APIs are a natural mechanism to capture observed facts about customer interactions. In this talk we showcase the link between observed and derived facts by using APIs for data and event publishing with reactive micro-services and run-time discovery acting as the glue that binds it all together. We will show that this method of modeling of systems results in true design time and run-time visibility into the system and its interactions and solves some interesting problems in areas of functional decomposition, monitoring and financial balancing and controls.