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.

{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.