A study of unavailability of software systems

System availability is a critical measure of complex system during the operational phase. It is especially important to those safety-critical systems. While in the last few decades many technologies have been developed for enhancing the hardware reliability and availability and there are plenty of literatures in this field, little has been done in the aspect of software availability. The objective of this paper is to investigate some simple models for the estimation of software unavailability and its impact on systems considering safety, risk and cost. In this paper, a stochastic framework to model software availability is proposed. We consider software availability as the percentage of time the software is operational and both the operation and repair process are studied under this framework. The unavailability is the percentage of downtime. The dynamic behaviours of software failure and maintenance are studied and the effects of scheduled service and break-down time on software unavailability are investigated. A numerical example is provided to show how software availability can be estimated.