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<h1 class="lined-header"><span>Protocol upgrades</span></h1>
<p>Cowboy features many different handlers, each for different purposes. All handlers have a common entry point: the <code>init/3</code> function.</p>
<p>The default handler type is the simple HTTP handler.</p>
<p>To switch to a different protocol, you must perform a protocol upgrade. This is what is done for Websocket and REST and is explained in details in the respective chapters.</p>
<p>You can also create your own protocol on top of Cowboy and use the protocol upgrade mechanism to switch to it.</p>
<p>For example, if you create the <code>my_protocol</code> module implementing the <code>cowboy_sub_protocol</code> behavior, then you can upgrade to it by simply returning the module name from <code>init/3</code>.</p>
<script type="syntaxhighlighter" class="brush: erlang"><![CDATA[
init(_, _, _Opts) ->
{upgrade, protocol, my_protocol}.
]]></script>
<p>The <code>cowboy_sub_protocol</code> behavior only requires one callback, <code>upgrade/4</code>. It receives the Req object, the middleware environment, and the handler and options for this request. This is the same module as the <code>init/3</code> function and the same options that were passed to it.</p>
<script type="syntaxhighlighter" class="brush: erlang"><![CDATA[
upgrade(Req, Env, Handler, HandlerOpts) ->
%% ...
]]></script>
<p>This callback is expected to behave like a middleware. Please see the corresponding chapter for more information.</p>
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<h3>See also</h3><ul><li><a href="/docs/en/cowboy/1.0/manual/">Function Reference</a></li><li><a href="/docs/en/cowboy/1.0/index.html">README</a></li></ul>
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