Analysis of Preventive Maintenance in Transactions Based Software Systems

Preventice maintenance of operational software systems is a novel technique for software fault tolerance, used speci cally to counteract the phenomenon of software \aging". However, it incurs some overhead. The necessity to do preventive maintenance not only in general purpose software systems of mass use but also in safety-critial and highly availabile systems clearly indicate the need to follow an analysis based approach in determining the optimal times to perform preventive maintenance. In this paper, we present an analytical model of a software system which serves transactions. Due to aging, not only the service rate of the software decreases with time but the software itself experiences crash/hang faliures which result in its unavailability for service. Two policies for preventive maintenance are evaluated for resulting steady state availability, probability that an arriving transaction is lost and an upper bound on the expected response time of a transition. Numerical examples are presented to illustrate the usefulness and applicability of the models.