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Active replication is a common approach to handle failures in distributed systems, including Event Stream Processing (ESP) systems. However, one weakness of conventional active replication is that replicas, being equal and in the same state, are susceptible to common-mode crashes due to software bugs. We propose a new approach to active replication that assumes a failure model stronger than fail-stop but weaker than models permitting arbitrary failures. We combine transactional memory and extended runtime checking to achieve: (i) low processing latency in failure-free runs by allowing downstream nodes to use speculative results and, thus, to circumvent the overhead added by the extended runtime checks; (ii) reduce the MTTR by enabling localized rollbacks (with word granularity) in several cases. We show that major limitations of n-variant active replication (e.g., multi-threading support, complex and slow recovery) can be overcome and tolerance to software bugs is orthogonal to Byzantine fault tolerance.
Date of Conference: 18-25 July 2010