The DEAT Business Rule Pattern Language

Russell R. Hurlbut, Ph.D.

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Rule Pattern 8: Uniqueness

Problem Describing Permissibility of Equivalent Elements

Business processes often rely on maintaining uniqueness over a collection of entities. Although an identification attribute may be assigned for internal use by a system, other attributes may still need to be unique across the collection that is independent of any implementation strategy. Uniqueness may apply to the whole collection of merely to a selected subset of the collection.


The first step in determining if elements are equivalent is to identify a set of one or more attributes of the elements of a collection that must be unique. Then, a Boolean equivalence comparison function must be defined. Finally, a scope for the uniqueness constraint must be defined as a function that selects a subset of the entire collection.

Example Real Estate Listing

A multiple listing service issues a listing number for each property placed on the market by one of its member brokers. This number must be unique across the entire collection of all listings ever submitted. However, another unique restriction is placed on the property that requires that a property can only be actively listed with one broker at a time. A second attribute, the real estate tax number, is identified to enforce this restriction.


Real Estate Listing Parameters
Parameters Attribute listing number
Scope <default:all>
Parameters Attribute real estate tax number
Scope is Active Listing

Example Employee Identification Number

The headquarters for a corporate conglomerate is consolidating the benefits administration for all of its divisions. Since many of these divisions were previously independent companies, each has it own representation for an employee identification number. A combination of attributes will not suffice, such as division and employee number, since it is desired to maintain a unique number for each employee. Therefore, a new attribute must be created to uniquely identify each employee at the conglomerate level to overcome different employee numbers that result from a transferred employee from one division to another.

Employee Identification Number Parameters
Parameters Attribute corporate employee number
Scope <default:all>
Parameters Attribute employee number
Scope Division


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