On Maintaining Priorities in a Production Rule System
Cochrane, Roberta J.
Lindsay, Bruce G.
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We present a priority system which is particularly suited for production rules coupled to databases. In this system, there are default priorities between all rules and overriding user-defined priorities between particular rules. Rule processing using this system is repeatable: for a given set of rules and priorities, the rules are considered for execution in the same order if the same set of transactions is executed twice on the same initial database state. The rule order adheres to the default order as closely as possible: rules are considered in the same order as the default order unless user-defined precedence constraints force an inversion.<P>We present data structures an efficient algorithms for implementing such a priority system. We show how the data structures can be incrementally maintained as user- defined priorities are altered. We also discuss how the proposed scheme can be extended to build a multi-level hierarchical priority system.