Theory of Software Reliability Based on Components1

We present a foundational theory of software system reliability based on components. The theory describes how component developers can design and test their components to produce measurements that are later used by system designers to calculate composite system reliability — without implementation and test of the system being designed. The theory describes how to make component measurements that are independent of operational profiles, and how to incorporate the overall system-level operational profile into the system reliability calculations. In principle, the theory resolves the central problem of assessing a component, which is: a component developer cannot know how the component will be used and so cannot certify it for an arbitrary use; but if the component buyer must certify each component before using it, component-based development loses much of its appeal. This dilemma is resolved if the component developer does the certification and provides the results in a form the component buyer can use. Our theory addresses the basic technical problems inherent in certifying components to be released for later use in an arbitrary system. Most component research has been directed at functional specification of software components; our theory addresses the other, equally important, side of the coin: component quality.