|To describe the systems that represent knowledge in the form of rules. Rule-based systems normally use a working memory that initially contains the input data for a particular run, and an inference engine to find applicable rules and apply them.|
|Keywords: backward chaining, condition-action rule, conflict resolution, expert system, fire, forward chaining, inference engine, match-resolve-act cycle, ripple-down rules, rule-based system, working memory|
"Production" in the title of these notes (or "production rule") is a synonym for "rule", i.e. for a condition-action rule (see below). The term seems to have originated with the term used for rewriting rules in the Chomsky hierarchy of grammar types, where for example context-free grammar rules are sometimes referred to as context-free productions.
These are also called condition-action rules.
These components of a rule-based system have the form:
if <condition> then <conclusion>
if <condition> then <action>
if patient has high levels of the enzyme ferritin in their blood
and patient has the Cys282→Tyr mutation in HFE gene
then conclude patient has haemochromatosis*
* medical validity of this rule is not asserted here
Rules can be evaluated by:
The match-resolve-act cycle is what the inference engine does.
match conditions of rules with contents of working memoryend loop
if no rule matches then stop
act (i.e. perform conclusion part of rule)
Step: Check order Bag1: <empty> Unpacked: Bread Glop Granola (2) Ice cream Chips
ITEM CONTAINER TYPE SIZE FROZEN? Bread Plastic bag Medium No Glop Jar Small No Granola Cardboard box Large No Ice cream Cardboard carton Medium Yes Pepsi Bottle Large No Chips Plastic bag Medium No
B1: if the step is check-order and there is a bag of chips and there is no soft-drink bottle then add one bottle of soft drink to the order B2: if the step is check-order then discontinue the check-order step and start the pack-large-items step
Which of these rules should be chosen when in the check order step?
|The most recently used rule has highest priority||or|
|the least recently used rule has highest priority||or|
|the most recently used datum has highest priority||or|
|the least recently used datum has highest priority.|
B3: if the step is pack-large-items and there is a large item to be packed and there is a large bottle to be packed and there is a bag with < 6 large items then put the bottle into the bag B4: if the step is pack-large-items and there is a large item to be packed and there is a bag with < 6 large items then put the large item into the bag B5: if the step is pack-large-items and there is a large item to be packed then get a new bag B6: if the step is pack-large-items then get a new bag
Step: pack-medium-items Bag1: Pepsi Granola (2) Unpacked: Bread Glop Ice cream Chips
B7: if the step is pack-medium-items and there is a medium item to be packed and there is an empty bag or a bag with medium items and the bag is not yet full and the medium item is frozen and the medium item is not in a freezer bag then put the medium item in a freezer bag B8: if the step is pack-medium-items and there is a medium item to be packed and there is an empty bag or a bag with medium items and the bag is not yet full then put the medium item in the bag B9: if the step is pack-medium-items and there is a medium item to be packed then get a new bag B10: if the step is pack-medium-items then discontinue the pack-medium-items step and start the pack-small-items step
Step: pack-small-items Bag1: Pepsi Granola (2) Bag2: Bread Ice cream (in freezer bag) Chips Unpacked: Glop
B11: if the step is pack-small-items and there is a small item to be packed and the bag is not yet full and the bag does not contain bottles then put the small item in the bag B12: if the step is pack-small-items and there is a small item to be packed and the bag is not yet full then put the small item in the bag B13: if the step is pack-small-items and there is a small item to be packed then get a new bag B14: if the step is pack-small-items then discontinue the pack-small-items step and stop
To use this code, copy it to your own directory, e.g. by
% cp ~cs9414/public_html/Examples/rules-swi.pro ~
then start Prolog and do the following dialogue:
% prolog -qs rules-swi.pro ?- wm(X).[Prolog will tell you which facts it knows (a, b, and c). Don't forget to type ";" after each solution is produced by Prolog.]
?- run. Yes ?- wm(X).[The answer tells you that Prolog still knows that a, b, and c are true, and also which other "facts" it now knows. Don't forget the ";"s.]
?- already_fired(X, Y).[This time the answer tells you that two rules have fired, and gives their names and their conditions. A rule with the name
nullis also mentioned - this is a workaround in the code to avoid Prolog complaining that
already_firedis undefined, in cases where no rules have yet been fired.]
You can also play with the code - e.g. by writing your own rules and facts, and running the system with them.
To understand the code fully, you need to read up on some
extra features of Prolog:
You can find out about these at the links below:
Two problems that became apparent in attempts at commercial use of rule-based systems were:
stepdetermining which rules are active at any given time.
The best known approach to the maintenance problem (and it also deals with the interaction problem) is to use Ripple Down Rules (RDRs). If you plan to use rule-based systems in a large-scale application, you should spend some time reading up on RDRs.
Paul Comptonwho developed Ripple-Down Rules while working at the Garvan Institute of Medical Research. He is now an Emeritus Professor in the School of Computer Science and Engineering at UNSW.
More on RDRs
You can find material on RDRs at http://www.cse.unsw.edu.au/~cs9416/06s1/lectures/rdr/RDR_links.html and Paul Compton's Home Page
The UNSW course COMP9416 Rule-based Systems usually covers Ripple-Down Rules in more detail.
|Summary: Rule-Based Systems|
|Rule-based systems consist of a set of rules, a working memory and an inference engine. The rules encode domain knowledge as simple condition-action pairs. The working memory initially represents the input to the system, but the actions that occur when rules are fired can cause the state of working memory to change. The inference engine must have a conflict resolution strategy to handle cases where more than one rule is eligible to fire.|
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Copyright (C) Bill Wilson, 1996-2012, except where another source is acknowledged. Much of the material on this page is based on an earlier version by Claude Sammut.