Late detection of new types of faults often results in the evolution of fault-tolerance requirements while developers have already created design artifacts. Thus, the reuse of an existing design in the development of a fault-tolerant version thereof has the potential to reduce the overall development costs. Moreover, the automation of such a reuse yields a fault-tolerant design that is correct by construction, given that the existing design is correct. To facilitate such an automation, we present an approach, where we add three levels of fault-tolerance, namely failsafe, nonmasking, and masking, to functional designs represented as state machines. Intuitively, failsafe fault-tolerance requires that safety specification is met even in the presence of faults. In the presence of faults, nonmasking fault-tolerance guarantees recovery to states from where safety and liveness specifications are satisfied. Masking fault-tolerance stipulates that (i) recovery is provided to states from where safety and liveness specifications are met, and (ii) safety specification is satisfied during such a recovery. Specifically, we present sound and complete deterministic algorithms for automated addition of (failsafe/nonmasking/masking) fault-tolerance to the functional design of concurrent programs. These polynomial-time algorithms are especially useful in model-driven development of fault-tolerant systems, where models are automatically checked and modified. We also discuss (1) the effect of distribution and safety specification model on the complexity of adding fault-tolerance, and (2) the impact of the proposed algorithms on the addition of multitolerance.
