MetricDefinition can be defined for any type of entity that a Metric measures (therefore the association to RootEntityType). The SID uses the EntitySpec/Entity pattern to dynamically instantiate entities.  MetricDefinition is used to measure these dynamically defined entities.  Therefore the association to Specification (as a base class to ResourceSpec and ServiceSpec)  has been included as shown in the following Figure.<br/>There is also option to extend the model by associating MetricDefinition with other entity specifications. <br/>Examples for ProductSpec and PerformanceSpec are shown. MetricDefinition has been sub-classed to explicitly show these two types of MetricDefintions. From an implementation perspective they can be rolled up into MetricDefintion if desired, or MetricDefinition could be rolled down into them. Often application boundaries dictate which of the two approaches to take.<br/>