Fault tolerance in a layered architecture: a general specification pattern in B

Dependable control systems are usually complex and prone to errors of various natures. Such systems are often built in a modular and layered fashion. To guarantee system dependability, we need to develop software that is not only fault-free but also is able to cope with faults of other system components. In this paper we propose a general formal specification pattern that can be recursively applied to specify fault tolerance mechanisms at each architectural layer. Iterative application of this pattern via stepwise refinement in the B method results in development of a layered fault tolerant system correct by construction. We demonstrate the proposed approach by an excerpt from a realistic case study - development of liquid handling workstation Fillwell.