Analysis of data replication with two levels of consistency

A system employing data replication is structured as a two-level logical hierarchy. Replicas at level 0 maintain strong consistency, while those at level 1 may contain out-of-date information. Accesses which change the data have to be executed at level 0, and require an appropriate quorum of replicas. Read requests may be executed either at level 0 or at level 1, depending on whether the most recent information is required or not. A read quorum is necessary in the former case, but not in the latter. The performance and reliability characteristics of the system are evaluated by means of two models-one with reliable servers and one with random breakdowns and repairs. Much of the analysis is exact, but there are also some approximations