Database and Expert System Technology - Information Systems Reengineering, Integration and Normalization - page 72

Start-Up

Pattern
Matching

Recognize
Cycle

No rules
could fire

one
rule

Conflict
Resolution

No No

No

Yes Yes

Rule
Applying

Stop

Action
Cycle

Actions Stop

Yes

Fig. 2.16 Recognize-act cycle

The “rule interpreter” is a program that identifies applicable rules (i.e., rules
whose condition part is satisfied), and determines the order in which applicable
rules should be applied. It follows the “recognize-act cycle” (see Fig. 2.16 ).
Working memory supplies the data for pattern matching and its structure may be
modified during the application of rules. Usually, a “start-up” element is inserted
into the working memory at the start of the computation to get the cycle going. The
computation halts if there is a cycle in which no rules become active, or if the action
of a fired rule contains an explicit command to stop.
Pattern matching identifies which rules could be fired. The interpreter matches
the calling patterns of rules against elements in working memory. Two major con-
trol strategies used for pattern matching are forward and backward chaining. We
can chain forward from those conditions that we know to be true, towards conclu-
sions that the facts allow us to establish, by matching data in working memory
against the IF-part of rules. However, we can also chain backward from a conclu-
sion that we wish to establish, towards the conditions necessary for its truth, to see
if they are supported by the facts.
Conflict resolution determines which rule to fire. There is no conflict resolution
problem in deterministic rule sets, because we can always determine the right rule
to fire at any point in the computation. The problem we need to solve is in the case
of nondeterministic rules. Good performance conflict resolution is dependent on
both sensitivity and stability from an expert system point of view. Sensitivity means
responding quickly to changes in the environment reflected in the working memory,
while stability means showing some kind of continuity in the line of reasoning