Abstract
This paper describes a way of using the process algebra CSP to enable controlled interaction between B machines. This approach supports compositional verification: each of the controlled machines, and the combination of controller processes, can be analysed and verified separately in such a way as to guarantee correctness of the combined communicating system. Reasoning about controlled machines separately is possible due to the introduction of guards and assertions into description of the controller processes in order to capture assumptions about other controlled machines and provide guarantees to the rest of the system. The verification process can be completely supported by difierent tools. The use of separate controller processes facilitates the iterative development and analysis of complex control flows within the system. The approach is motivated and illustrated with a non-trivial running example.