CONTEXT: A CONstrainT EXplanation Tool

摘要

Encapsulation is an important concept within the object-oriented paradigm. A problem with encapsulation in an object-oriented database, however, is that important semantic information may be hidden within database operations, stripping the database system of information needed for intelligent database activities such as query processing or the ability to reason about objects. This paper describes the functionality of a constraint explanation tool known as CONTEXT. CONTEXT was developed to be used within an environment where database semantics are expressed in a declarative manner, rather than encapsulating constraints within database operations. The database system is therefore responsible for managing information about constraints and checking constraints at execution time. CONTEXT provides a way of sharing information about constraints with the designer at the time database operations are formed, explaining how constraints affect operations and assisting the designer in specifying propagation actions for responding to constraint violations. As a result, operations on objects encapsulate information about how to respond to constraints (in the form of rules), rather than encapsulating information about constraints themselves. The internal processing required to support the explanation of constraints and the identification of alternative propagation actions for transforming constraints to rules is described.