Dependability of Modular Software in a
Multiuser Operational Environment

J. F. Meyer, B. Littlewood, and D. R. Wright


Effects of shared use on the dependability of modular software are evaluated in terms of a generally defined stochastic model. The total system in question consists of a community of n users who share a software system with m modules. The input aspect of the operational environment, reflecting user demands at the module level, is represented by a continuous-time, finite-state Markov process, called the operational profile. The profile's construction is based on the isolated profiles of individual users which, in the case of heterogeneous use, are pairwise-distinct processes. Moreover, in the presence of other users, an individual profile can differ from its isolated version due to "slowdowns'' caused by sharing. This multiuser profile is combined with a failure model which, among other things, captures "stress'' due to shared use. A number of basic issues are then addressed and resolved in terms of closed-form dependability solutions obtained for elementary 2-user, 1-module systems. Specifically, three measures are investigated in this manner, providing considerable insight into how dependability, as perceived by a subject user, is affected by the profile of an interfering user.

Keywords: Dependability, model-based evaluation, modular software, design faults.

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