Sun. Jan 22nd, 2023

Customer Requirements

At the start of the project, you will typically meet with the client to discuss what they require. There will inevitably be a long list of requirements – that is, after all, the point of the meeting.

The requirements are separated into two categories: functional requirements, and non-functional requirements.

Functional Requirements

A functional requirement is a function that the software is expected to provide. For example, a system which allows a travel-agent to look for holidays available in a selected country must allow a country to be input, and must show all available holidays in that country. It does not discuss anything about how it should happen.

Put simply, functional requirements are what a system is expected to do.

Some further examples are (obviously it is impossible to give a full list of functional requirements as they are dependent on the system):

  • User can create an account
  • Password can be reset

Non-functional Requirements

Everything else. Anything that a client says the system should do/have is ultimately a requirement. However, unless that requirement is one of the things that it should do (a functional requirement), it is a non-functional requirement. Non-functional requirements deal with the how factor. Using the example from above, we have a functional requirement for the travel agent. However, it would be reasonable to expect that the client would also specify a time limit: perhaps a response must be received by the user within 5 seconds of the request being made.

Here is a list of requirement-types that are all non-functional:

  • 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

Further Reading

Non-functional requirements (Wikipedia)

Functional requirements (Wikipedia)