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Zones, contracts and absorbing changes: an approach to software evolution

This paper describes a novel approach to managing the evolution of distributed, persistent systems at run-time. This is achieved by partitioning a system into disjoint zones, each of which can be evolved without affecting code in any other. Contracts are defined between zones, making type-level interdependencies and inter-zone communication explicit. Programmer supplied code is added to the running system, at the boundary between zones, to constrain the scope of changes. A change methodology is presented which the software engineer uses to help describe and manage the evolution of the system. Knowledge of the application semantics is essential when evolving a system and our approach allows the engineer to concentrate on these semantic aspects of change. Our Java-based demonstration platform and methodology reduce or remove some of the burdensome tasks the software engineer is normally expected to perform when changing a system, making evolution more tractable.