We have a C# WebAPI in front of R for our use. Each application can either do ad-hoc requests or register itself with the api, reserve N instances of R and a set of packages to be loaded on each and they get enacted as a queue for the app to handle incoming requests. I built the first version using rApache (http://rapache.net/) and then did some testing with OpenCPU (https://www.opencpu.org/) but we had to make it run on Windows and IIS so we built our own in the end. http://deployr.revolutionanalytics.com/ could be a ready made option and seems to be pretty solid but we deemed it to complicated for our needs when we looked at it.