Describing consistent component extension with expression of dependence

The component technology will promise to cope with software evolution by extending and adapting only the affected parts. It is important to ensure consistency of extended and extending components. The consistency is generally determined with respect to some desired properties. The paper concentrates on the issue of composition safety: system-wide rules must not be violated by an extension. A component dependence model is presented in the paper. Besides the interface specification, we also require the model to explicitly express the needs provided by other components. Semantically, the dependence relationship asserts that a component satisfies property P if its depended component preserves property Q. The paper shows a crucial requirement to ensure composition safety is that an extending component should enhance functionality of the extended component but weaken the dependence. The logical framework is based on refinement calculus.