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<h1 class="lined-header"><span>cowboy_loop(3)</span></h1>
<div class="sect1">
<h2 id="_name">Name</h2>
<div class="sectionbody">
<div class="paragraph"><p>cowboy_loop - Loop handlers</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_description">Description</h2>
<div class="sectionbody">
<div class="paragraph"><p>The module <code>cowboy_loop</code> defines a callback interface for
long running HTTP connections.</p></div>
<div class="paragraph"><p>You should switch to this behavior for long polling,
server-sent events and similar long-running requests.</p></div>
<div class="paragraph"><p>There are generally two usage patterns:</p></div>
<div class="ulist"><ul>
<li>
<p>
Loop until receiving a specific message, then send
a response and stop execution (for example long polling);
</p>
</li>
<li>
<p>
Or initiate a response in <code>init/2</code> and stream the
body in <code>info/3</code> as necessary (for example server-sent events).
</p>
</li>
</ul></div>
</div>
</div>
<div class="sect1">
<h2 id="_callbacks">Callbacks</h2>
<div class="sectionbody">
<div class="paragraph"><p>Loop 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">cowboy_loop</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>}
| {<span style="color: #FF6600">cowboy_loop</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>, <span style="color: #FF6600">hibernate</span>}
<span style="font-weight: bold"><span style="color: #000000">info</span></span>(<span style="color: #009900">Info</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: #FF6600">ok</span>, <span style="color: #009900">Req</span>, <span style="color: #009900">State</span>, <span style="color: #FF6600">hibernate</span>}
| {<span style="color: #FF6600">stop</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">Info</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="color: #FF6600">stop</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>The <code>init/2</code> callback is common to all handlers. To switch
to the loop behavior, it must return <code>cowboy_loop</code> as the
first element of the tuple.</p></div>
<div class="paragraph"><p>The <code>info/3</code> callback will be called for every Erlang message
received. It may choose to continue the receive loop or stop
it.</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 loop handlers:</p></div>
<div class="dlist"><dl>
<dt class="hdlist1">
stop
</dt>
<dd>
<p>
The handler requested to close the connection by returning
a <code>stop</code> tuple.
</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="_changelog">Changelog</h2>
<div class="sectionbody">
<div class="ulist"><ul>
<li>
<p>
<strong>2.0</strong>: Loop handlers no longer need to handle overflow/timeouts.
</p>
</li>
<li>
<p>
<strong>1.0</strong>: Behavior introduced.
</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>,
<a href="../cowboy_handler">cowboy_handler(3)</a></p></div>
</div>
</div>
</div>
<div class="span3 sidecol">
<h3>
Cowboy
2.2
Function Reference
</h3>
<ul>
<li><a href="/docs/en/cowboy/2.2/guide">User Guide</a></li>
<li><a href="/docs/en/cowboy/2.2/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/manual">2.4</a></li>
<li><a href="/docs/en/cowboy/2.3/manual">2.3</a></li>
<li><a href="/docs/en/cowboy/2.2/manual">2.2</a></li>
<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>
</ul>
</div>
</div>
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