Quorum consensus in nested-transaction systems

Abstract

Gifford's Quorum Consensus algorithm for data replication is studied in the context of nested transactions and transaction failures (aborts), and a fully developed reconfiguration strategy is presented. A formal description of the algorithm is presented using the Input/Output automaton model for nested-transaction systems due to Lynch and Merritt. In this description, the algorithm itself is described in terms of nested transactions. The formal description is used to construct a complete proof of correctness that uses standard assertional techniques, is based on a natural correctness condition, and takes advantage of modularity that arises from describing the algorithm as nested transactions. The proof is accomplished hierarchically, showing that a fully replicated reconfigurable system “simulates” an intermediate replicated system, and that the intermediate system simulates an unreplicated system. The presentation and proof treat issues of data replication entirely separately from issues of concurrency control and recovery.