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Optimization of Rule-Based Systems Using State Space Graphs
March/April 1998 (vol. 10 no. 2)
pp. 238-254

Abstract—Embedded rule-based expert systems must satisfy stringent timing constraints when applied to real-time environments. The paper describes a novel approach to reduce the response time of rule-based expert systems. Our optimization method is based on a construction of the reduced cycle-free finite state space graph. In contrast with traditional state space graph derivation, our optimization algorithm starts from the final states (fixed points) and gradually expands the state space graph until all of the states with a reachable fixed point are found. The new and optimized system is then synthesized from the constructed state space graph. We present several algorithms implementing the optimization method. They vary in complexity as well as in the usage of concurrency and state-equivalency—both targeted toward minimizing the size of the optimized state space graph. Though depending on the algorithm used, optimized rule-based systems: 1) in general have better response time in that they require fewer rule firings to reach the fixed point; 2) are stable, i.e., have no cycles that would result in the instability of execution; and 3) have no redundant rules. We also address the issue of deterministic execution and propose optimization algorithms that generate the rule-bases with single corresponding fixed points for every initial state. The synthesis method also determines the tight response time bound of the new system and can identify unstable states in the original rule-base. No information other than the rule-based real-time decision program itself is given to the optimization method. The optimized system is guaranteed to compute correct results independent of the scheduling strategy and execution environment.

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Index Terms:
Computer-aided software engineering, real-time decision systems, response time, rule-based programs, optimization, state-space graphs, synthesis.
Blaz Zupan, Albert Mo Kim Cheng, "Optimization of Rule-Based Systems Using State Space Graphs," IEEE Transactions on Knowledge and Data Engineering, vol. 10, no. 2, pp. 238-254, March-April 1998, doi:10.1109/69.683755
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