Non-functional requirements

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In systems engineering and requirements engineering, non-functional requirements are 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. Typical non-functional requirements are reliability, scalability, and cost. Non-functional requirements are often called the ilities of a system. Other terms for non-functional requirements are "quality attributes" and "quality of service requirements".

[edit] Examples

A system may be required to present the user with a real-time display of the number of records in a database. This is a functional requirement. In order to fulfill this requirement, the system architects must ensure that the database is capable of updating its record count within a predetermined response time - this is a non-functional requirement.

Sufficient network bandwidth may also be a non-functional requirement of a system.

Other examples:

Availability (see Service Level Agreement)
Certification
Dependency on other parties
Documentation
Efficiency (resource consumption for given load)
Escrow
Extensibility (adding features, and carry-forward of customizations at next major version upgrade)
Legal and licensing issues
Maintainability
Performance / Response time (see Performance Engineering)
Platform compatibility
Price
Quality (e.g. Faults Discovered, Faults Delivered, Fault Removal Efficacy)
Reliability (e.g. Mean Time Between Failures - MTBF)
Resiliance
Resource constraints (processor speed, memory, disk space, network bandwidth etc. )
Robustness
Safety
Scalability (horizontal, vertical)
Security
Software, tools, standards etc. Compatibility
Stability
Support issues
Usability by target user community