Requirements can be divided into two in the following way:
- Functional requirements specify what the system should do.
- Non-functional requirements specify the constraints under which system is developed and operated.
Some examples of non-functional requirement categories:
- Data requirements e.g. size,
- Environment requirements e.g. technical environment in which system would operate or need to be compatible with.
- Accessibility, Capacity, Compliance with regulations, Documentation, Disaster recovery, Efficiency, Extensibility, Fault tolerance, Interoperability, Maintainability, Privacy, Portability, Quality, Reliability, Response time, Robustness, Scalability, Security, Stability, Testability, and more ...
- Business/domain rules: e.g. the size of the minefield cannot be smaller than five.
- Constraints: e.g. the system should be backward compatible with data produced by earlier versions of the system; system testers are available only during the last month of the project; the total project cost should not exceed $1.5 million.
- Technical requirements: e.g. the system should work on both 32-bit and 64-bit environments.
- Performance requirements: e.g. the system should respond within two seconds.
- Quality requirements: e.g. the system should be usable by a novice who has never carried out an online purchase.
- Process requirements: e.g. the project is expected to adhere to a schedule that delivers a feature set every one month.
- Notes about project scope: e.g. the product is not required to handle the printing of reports.
- Any other noteworthy points: e.g. the game should not use images deemed offensive to those injured in real mine clearing activities.
We may have to spend an extra effort in digging NFRs out as early as possible because,
- NFRs are easier to miss e.g., stakeholders tend to think of functional requirements first
- sometimes NFRs are critical to the success of the software. E.g. A web application that is too slow or that has low security is unlikely to succeed even if it has all the right functionality.
Given below are some requirements of TEAMMATES (an online peer evaluation system for education). Which one of these are non-functional requirements?
- a. The response to any use action should become visible within 5 seconds.
- b. The application admin should be able to view a log of user activities.
- c. The source code should be open source.
- d. A course should be able to have up to 2000 students.
- e. As a student user, I can view details of my team members so that I can know who they are.
- f. The user interface should be intuitive enough for users who are not IT-savvy.
- g. The product is offered as a free online service.
Explanation: (b) are (e) are functions available for a specific user types. Therefore, they are functional requirements. (a), (c), (d), (f) and (g) are either constraints on functionality or constraints on how the project is done, both of which are considered non-functional requirements.