# How to Create a Cowboy Middleware

Add an entry to the `middlewares`. Usually you want to keep the standard Cowboy
handlers afterwards, but you can replace them if you want.

```erlang
{ok, Pid} = cowboy:start_clear(ListenerName,
    [{port, Port}],
    #{
        env => #{dispatch => Dispatch},
        middlewares => [my_middleware, cowboy_router, cowboy_handler]
    }
),
```

A middleware implements the behavior `cowboy_middleware`.

## Caveats

* You cannot change a response after it has been sent. For example, if you add a
  middleware after the generic `cowboy_handler` middleware. If a previous
  middleware has called the `cowboy_req:reply/4` function the response has
  already been sent on the wire. For that purpose you need to use a stream
  handler instead.