Software systems in which many user's or programmers intervene may easily contain software items — such as viruses — which will endanger the integrity of the system. This paper proposes that in addition to the conventional recovery techniques, such as dumps and roll-back recovery, system availability be enhanced by the introduction of virus tests or other types of “failure tests”. We present a model to analyze the effect of the failure rate, the frequency of virus and failure testing, and the frequency of periodic dumps, on global system availability. We assume that the “failure” rate of the system increases as time elapses beyond any individual instant at which a virus test or failure test has been carried out. Thus, we are dealing with a system in which failures will be naturally time-dependent. We compute the optimum value of the interval between dumps, and also the best time interval between virus or failure tests for this system. In order to illustrate the methodology of this work, numerical examples are presented for various time-dependent failure statistics.
