Transcript PPT

Jess: A Rule-Based Programming Environment
Expert System
Reporter: Yu Lun Kuo
E-mail: [email protected]
Date: April 10, 2006
Why Expert System
Question of project developer
– Program = algorithm + Data Structure (Hard)
– Software Engineering
• Demand  Design  Coding (Demand Phase)
– Programmer
Expert Systems
– Rule-based computer programs that capture the
knowledge of human experts in their own fields of
experience
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Rules
A rule is a kind of instruction or command that
applies in certain situations
Rules are a lot like the if-then statements of
traditional programming languages
– if part of a rule is often called its left-hand side
– then part of a rule is often called its right-hand side
if
predicate or premises
then
actions or conclusions
• A rule-based system is a system that uses rules to
derive conclusions from premises
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Rule Engine
A rule engine doesn’t contain any rules until
they are programmed in
– A rule engine knows how to follow rules, without
containing any specific knowledge itself
– deployment environment
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Architecture of a Rule-Based System
A typical rule engine contains
– Inference engine
• Forward Chaining – LISP, CLIPS
• Backward Chaining - PROLOG
– Rule base (knowledge base)
– Working memory (fact base)
Inference engine consists of
– Pattern matcher
– Agenda
– Execution engine
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Pattern Matcher
The working memory contains thousands of
facts, and each rule has two or three premises
The pattern matcher need to search through
millions of combinations of facts to find those
combinations that satisfy rules
– All the rules are compared to working memory
– Decide which ones should be activated during this
cycle
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Agenda
Inference engine figures out which rules should
be fired, it still must decide which rule to fire
first
– The list of rules that could potentially fire is stored
on the agenda
– Ex.
Give high priority
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Execution Engine
The execution engine is the component of a rule
engine that fires the rules
Some modern rule engines offer a complete
programming language you can use define
– What happen when a give rules fires
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Conflict Set
Conflict
Resolution
Fired
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Jess (Java Expert System Shell)
The Java Rule Engine API defined by the
javax.rules package
– A standard enterprise API for accessing rule engines
– Site: http://www.jcp.org/jsr/detail/94.jsp
Jess rule engine
– A rule engine and scripting language developed at
Sandia National Laboratories in Livermore,
California in the late 1990s
– It can access to all of Java’s powerful APIs for
networking, graphics, database access, and so on
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Jess (Cont.)
The Jess language can directly access all Java
classes and libraries
– This allows you to experiment with Java APIs
interactively and build up large programs
incrementally
– Jess is therefore useful in a wide range of situations
– Jess can be used in command-line applications, GUI
applications, servlets, and applets.
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The most important step in developing a Jess
application is to choose an architecture
– Pure Jess language, with no Java code
– Pure Jess language, but the program accesses Java APIs
– Mostly Jess language code, but with some custom Java
code in the form of new Jess commands written in Java
– Half Jess language code, with a substantial amount of
Java code providing custom commands and APIs. Jess
provides the main() function
– Half Jess language code, with a substantial amount of
Java code providing custom commands and APIs. You
write the main() function
– Mostly Java code, which loads Jess language code at
runtime
– All Java code, which manipulates Jess entirely through its
Java API
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Scripting Java with Jess
From Java code, you can access all parts of the
Jess library
– Easy to embed Jess in any Java application
– Use Jess for experimenting with Java APIs
• Jess is therefore a kind of scripting language for Java
You can create the windows, buttons, and other
graphical components with a few lines of Jess
code
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Representing facts in Jess
The contents of Jess’s working memory are held
in your computer’s RAM
Most other constructs in Jess, facts are stored as
lists
Jess offers a set of functions to let a program
perform the basic collection operations on the
working memory
– add, remove, modify, duplicate
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Working memory
assert – Add facts to working memory
Clear – Clears all of Jess
Deffacts – Defines the initial contents of
working memory
Facts – Display the content of working memory
Reset – Initializes the working memory
Retract – Removes facts from working memory
Watch – Tells Jess to print diagnostics when
interesting things happen
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