In systems engineering
and requirements engineering
, non-functional requirements
which specify criteria that can be used to judge the operation of a system, rather than specific behaviors. This should be contrasted with functional requirements
that specify specific behavior or functions. In general, functional requirements define what a system is supposed to do
whereas non-functional requirements define how a system is supposed to be
. Non-functional requirements are often called qualities
of a system. Other terms for non-functional requirements are "constraints
", "quality attributes", "quality goals" and "quality of service requirements". Qualities, aka. non-functional requirements, can be divided into two main categories.
- Execution qualities, such as security and usability, are observable at run time.
- Evolution qualities, such as testability, maintainability, extensibility and scalability, are embodied in the static structure of the software system.
A system may be required to present the user with a display of the number of records in a database. This is a functional requirement. How up-to-date this number needs to be is a non-functional requirement. If the number needs to be updated in real time, the system architects must ensure that the system is capable of updating the displayed record count within an acceptably short interval of the number of records changing.
Sufficient network bandwidth may also be a non-functional requirement of a system.
For example: Online response times, timing of the company offices, security, back-up etc...
Templates and Examples
Modeling Non-Functional Properties in SOA