Composite Structure Diagrams

This is a relatively unknown diagram that I have rather taken a fancy to.

A composite structure diagram is an object diagram (i.e. it depicts instances of classes). It represents the run time structure and relationships of objects. It’s information content is pretty much identical to that of class diagrams but in some situations it can be significantly more informative than class diagrams.

Consider the following example.

Health Insurance Eligibility Structure

This diagram represents the structure of an eligibility object (in health insurance eligibility is a term used to represent what an insured is eligible for – what is covered, what is not covered, what is his copay, coinsurance, deductibles, what are his in-network coverages, what are his out-of-network coverages etc. etc).

What does this diagram say? Quite a lot, really.

• An eligibility object holds references to one payer object, one subscriber object, zero or up to four dependent objects and zero or one physician object (this being the primary care physician of the insured) (reference is indicated by the dotted outline).
• It holds one plan object (composition is indicated by the solid outline). This is nothing but the Plan class instantiated with values for a particular insured.
• The plan object, in turn, holds one or more benefit set objects, one in-network deductible object and one out of network deductible object.
• Each benefit set object, in turn, holds one in-network coverage object, zero or one out-of-network coverage objects and a reference to one service object. In other words, a benefit set specifies a particular treatment and the monies that the insurance company will pay for it. The monies vary depending on which hospital delivers the treatment – whether the hospital is part of the provider network of that insurance company (in-network) or not (out-of-network).

The same information can also be conveyed by a class diagram.The class diagram below models all the key entities involved in a health insurance plan and the relationships between those entities.

Health Insurance Plan Domain Model

One key difference between this class diagram and the earlier composite structure diagram is, the class diagram does not refer to eligibility at all. It is entirely possible to add a class called Eligibility to the class diagram and depict all its relationships, but try it and you will quickly see how messy the diagram would become. The composite structure diagram expresses the structure of Eligibility elegantly and clearly by using a nested notation. Therein lies its expressive power.

Thus, this example also illustrates the importance of choosing the correct type of diagram to express a given idea or concept. The right choice of diagram makes all the difference between “Wow!” and “Yuck!”

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