cowboy/doc/src/guide/hooks.ezdoc

::: Hooks

Hooks allow the user to customize Cowboy's behavior during specific
operations.

:: Onresponse

The `onresponse` hook is called right before sending the response
to the socket. It can be used for the purposes of logging responses,
or for modifying the response headers or body. The best example is
providing custom error pages.

Note that this function MUST NOT crash. Cowboy may or may not send a
reply if this function crashes. If a reply is sent, the hook MUST
explicitly provide all headers that are needed.

You can specify the `onresponse` hook when creating the listener.

``` erlang
cowboy:start_http(my_http_listener, 100,
    [{port, 8080}],
    [
        {env, [{dispatch, Dispatch}]},
        {onresponse, fun ?MODULE:custom_404_hook/4}
    ]
).
```

The following hook function will provide a custom body for 404 errors
when it has not been provided before, and will let Cowboy proceed with
the default response otherwise.

``` erlang
custom_404_hook(404, Headers, <<>>, Req) ->
    Body = <<"404 Not Found.">>,
    Headers2 = lists:keyreplace(<<"content-length">>, 1, Headers,
        {<<"content-length">>, integer_to_list(byte_size(Body))}),
    cowboy_req:reply(404, Headers2, Body, Req);
custom_404_hook(_, _, _, Req) ->
    Req.
```

Again, make sure to always return the last request object obtained.