[emidln]

I suspect you might have encountered the SPEC if you opened the README of the ring project or visited its github page (which displays the README by default). It's mentioned and linked in that README:

From https://github.com/ring-clojure/ring:

    Ring is a Clojure web applications library inspired by Python's WSGI and Ruby's Rack. By abstracting the details of HTTP into a simple, unified API, Ring allows web applications to be constructed of modular components that can be shared among a variety of applications, web servers, and web frameworks.
    The SPEC file at the root of this distribution provides a complete description of the Ring interface.