Reengineering of distributed systems using formal methods

For complex industrial software systems, it is often necessary to add or reimplement components for future releases. This task has not been treated with formal methods so far. We show how a formal development technique can be used to develop a specification of an existing distributed system based on a set of allowed sequences of communications taken from the documentation or observations of the running system. An industrial case study (a PABX) is taken as an example to document the applicability of the approach as part of a reengineering technique. It is shown that earlier design steps can be repeated in a formal way, with an incremental development technique that preserves deadlock-freedom.