classification of functional and nonfunctional system requirements.

Requirements

A requirements analysis involves a systematic evaluation of needs derived from a company’s

business goals and strategy. One approach to determining requirements involves the

classification of functional and nonfunctional system requirements.

Functional Requirements

Functional requirements are at the heart of systems or technology development. Functional

requirements specify the behavior of a system and determine what the system should do. These

requirements are developed well before the acquisition or development of systems. The activity

can be performed during the system design and can be application- or architecture-driven. For

example, a functional requirement for a calculator would be “must be able to add, subtract,

multiply, and divide numbers.”

Typical functional requirements are (Eriksson, 2012):

• business rules
• transaction corrections, adjustments, and cancellations
• administrative functions
• authentication
• authorization levels
• audit tracking
• external interfaces
• certification requirements
• reporting requirements
• historical data
• legal or regulatory requirements

These rules determine things such as operations, systems reports, and workflow. These

requirements are also the determining factor when evaluating system compliance. Typically, the

flow of functional requirements incorporates such things as user request, feature, use case, and

business rule.

Examples of Functional Requirements

User request: I need to send secure messages in the new application.

Feature: End-to-end encryption when sending messages.

Use case: This feature will be enabled when the application sends communications to the

database.

rule: All encryption must be FIPS 140-2 compliant.

Nonfunctional Requirements

Nonfunctional requirements specify how the system will support the functional requirements.

They may include ways of verification of functional requirements fulfillment, as well as any

remaining requirements that are not covered by the functional requirements.

While functional requirements are necessary to make the system or component work

nonfunctional requirements can determine how well the system or component works because

they describe the “quality characteristics” or “quality attributes” of the functional requirements

(Eriksson, 2012).

Typical nonfunctional requirements include (Eriksson, 2012):

• performance (e.g., response time, throughput, utilization, static, volumetric)
• scalability
• capacity
• availability
• reliability
• recoverability
• maintainability
• serviceability
• security
• regulatory
• manageability
• environmental
• data integrity
• usability
• interoperability

Examples of Nonfunctional Requirements

Availability: The Online Payment System will be available for users between the hours of 5:00

a.m. and 11:00 p.m. EST.

Capacity: Up to 1,000 total users can use the system.

Maintainability: Changes required by law will applied at least 3 months before the law becomes

enforceable.

Reliability: Will be available to users 98 percent of normal working hours.

Security: Only users with the role “Administrator” or “Supervisor” can make changes to the

system.

References

Eriksson, U. (2012, April 5). Functional vs. non functional requirements [Blog post]. Retrieved

from http://reqtest.com/requirements-blog/functional-vs-non-functional-requirements/

Requirements. (n.d.). Non functional requirements. Retrieved from

http://www.requirements.com/glossary/nonfunctionalrequirements/tabid/91/default.aspx