8.3.5 Element Comparison for Rules

As an example for an application of the compare procedure in the context of rules, we consider the Stable Dependencies Principle (SDP) [Mar03]. The SDP says that the instability (ratio of efferent couplings to total couplings) of a package should be larger than the instability of the packages that it depends on. Assume we already have defined the instability metric I for packages, and the set DepPackages of packages that a given package depends on. We can then define a rule reporting violations of the SDP as follows:

<rule name="SDP" domain="package" category="Style" severity="2-med">
  <description>Stable dependencies principle: Instability of a package 
   should be larger than that of the packages it depends on.</description>
  <compare term="I" set="DepPackages" with="I" comp="term<with" 
   value="qualifiedname(self)" />
</rule>

In the context of rules, the report_element attribute is not allowed, and the compare procedure supports two additional attributes:

• Attribute precondition to specify that the rule is only applicable to certain elements. The precondition is evaluated for the subject model element (the one for which the term expression applies).
• Attribute value defines a metric expression that is applied to matching elements as the value of the rule.