Pre-Orders for Fault Tolerance

We describe a process algebraic approach to the semantics of robust systems. We extend a subset of CCS [14] with multi-set pre xes to model systems with replicated synchronous majority voting. Based on an operational semantics, we de ne pre-orders which introduces a hierarchy of faulty processes and fault-tolerant processes. We then show how a similar ordering on modalformulae [18] can characterise the fault pre-orders. Department of Computer Science University of Canterbury, Private Bag 4800 Christchurch, New Zealand Email: paddy@cosc.canterbury.ac.nz

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