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<title>Nine Nines: Handlers</title>
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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 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">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 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">Req0</span>, <span style="color: #009900">State</span>) <span style="color: #990000">-></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"><<</span><span style="color: #FF0000">"content-type"</span><span style="color: #990000">>></span>, <span style="color: #990000"><<</span><span style="color: #FF0000">"text/plain"</span><span style="color: #990000">>></span>}
], <span style="color: #990000"><<</span><span style="color: #FF0000">"Hello World!"</span><span style="color: #990000">>></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 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>You can also switch to your own custom handler type:</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_handler_type</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>How to implement a custom handler type is described in the
<a href="../sub_protocols">Sub protocols</a> chapter.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_cleaning_up">Cleaning up</h2>
<div class="sectionbody">
<div class="paragraph"><p>With the exception of Websocket handlers, all handler types
provide the optional <code>terminate/3</code> callback.</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">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="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>
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<a style="float:left" href="http://ninenines.eu/docs/en/cowboy/2.0/guide/constraints/">
Constraints
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<a style="float:right" href="http://ninenines.eu/docs/en/cowboy/2.0/guide/loop_handlers/">
Loop handlers
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