[ http://jira.codehaus.org/browse/DROOLS-38?page=all ]
bob mcwhirter updated DROOLS-38:
--------------------------------
Comment: was deleted
> conflict resolution
> -------------------
>
> Key: DROOLS-38
> URL: http://jira.codehaus.org/browse/DROOLS-38
> Project: drools-legacy
> Issue Type: Improvement
> Components: reteoo
> Affects Versions: 2.0-beta-15
> Reporter: bob mcwhirter
> Assigned To: Mark Proctor
> Priority: Critical
> Fix For: 2.0-beta-16
>
> Original Estimate: 1 week, 3 days
> Remaining Estimate: 1 week, 3 days
>
> from Pete Carapetyan:
> Conflict Resolution:
> Each of the engines he compared uses an algorthim for rules conflict
> resolution. All but JRules use MEA, and JRules uses LEX, which is
> similar but not quite as efficient
> MEA and LEX are very similar, and each does about 5 things to establish
> which should fire first. Since MEA is both published and a long term
> accepted standard, is there a reason for not going with the flow on
> this, long term?
> -----
> from Tom McGuire:
> I was browsing through the archives and saw a post about Rule Priorities.
> Bob had asked for a design or some rough ideas. This will be my quick stab
> at it.
> BACKGROUND
> This topic is generally termed Conflict Resolution. Multiple Rules may be
> able to fire based on the set facts entered into the Rule Engine. The Rule
> Engine must apply some strategy to determine which rule to fire first. Bob
> had indicated in his response to the original posting that drools uses
> "rule-load-order" to determine which rules to fire. I take that mean the
> rules can be thought of as numbered based on where they appear in the .drl
> file. If multiple rules can be fired drools will fire from the lowest number
> first and then move up the chain through each of the higher numbered rules.
> This ordinal numbering of Rules is defined as SALIENCE and is used by JESS
> and CLIPS as the basis for various strategies.
> Jackson describes 3 conflict resolution mechanisms in his book (See
> references): Refactoriness, Recency, and Specificity.
> Refactoriness - is that a rule shouldn't fire more than once on the same set
> of data.
> Recency - Working memory objects are time tagged and rules that can fire are
> prioritized based on whether they are triggered by more recent data.
> Specificity - the rules with more conditions (therefore more specific) fire
> first.
> CONFLICT RESOLUTION STRATEGIES
> Given the above background here are my suggestions on improving drools
> conflict resolution:
> SALIENCE
> The easiest suggestion would be to add a salience parameter to the
> definition of the Rule. This would allow Rules to be added in any order and
> still let the developer to have some control over how those rules will be
> processed when their are conflicts. There would still be a need for the
> rule-load-order because a developer could end up with Rules with the same
> SALIENCE. Rules added dynamically could then be inserted into the Rule base
> with higher or lower prioirity based on data collected during runtime. It's
> also a first step at providing information into a rule to allow more complex
> strategies.
> DEPTH
> This is an add on to SALIENCE. Rules triggered by new data are placed above
> rules of the same SALIENCE triggered by old data. This provides a depth
> first search over the data.
> BREADTH
> Rules triggered by new data are placed below rules of the same SALIENCE.
> This allows all the rules triggered first to be completed before the next
> cycle of rules come into play.
> SIMPLICITY
> If the Rules provide a number for the amount of conditions they have. Now
> when multiple Rules with the same SALIENCE conflict we can use the simpler
> rules first ie. the ones with lower specificity. This is a good startegy if
> we always want to do something or trigger something on a related set of
> rules.
> COMPLEXITY
> This is the reverse of SIMPLICITY choose the higher specificity first.
> CONCLUSION
> Once the information is available within the Rule we can then make the
> strategy user selectable and allow the developer to provide their own
> Conflict resolution strategy.
> REFERENCES
> Nambiar, Ullas B. "Study of JAVA based Expert System Shell - JESS" Dept of
> Comp. Sci. Arizona State University
> Jackson Peter "Introduction to Expert Systems" Addison-Wesley 1999
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