In recent studies, the phenomenon of software ''aging'' has come to li
ght which causes performance of a software to degrade with time. Softw
are rejuvenation is a fault tolerance technique which counteracts agin
g. In this paper, we address the problem of determining the optimal ti
me to rejuvenate a server type software which experiences ''soft failu
res'' (witnessed in telecommunication systems) because of aging. The s
ervice rate of the software gradually decreases with time and settles
to a very low value. Since the performability in this state is unaccep
table, it is necessary to ''renew'' the software to its peak performan
ce level. We develop Markov decision models for such a system for two
different queuing policies. For each policy, we define the look-ahead-
n cost functions and prove results on the convergence of these functio
ns to the optimal minimal cost function. We also prove simple rules to
determine optimal times to rejuvenate for a realistic cost criterion.
Finally, the results are illustrated numerically and the effectivenes
s of the MDP model is compared with that of the simple rules.