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## Chapter 10. Handlers

Handlers are Erlang modules that handle HTTP requests.

## 10.1. 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 Req object and the initial state defined in the 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`.

## 10.2. 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: cowboy_rest, cowboy_websocket and 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}.

## 10.3. Cleaning up

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

    terminate(_Reason, _Req, _State) ->
        ok.

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.

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