Controlling hazards and safety in complex systems: a multi-layered part-whole approach to system safety

The behavior of complex dependable systems poses severe safety issues due to hazards which may result from incorrect and unpredictable behavior. In order to prevent such hazards, system behavior has to be specified and checked incrementally, in order to defeat the overall system’s complexity. Modularity in system design is however not trivial due to the intrinsic monolithic nature of the control loop, typical of such systems. An additional problem is given by the fact that the current modeling paradigm tends at introducing additional interactive complexity due to the direct communication and synchronization mechanism among decomposed modules. It can be shown, however, that modular decomposition is feasible by revising the current communication and interaction paradigm. Physical interactions in physical systems denote in fact less evident conceptual structures, which host the overall interaction and synchronization knowledge among the component parts. By introducing additional system entities with the aim of hosting such knowledge in a localized and compact manner, we obtain a part-whole hierarchy of systems, called holarchy. Such systems are, at the same time, both parts and wholes within a holarchy, thus giving a formal characterization to Koestler’s holons.