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Conflict Resolution in Forward Chaining Expert Systems

In a forward chaining expert system, during a consultation information collected is added to the working memory. Information added to the working memory can result in rules in the knowledge base becoming ready to fire ( a rule that can fire is one where all the conditions attached to the rule are known to be true. If all the conditions are true, then the advice from the rule can be generated).

In an expert system (especially a large one with a large amount of rules), at any one moment in time there may be a series of rules that are ready to fire. Which one should be fired first?

The rules which could fire at any moment in time are known as the conflict set.

A Conflict Resolution Strategy is required to make the decision as to which rule should be fired first. A list of these strategies is shown:

Rule Ordering (First Come, First Served) - Simple. The rule fired is the first rule in the conflict set. If the set contains rules 2, 5, 7 and 9, then fire rule 2.

Recency - The rule is fired which uses the data added most recently to the working memory (a common mistake is to use the rule added most recently to the knowledge base - wrong). E.g.
Rule 1: IF A AND B THEN C
Rule 2: IF D AND E THEN F
Rule 4: IF G AND H THEN I

If the contents of working memory are B, A, H, G, E, D (added in that order with D most recent addition) then rule 2 will fire, as D and E are the most recent additions. (If you will, the 'freshest' evidence in the trail).

Specificity - Fire the rule with the most conditions attached. E.g.
Rule 1: IF A AND B THEN C
Rule 2: IF D AND E AND F THEN G

Rule 2 is fired because it has 3 conditions attached as opposed to the 2 conditions in rule 1.

Refractoriness - This prevents any rule rule that has already fired from firing again, which can cause an infinite loop in the system. Once fired, a rule should be rpermanently removed from the conflict set.

The RETE Algorithm

A great burden can be placed on the inference engine in resolving conflicts resolution in expert systems. To alleviate this burden, the RETE algoritm can be used. The RETE algorithm is a method which maintains a list of rules whose conditions match the facts in the working memory, alleviating the strain on the inference engine. The RETE algorithm in memory-intensive, but can improve the efiiciency of an expert system.

Conflict Resolution and Rete Algorithm Questions


 

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