Bounds on the effects of replication on availability

The author discusses some theoretical limitations on the potential benefits of replication. In particular, he investigates two fundamental questions: (1) does placing copies of data around a network increase the probability that the data will be available, and (2) does such a technique decrease the mean duration of unavailability of the data. Given that many applications require mutually exclusive access to the data, he shows that the potential benefits of replication are rather low with respect to both of these metrics. Although these results are not necessarily surprising, it is interesting that the proofs are protocol independent and, in the case of the availability measure, topology independent. They are useful, therefore, in focusing attention on the replica consistency model and performance measures and away from any particular protocol or network configuration.<>