A flexible contracts approach to system resiliency

Contract-based design (CBD) employs formalisms that explicitly define system requirements, constraints, and interfaces. This paper explores a contract-based design paradigm for expressing system resiliency features. Specifically, resilience formalisms are defined in terms of invariant and flexible assertions. A flexible assertion is one that is learned during system operation and can accommodate unpredicted system behaviors. Invariant assertions are fixed system constraints that are known a priori. A general model structure comprising four key features that contribute to system resilience is presented. In particular, the concept of flexible contracts is operationalized using the Hidden Markov Model (HMM) construct. A system architecture based on flexible contracts and lightweight error monitoring and resiliency response mechanisms is also presented. The proposed framework can serve as a testbed to experiment with different systems resiliency approaches.