Invariant Requirements

Definition

An invariant requirement is a data requirement that specifies a mandatory relationship among logical data values of an application, component, or business object.

Objectives

The typical objectives of an invariant requirement are to:

• Ensure that mandatory relationships among data values hold.

Examples

The following are typical examples of invariant requirements:

• “The sum of the prices of the order items equals the total price of the order.”

Guidelines

The following guidelines have been found to be useful when producing invariant requirements:

• Invariant requirements are often derived from business rules or physical laws.