Realtime web comms for Clojure/Script – Sente (by Peter Taoussanis)

From its documentation

Sente is a small client+server library that makes it easy to build reliable, high-performance realtime web applications with Clojure + ClojureScript.

You can ignore the underlying protocol and deal directly with Sente's unified API. It's simple, and exposes the best of both WebSockets (bidirectionality + performance) and Ajax (optional evented ack/reply model).