Have a look at the fiery package (https://cran.r-project.org/package=fiery), which utilizes futures and that Thomas Pedersen just released:

"A very flexible framework for building server side logic in R. The framework is unoppinionated when it comes to how HTTP requests and WebSocket messages are handled and supports all levels of app complexity; from serving static content to full-blown dynamic web-apps. Fiery does not hold your hand as much as e.g. the shiny package does, but instead sets you free to create your web app the way you want."

Bob Rudis wrote an interesting blog post (https://rud.is/b/2016/07/05/a-simple-prediction-web-service-...) about it the other day. As you can see there, Thomas has some interesting extensions planned (e.g. routr).