Reliability, Availability, Dependability and Performability: a User-centered View

Reliability and availability have long been considered twin system properties that could be enhanced by distribution. Paradoxically, the traditional de nitions of these properties do not recognize the positive impact of recovery|as distinct from simple repair and restart|on reliability, nor the negative e ect of recovery, and of internetworking of clients and servers, on availability. As a result of employing the standard de nitions, reliability would tend to be underestimated, and availability overestimated. We o er revised de nitions of these two critical metrics, which we call service reliability and service availability, that improve the match between their formal expression, and intuitive meaning. A fortuitous advantage of our approach is that the product of our two metrics yields a highly meaningful gure of merit for the overall dependability of a system. But techniques that enhance system dependability exact a performance cost, so we conclude with a cohesive de nition of performability that rewards the system for performance that is delivered to its client applications, after discounting the following consequences of failure: service denial and interruption, lost work, and recovery cost.