Tools for the definition and enforcement of dynamic constraints

Whenever a user or an application updates the contents of a database or knowledge base, it has to be guaranteed that the DB or KB is left in a consistent and meaningful state. The admissibility of a state is defined by means of consistency constraints (or semantic integrity constraints). For static constraints, which are defined on a single database state, J.M. Nicolas (1982) proposed an enforcement method in which constraints are defined in first order logic, and the enforcement procedure is efficient. Dynamic constraints concern several database states, associated to possibly distinct time points. They allow us to impose restrictions on the database transitions from a state to another: for example, in personnel management, to ensure that salaries are never decreased; or to monitor instalments for a sale made by a warehouse. The semantics of dynamic constraints is complex because of the temporal dimension. We propose a method for constraints definition and enforcement which is able to deal with dynamic constraints, introducing a distinction between imperative and non imperative constraints. A temporal first order logic is used for constraints definition and the enforcement procedure is extended through the use of active rules. The method extends Nicolas' proposal in that it is able to cope with the static constraints handled by Nicolas' method.