Business Rules

Definition

A business rule is a constraint that specifies a policy, business practice, rule, relevant governmental regulation, or physical fact concerning the customer organization’s current business enterprise.

Objectives

The typical objectives of a business rule are to:

• Ensure that the members of the development organization understand and do not violate any business rules.
• Ensure that the application or component does not violate any business rules.

Examples

The following are typical examples of business rules:

• Calculation (i.e., derivation) Rules:
  • “The order line item subtotal equals the sum for all order line items of the associated line item unit price at the time of receiving the order multiplied by the number of associated line items ordered.”
  • “The total order payment amount equals the sum of the order line item subtotal plus the associated sales tax plus the associated shipping fee plus the associated insurance fee (if selected).”
• Logical Preconditions:
  • “If a member’s current fees are paid in full, then the members may register new auctions or sales.”
  • “If a customer’s accound is overdrawn, then the customer may not use an ATM to withdraw money from his or her account.”
  • “If a potential customer’s credit report is satisfactory, then the potential customer may become an actual customer and open an account only.”
• Logical Postconditions (Inferences):
  • “If the member of the customer organization’s frequent flyer program flys 100,000 or more miles on the customer organization’s airline or on any of the customer airline’s partner airlines within the preceding calendar year, then the member of the customer organization’s frequent flyer program shall be a gold member and retain that status for one calendar year unless the member obtains platinum status.”
  • “If the member of the customer organization’s frequent flyer program flys 250,000 or more miles on the customer organization’s airline or on any of the customer airline’s partner airlines within the preceding calendar year, then the member of the customer organization’s frequent flyer program shall become a platinum member and retain that status for one calendar year.”
• Logical Invariants:
  • “The account balance must be greater than or equal $0.00 USD.”
  • “The overdrawn fee must be greater than or equal to $5.00 USD.”
• Physical Facts:
  • “Chemicals cannot move from one vat to another without passing through the intermediate pipes, valves, and pumps.”
  • “For crude oil, the relevant boiling range cut points are given by the folloWing:
    • Gasoline will distill out of crude oil between 90-220° Fahrenheit.”
    • Naphtha will distill out of crude oil between 220-315° Fahrenheit.”
    • Kersosene will distill out of crude oil between 315-450° Fahrenheit.”
    • Light Gas Oil (LGO) will distill out of crude oil between 450-650° Fahrenheit.”
    • Heavy Gas Oil (HGO) will distill out of crude oil between 650-800° Fahrenheit.”
  • “The specific gravity of sea water is 1.025.”
  • “An elevator cannot travel from one floor to another without passing through the intermediate floors.”
  • “An elevator weighs 1,240 pounds.”
  • “The elevator cable weighs 5.4 pounds per foot.”
  • “The maximum safe tensile strength of the elevator cable is 6,500 pounds.”
• Triggering Events:
  • “Members shall be notified if their credit card payments cannot be authorized.”

Guidelines

The following guidelines have been found to be useful when producing business rules:

• Business rules should be specified in the language of the business rather than technical jargon in order to facilitate understanding and consensus by their stakeholders.
• Business rules for different business enterprises within the same business domain will be similar, yet different. Business rules may differ within different parts of the same business enterprise. The business rules of international business enterprises my vary by country or state.
• Business rules specify the required actions to be taken or constraints to be enforced by that business’s applications.
• Business rules can be obtained from customer representatives, domain experts, and user representatives, as well as customer and industry documentation.
• Business rules must be carefully evaluated for correctness and relevancy by subject matter experts.
• Functional requirements should not repeat business rules, but should reference relevant business rules.
• Business rules can be implemented and enforced by commercially-available business rule engines (BRE) that provide the following benefits:
  • Applications using BREs can be developed faster, and thus developers can decrease time to market.
  • BREs can decrease maintenance costs by enabling requirements engineers to change the business rules without making changes to the architecture, design, or implementation of the application(s).
  • BREs can improve consistency of business rules across a business enterprise by providing a single point of use and modification.
  • The development organization can concentrate their limited resources on more business-critical tasks like new applications or making significant modifications to existing applications.