This paper addresses some issues involved in applying the eventcondition-action (ECA) rule paradigm of active databases to policies-collections of general principles specifying the desired behavior of a system. We use a declarative policy description language PDL, in which policies are formulated as sets of ECA rules. The main contribution of the paper is a framework for detecting action conflicts and finding resolutions to these conflicts. Conflicts are captured as violations of action constraints. The semantics of rules and conflict detection and resolution are defined axiomatically using logic programs. Given a policy and a set of action constraints, the framework defines a range of monitors that filter the output of the policy to satisfy the constraints.
The LDL system provides a declarative logic-based language and integrates relational database and logic programming technologies so as to support advanced data and knowledge-based applications. This paper contains a comprehensive overview of the system and contains a description of the LDL language and the compilation techniques employed to translate LDL queries into target queries on the stored data. The paper further contains a description of the architecture and runtime environment of the system and the optimization techniques employed in order to improve the performance and assure the safety of the compiled queries. The paper concludes with an account of the experience gained so far with the system, and discusses application areas where the LDL approach appears to be particularly effective.
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