An Analysis of a Virtually Synchronous Protocol

Enterprise-scale systems such as those used for cloud computing require a scalable and highly available infrastructure. One crucial ingredient of such an infrastructure is the ability to replicate data coherently among a group of cooperating processes in the presence of process failures and group membership changes. The last few decades have seen prolific research into efficient protocols for such data replication. One family of such protocols are the virtually synchronous protocols. Virtually synchronous protocols achieve their efficiency by limiting their synchronicity guarantee to messages that bear a causal relationship to each other. Such protocols have found wide-ranging commercial uses over the years. One protocol in particular, the CBCAST protocol developed by Birman, Schiper and Stephenson in 1991 and used in their ISIS platform was particularly promising due to its unique no-wait properties, but has suffered from seemingly intractable race conditions. In this paper we describe a corrected version of this protocol and prove its formal properties.