By end-to-end i mean that you can: (1) read data from SQL or NoSQL DB, excel file, csv, JSON through some REST service, etc. (2) process it in R (3) display it to the user through some interactive web interface e.g. using Shiny

It is relatively easy if your app fit the whole loop. I would recommend with some Shiny tutorial to get you started: http://shiny.rstudio.com/tutorial/

But if you need e.g. some stream processing or more complex guis, then R might be not enough I guess.