Entity Definitions

Definition References and Circularity

An individual definition can look good, but when viewed together they can be circular.

Without some care, this can happen with entity and attribute definitions.

Example:

■CUSTOMER: Someone who buys one or more of our PRODUCTs

■PRODUCT: Something we offer for sale to CUSTOMERs

It is important when you define entities and attributes in your data model that you avoid these circular references.

Business Glossary Construction

It is often convenient to make use of common business terms when defining an entity or attribute. For example, “A CURRENCY-SWAP is a complex agreement between two PARTYs where they agree to exchange cash flows in two different CURRENCYs over a period of time. Exchanges can be fixed over the term of the swap, or may float. Swaps are often used to hedge currency and interest rate risks.”

In this example, defined terms within a definition are highlighted. Using a style like this makes it unnecessary to define terms each time they are used, since people can look them up whenever needed.

If it is convenient to use, for example, common business terms that are not the names of entities or attributes, it is a good idea to provide base definitions of these terms and refer to these definitions. A glossary of commonly used terms, separate from the model, can be used. Such common business terms are highlighted with bold-italics, as shown in the previous example.

It may seem that a strategy like this can at first lead to a lot of going back and forth among definitions. The alternative, however, is to completely define each term every time it is used. When these internal definitions appear in many places, they need to be maintained in many places, and the probability that a change will be applied to all of them at the same time is very small.

Developing a glossary of common business terms can serve several purposes. It can become the base for use in modeling definitions, and it can, all by itself, be of significant value to the business in helping people to communicate.

Chapter 5: Naming and Defining Entities and Attributes 41