Building open distributed systems is an even more challenging task than building distributed systems, as their components are loosely synchronised, can move, become disconnected, and their behaviour may depend on the changing context. The approach we are putting forward relies on using a combination of formal methods applied for rigorous development of the critical parts of the system and a set of design abstractions proposed specifically for the open context-aware applications and supported by a special middleware. Our middleware provides system structuring through the concepts of roles, agents, locations and scopes, making it easier for application developers to achieve fault tolerance. We demonstrate our approach using a case study, in which we show the whole process of developing an ambient campus application - an example of open distributed systems - including its formal specification, refinement, and implementation.