+
+<h1 class="lined-header"><span>Handlers</span></h1>
+
+<div class="paragraph"><p>Handlers are Erlang modules that handle HTTP requests.</p></div>
+<div class="sect1">
+<h2 id="_plain_http_handlers">Plain HTTP handlers</h2>
+<div class="sectionbody">
+<div class="paragraph"><p>The most basic handler in Cowboy implements the mandatory
+<code>init/2</code> callback, manipulates the request, optionally
+sends a response and then returns.</p></div>
+<div class="paragraph"><p>This callback receives the <a href="../req">Req object</a> and the initial
+state defined in the <a href="../routing">router configuration</a>.</p></div>
+<div class="paragraph"><p>A handler that does nothing would look like this:</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">-&gt;</span>
+    {<span style="color: #FF6600">ok</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>Despite sending no reply, a <code>204 No Content</code> response will be
+sent to the client, as Cowboy makes sure that a response is
+sent for every request.</p></div>
+<div class="paragraph"><p>We need to use the Req object to reply.</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">Req0</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-&gt;</span>
+    <span style="color: #009900">Req</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:reply</span></span>(<span style="color: #993399">200</span>, #{
+        <span style="color: #990000">&lt;&lt;</span><span style="color: #FF0000">"content-type"</span><span style="color: #990000">&gt;&gt;</span> <span style="color: #990000">=&gt;</span> <span style="color: #990000">&lt;&lt;</span><span style="color: #FF0000">"text/plain"</span><span style="color: #990000">&gt;&gt;</span>
+    }, <span style="color: #990000">&lt;&lt;</span><span style="color: #FF0000">"Hello World!"</span><span style="color: #990000">&gt;&gt;</span>, <span style="color: #009900">Req0</span>),
+    {<span style="color: #FF6600">ok</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>Cowboy will immediately send a response when <code>cowboy:reply/4</code>
+is called.</p></div>
+<div class="paragraph"><p>We then return a 3-tuple. <code>ok</code> means that the handler ran
+successfully. We also give the modified Req back to Cowboy.</p></div>
+<div class="paragraph"><p>The last value of the tuple is a state that will be used
+in every subsequent callbacks to this handler. Plain HTTP
+handlers only have one additional callback, the optional
+and rarely used <code>terminate/3</code>.</p></div>
+</div>
+</div>
+<div class="sect1">
+<h2 id="_other_handlers">Other handlers</h2>
+<div class="sectionbody">
+<div class="paragraph"><p>The <code>init/2</code> callback can also be used to inform Cowboy
+that this is a different kind of handler and that Cowboy
+should switch to it. To do this you simply need to return
+the module name of the handler type you want to switch to.</p></div>
+<div class="paragraph"><p>Cowboy comes with three handler types you can switch to:
+<a href="../rest_handlers">cowboy_rest</a>, <a href="#ws_handlers">cowboy_websocket</a>
+and <a href="../loop_handlers">cowboy_loop</a>. In addition to those you
+can define your own handler types.</p></div>
+<div class="paragraph"><p>Switching is simple. Instead of returning <code>ok</code>, you simply
+return the name of the handler type you want to use. The
+following snippet switches to a Websocket handler:</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">-&gt;</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>
+</div>
+<div class="sect1">
+<h2 id="_cleaning_up">Cleaning up</h2>
+<div class="sectionbody">
+<div class="paragraph"><p>All handler types provide the optional <code>terminate/3</code> callback.</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">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">-&gt;</span>
+    <span style="color: #FF6600">ok</span><span style="color: #990000">.</span></tt></pre></div></div>
+<div class="paragraph"><p>This callback is strictly reserved for any required cleanup.
+You cannot send a response from this function. There is no
+other return value.</p></div>
+<div class="paragraph"><p>This callback is optional because it is rarely necessary.
+Cleanup should be done in separate processes directly (by
+monitoring the handler process to detect when it exits).</p></div>
+<div class="paragraph"><p>Cowboy does not reuse processes for different requests. The
+process will terminate soon after this call returns.</p></div>
+</div>
+</div>
+
+
+
+	
+		
+		
+		
+		
+		
+
+		<nav style="margin:1em 0">
+			
+				<a style="float:left" href="https://ninenines.eu/docs/en/cowboy/2.3/guide/constraints/">
+					Constraints
+				</a>
+			
+
+			
+				<a style="float:right" href="https://ninenines.eu/docs/en/cowboy/2.3/guide/loop_handlers/">
+					Loop handlers
+				</a>
+			
+		</nav>
+	
+
+
+
+</div>
+
+<div class="span3 sidecol">
+
+
+<h3>
+	Cowboy
+	2.3
+	
+	User Guide
+</h3>
+
+<ul>
+	
+		<li><a href="/docs/en/cowboy/2.3/guide">User Guide</a></li>
+	
+	
+		<li><a href="/docs/en/cowboy/2.3/manual">Function Reference</a></li>
+	
+	
+</ul>
+
+<h4 id="docs-nav">Navigation</h4>
+
+<h4>Version select</h4>
+<ul>
+	
+	
+	
+		<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]>	2018-04-04 13:13:37 +0200
committer	Loïc Hoguin <[email protected]>	2018-04-04 13:13:37 +0200
commit	d2a3f2cedd7c00d0933222aed9c06b3149aa4db4 (patch)
tree	4f1499bac639c782b250af0fbf0dec2064315813 /docs/en/cowboy/2.3/guide/handlers/index.html
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