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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.

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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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first two ancedor rules and the parent base rel&on and a second layer conslstmg of the ezcl-ancestor rule The second layer may be computed once the first one has been computed The followmg 19 an madmlssrble LDL program smce it csnnot be strtified, the reason bemg that even must be m a layer below even (&Q denotes the successor of Xj m 0 in 91 s X)) <--in(X) even 0 even 11 s X)) <--int(X), 7 even(X) Set terms as arguments of LDL predicates can be used for two different purposes s&enumemtwn and set-groupmg Set enumeration 15 the process of constructing a set from ib elements as in the followmg example t-We4 Qc, Y, Z)) <--bd(X, I+), took(Y, Py), book(Z, Pz), Px+Py+Pz) <-supplier(S#, P#) Unhke sebenumerataon, where the set B constructed by hstmg its elements, m grouping the set K+ constructed by definmg lla elements by a properly (I e , a coqunctaon of predicates) that they satrsfy It follows that the cardma& of the grouped set m a derived relataon B unbounded (but finite) 'Ihe la& dlustr&ve example of an admissible LDL program 19 Included to demon&&?

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