System Fault Tolerance Specification: Proposal of a Method Combining Semi-formal and Formal Approaches

The topic of the present work is the specification of system Fault Tolerance (FT). FT is considered a valid technique for increasing the dependability of critical automation systems by adding them the ability to operate in presence of faults. Two basic considerations stimulated the development of the present work. Firstly although a considerable amount of concepts and theory have been published around FT, a full-organized method supporting their application to the FT needs of a specific system is still missing. Furthermore, the availability of a methodology oriented to the specification of system FT is especially useful in view of integrating available FT software layers according to specific system needs. Goal of the present work is therefore to develop a methodology for the FT specification, to be used as a tool supporting the configuration of the tailorable FT software layer, which is currently under development within the TIRAN Project1. The presented approach to the FT specification is based on a combined use of two generalpurpose specification methods: the UML (Unified Modeling Language) graphical method and the TRIO (Tempo Reale ImplicitO) temporal logic. The main novelty of the proposed method consists in the identification and organization of a sequence of specification steps, which drive the industrial user in collecting and analyzing system dependability requirements and then in designing FT solutions, possibly tailoring already existing and configurable FT mechanisms.