Adaptive software fault tolerance policies with dynamic real-time guarantees

Real time applications with high dependability requirements demand fault tolerance strategies. While for small systems with static behaviour policies, worse case execution times can be used, this is not true for more complex systems, in which worst case execution times are partially unknown or differ drastically from their average execution time. In such cases often only a minimum of quality can be achieved. The paper proposes to combine fault tolerant policies described by the FERT (Fault tolerant Entity for Real Time) notation with the dynamic scheduling scheme TPS (TaskPair Scheduling). TPS alleviates FERT's precondition of completely known WCETs and provides a flexible implementation base to enable an easy mapping of FERT strategies to a runtime system. In a first step, a significant subset of FERT is investigated, which implies: the recovery block scheme, N Version programming, and imprecise computations. TPS is utilised to guarantee different levels of quality, tailored to the application and the required level of fault tolerance, while guaranteeing that a common deadline is met.