In this paper, we explore new failure models for multi-site systems, which are systems characterized by a collection of sites spread across a wide area network, each site formed by a set of computing nodes running processes. In particular, we introduce two failure models that allow sites to fail, and we use them to derive coteries. We argue that these coteries have better availability than quorums formed by a majority of processes, which are known for having best availability when process failures are independent and identically distributed. To motivate introducing site failures explicitly into a failure model, we present availability data from a production multi-site system, showing that sites are frequently unavailable. We then discuss the implementability of our abstract models, showing possibilities for obtaining these models in practice. Finally, we present evaluation results from running an implementation of the Paxos algorithm on PlanetLab using different quorum constructions. The results show that our constructions have substantially better availability and response time compared to majority coteries.
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