Backend technology of Unity application

Hello all,

I am going work on a Unity project that provides a customized solution for different users. The project contains a GUI-based interface made by Unity3D and a cloud database. I am curious about the backend that I can use for my purpose. Of course, I can used MySQL+PHP, but how to define a cloud gamelogic ? For example, I would like to provide an interface to my client. They can define their desired game rules via a web interface. Is there existing solutions for that ? Gamesparkes ?or Playfab ?
Any open source solution such as Parse Server can be used for that ? then I have build some interfaces on Parse Server ?

thanks to all