Improving System Dependability with Functional Alternatives

We present the concept of alternative functionality for improving dependability in distributed embedded systems. Alternative functionality is a mechanism that complements traditional performability and graceful degradation techniques. Rather than providing reduced performance or functionality when components or subsystems fail, alternative functionality replaces a lost feature with another existing system function that can substitute for the lost service. This can provide improved system dependability when it is not feasible to allocate dedicated backup systems for fault tolerance. We show how alternative functionality can be applied to enhance system dependability with a case study of an elevator control system. In simulation, an elevator design that implemented alternative functionality in some of its subsystems tolerated many combinations of component failures that caused system failures in the original design.