Dependability of modular software in a multiuser operational environment

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 then 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.