Page 92 - DCAP305_PRINCIPLES_OF_SOFTWARE_ENGINEERING

Principles of Software Engineering

Notes This example illustrates two points. First, in a sophisticated rule-based system the elements
of the simple rule-based system are elaborated in response to execution characteristics of
the particular class of languages being interpreted. If the design is presented in this way,
the original concept is retained to guide understanding and later maintenance. Second, as
the design is elaborated, different components of the simple model can be elaborated with
different idioms.

Figure 3: Simplified Sophisticated Rule-based System
Knowledge
Working Base
Memory
Rule Memory Fact Memory
Triggering
Multi- data Inactive Inactive
dimensional Activation/
Inputs deactivation rules facts
working
memory Active Active
rules facts
Outputs
Active rules
and facts
Data Update
Rule and
fact
compiler

Rule
antecedent
subexpressions

Unfinished Data-flow
actions network
Execution Interpreter by partially
stack Next action evaluated
Delete rule activations
completed
activations
Selected Matching
Incomplete action <rule, data>
procedures pairs
Prioritized
activations Candidate
Control <rule, data> Rule and
procedures Scheduler Agenda activations fact
compiler
Preferences
Rule Interpreter and
priorities
Rule and data
element Metarules
selection

Note that the rule-based model is itself a design structure: it calls for a set of rules whose
control relations are determined during execution by the state of the computation. A rule-
based system provides a virtual machine a rule executor to support this model.
Questions
1. What is sophisticated rule-based system?

2. Explain the simplified sophisticated rule-based system.

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