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== Handlers

Handlers are Erlang modules that handle HTTP requests.

=== Plain HTTP handlers

The most basic handler in Cowboy implements the mandatory
`init/2` callback, manipulates the request, optionally
sends a response and then returns.

This callback receives the xref:req[Req object] and the initial
state defined in the xref:routing[router configuration].

A handler that does nothing would look like this:

init(Req, State) ->
    {ok, Req, State}.

Despite sending no reply, a `204 No Content` response will be
sent to the client, as Cowboy makes sure that a response is
sent for every request.

We need to use the Req object to reply.

init(Req0, State) ->
    Req = cowboy_req:reply(200, #{
        <<"content-type">> => <<"text/plain">>
    }, <<"Hello World!">>, Req0),
    {ok, Req, State}.

Cowboy will immediately send a response when `cowboy:reply/4`
is called.

We then return a 3-tuple. `ok` means that the handler ran
successfully. We also give the modified Req back to Cowboy.

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 `terminate/3`.

=== Other handlers

The `init/2` 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.

Cowboy comes with three handler types you can switch to:
xref:rest_handlers[cowboy_rest], xref:ws_handlers[cowboy_websocket]
and xref:loop_handlers[cowboy_loop]. In addition to those you
can define your own handler types.

Switching is simple. Instead of returning `ok`, you simply
return the name of the handler type you want to use. The
following snippet switches to a Websocket handler:

init(Req, State) ->
    {cowboy_websocket, Req, State}.

=== Cleaning up

All handler types provide the optional `terminate/3` callback.

terminate(_Reason, _Req, _State) ->

This callback is strictly reserved for any required cleanup.
You cannot send a response from this function. There is no
other return value.

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).

Cowboy does not reuse processes for different requests. The
process will terminate soon after this call returns.