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A business rule is said to subsume another, if the conditions of the subsumed rule are included in the conditions of the subsuming rule. There are two types of subsumption:
Subsumption is a probable consequence of redundancy.
Any object that matches the subsumed rule (or group of objects, if the rule matches several objects) will also match the subsuming rule. The following two BAL rules, MoreThan100
and MoreThan1000
, cause a warning of this type. See Figure 14.7 for the warning text.
In this example, the MoreThan100
rule subsumes the MoreThan1000
rule, because if the value is greater than 1000 it will also be greater than 100.
If the purchase value is greater than $100 Then apply a 10% discount
If the purchase value is greater than $1000 Then apply a 20% discount
Potential rule subsumption happens when the subsuming rule contains conditions concerning classes that are not tested in the subsumed rule. The rule is considered to be subsumed, provided that the extra conditions of the subsuming rule are not incompatible with the subsumed rule.
In the following two BAL rules, MoreThan1000
and MoreThan500Gold
, consistency checking reports that the first rule is potentially subsumed by the second rule. See Figure 14.7 for the warning text.
The subsumption is true because any shoppingCart
matched by the MoreThan500Gold
rule is matched by the MoreThan1000
rule, since a value greater than 500 may also be greater than 1000. However, in this case there is potential subsumption due to the second condition on Customer
, which may or may not be true.
If the purchase value is greater than $1000 Then apply a 20% discount
If the purchase value is greater than $500 and the customer category is Gold Then apply a 15% discount
Business Rule Structure | Checking Rule Consistency
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