Handlers
========
Purpose
-------
Handlers are Erlang modules that represent a resource.
Handlers must process the request and send a reply. The nature of the
reply will vary between handlers.
Different kinds of handlers can be combined in a single module. This
allows a module to handle both websocket and long-polling code in a
single place, for example.
Protocol upgrades
-----------------
Cowboy features many different handlers: HTTP handlers, loop handlers,
websocket handlers, REST handlers and static handlers. All of them
have a common entry point: the `init/3` function.
By default, Cowboy considers your handler to be an HTTP handler.
To switch to a different protocol, like, for example, Websocket,
you must perform a protocol upgrade. This is done by returning
a protocol upgrade tuple at the end of `init/3`.
The following snippet upgrades the handler to `my_protocol`.
``` erlang
init(_Any, _Req, _Opts) ->
{upgrade, protocol, my_protocol}.
```
Cowboy comes with two protocol upgrades: `cowboy_rest` and
`cowboy_websocket`. Use these values in place of `my_protocol`
to use them.
Custom protocol upgrades
------------------------
The `my_protocol` module above will be used for further processing
of the request. It should use the `cowboy_sub_protocol` behaviour,
which requires only one callback, `upgrade/4`.
It receives the request object, the middleware environment, and
the handler this request has been routed to along with its options.
``` erlang
upgrade(Req, Env, Handler, HandlerOpts) ->
%% ...
```
This callback is expected to behave like any middleware. Please
see the corresponding chapter for more information.
