An indicator of robustness for distributed databases

Wide area distributed databases (DDB) can increase their robustness by copying data at a number of sites around the network. This redundancy lets clients access the most convenient server site which holds the desired data. If a communication link to a particular server is lost or a server crashes, the client can access another server that holds the same data. We propose a simple measure of the robustness of data in a wide area network, probability of an isolated node, P_isol . This indicator is dependent on the topology of the network and the number and position of data copies within the network. Due to the complexity of the algorithm for calculating this indicator, we investigate the method of Δ-Y transformations and how they can be applied to calculate P_isol and look at some simpler indicators of robustness such as the number of disjoint paths. P_isol=0 if every site was a server. However the problem of replica control reduces the performance of this scenario. We discuss the concept of replica control and its relationship to robustness. We hope that this indicator can be used by designers of DDB systems and in DDB management algorithms to measure data availability