How to arhitect a machine learning web service?

As far as I know, it is not really popular to implement Machine Learning (ML) algorithms in nodejs backend or frontend because of various reasons (e.g., callback). If I want to build a nodejs application that will allow the user to upload their data, how should I arhitect my application? Where is the ML engine? (Fro example, I forsee I may use python to build my ML engine).