How to Create a Cowboy Middleware

COWBOY_MIDDLEWARE.md

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.

my_middleware.erl

-module(my_middleware).

-behaviour(cowboy_middleware).

-export([execute/2]).

execute(Req, Env) ->
    io:format("~p request~n", [cowboy_req:method(Req)]),
    {ok, Req2, Env}.

Thanks for showing, what the official documentation doesn't :)

Do you have a source for this?

Middlewares are "deprecated"

It appears to me that stream handlers and middleware accomplish different goals. Middleware are a feature that the default cowboy_stream_h stream handler supports, right? Where you could hook in between, say, the router and the underlying application handler. If you wanted to do that with a stream handler, you'd need to reimplement the routing logic...

Do you have a source for this?

Middlewares are "deprecated"

Trying to find now where I read something about this but cannot remember. I thought it was somewhere in the official docs, repo or some comment from Loïc, but alas…

There is the slight possibility that I mixed it up with hooks which were explicitly deprecated in favor of stream handlers.