A Declarative Approach to Active Databases

This position paper provides a general description of the field of active databases, focusing on the main problems yet to be solved; suggests that deductive databases may contribute to understand some of these problems; and indicates classes of applications that can be specified in a declarative way. Premise The field of active databases has recently emerged as one of the most important directions of evolution of database technology. Significant progress has been achieved both in the research environment and in the commercial world; these are reflected on one side by the relative1 large number of recent articles on the subject (see [12]), and on the other side by the increasing number of research prototypes and commercial systems which provide active behavior (see [ 13,16,19]). In general, this behavior is supported through integrated, low-level production rule facilities allowing for the automatic execution of data manipulation operations when certain events occur and/or certain conditions are met. In spite of the rapid development of this field, several problems remain to be solved in order for active databases to become widespread and fully accepted. A first problem concerns the understanding of the semantics of a collection of production rules. This amounts to underst anding precisely under which conditions rules are executed and the effect of their execution. Difficulties are mostly due to the variety of production rules which have been proposed; differences are due to: 0 The event upon which a rule is triggered: delete, or update operations). A database modification (through insert, A retrieval operation. A time-related event. 0 The rule consideration time with respect to transactions. Rules can be executed in the context of the transactions that update the database, or be executed asynchronously. In the former case, rule consideration can be: After each update operation. At user-defined rule execution points. At the end of transaction processing. In the case of asynchronous rule processing, rule activity may be delayed: Until the execution of a separate transacUntil the execution of queries on the data 0 The granularity of changes considered by a rule: e.g., individual records or collections of records. 0 The evaluation of database changes which are The prioritization of rule execution, based on tion. which are affected by rules. considered at each rule activation.