cowboy/guide/handlers.md

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.

init(_Any, _Req, _Opts) ->
    {upgrade, protocol, my_protocol}.

The my_protocol module will be used for further processing of the request. It requires only one callback, upgrade/4.

@todo Describe upgrade/4 when the middleware code gets pushed.