What is Functional and Non Functional Requirement


A functional requirement describes what a software system should do, while non-functional requirements place constraints on how the system will do so.

Let me elaborate.

An example of a functional requirement would be that a system must send a an email whenever a certain condition is met (e.g. an order is placed, a customer signs up, etc).

A related non-functional requirement for the system may be that emails should be sent with a latency of no greater than 12 hours from such an activity.

The functional requirement is describing the behavior of the system as it relates to the system's functionality. The non-functional requirement elaborates a performance characteristic of the system.

Typically non-functional requirements fall into areas such as:
  • Accessibility
  • Capacity, current and forecast
  • Compliance
  • Documentation
  • Disaster recovery
  • Efficiency
  • Effectiveness
  • Extensibility
  • Fault tolerance
  • Interoperability
  • Maintainability
  • Privacy
  • Portability
  • Quality
  • Reliability
  • Resilience
  • Response time
  • Robustness
  • Scalability
  • Security
  • Stability
  • Supportability
  • Testability

A more complete list is available at Wikipedia's entry for non-functional requirements.

Non-functional requirements are sometimes defined in terms of metrics (something that can be measured about the system) to make them more tangible. Non-functional requirements may also describe aspects of the system that don't relate to it's execution, but rather to it's evolution over time (e.g. maintainability, extensibility, documentation, etc).






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