Modeling Perfect and Minimal Rejuvenation for Client Server Systems with Heterogeneous Load

In the present study a client-server system is considered, which experiences resource degradation as clients´ requests increase. Software rejuvenation is performed in order to counteract resource exhaustion. Two different levels of rejuvenation actions are implemented, perfect, and minimal. Moreover the concept of a failed rejuvenation is introduced to model the fact that rejuvenation due to some circumstances cannot be accomplished. As the load of such a system varies from hour to hour but reveals a cyclic behavior from day to day, different rejuvenation policies for each period of the day are proposed using a cyclic non-homogeneous Markov (CNHM) model. As a measure of performance, the steady-state expected downtime cost is considered. Additionally, to set off the need of CNHM modeling, the system is also modeled by a Homogeneous Markov Chain (HMC) and the performance results are compared.