The Effect of Failure and Repair Distributions on Consistency Protocols for Replicated Data Objects

The accessibility of vital information can be enhanced by replicating the data on several sites, and employing a consistency control protocol to manage the copies. Various protocols have been proposed to ensure that only current copies of the data can be accessed. The effect these protocols have on the accessibility of the replicated data is investigated by simulating the operation of the network and measuring the performance. Several strategies for replica maintenance are considered, and the benefits of each are analyzed. The details of the simulations are discussed. Measurements of the reliability and the availability of the replicated data are compared and contrasted. The sensitivity of the Available Copy and Dynamic-linear Voting protocols to common patterns of site failures and repairs is studied in detail. Exponential, Erlang, uniform, and hyperexponential distributions are considered, and the effect the second moments have on the results is analyzed. The relative performance of competing protocols is shown to be only marginally affected by non-exponential distributions, validating the robustness of the exponential approximations.

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