What is the difference between functional and non-functional requirements?
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:
- A system must send 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:
- 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 (i.e. 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 its execution, but rather to its evolution over time (e.g. maintainability, extensibility, documentation, etc.).
functional requirements are the main things that the user expects from the software for example if the application is a banking application that application should be able to create a new account, update the account, delete an account, etc. functional requirements are detailed and are specified in the system design
Non-functional requirement are not straight forward the requirement of the system rather it is related to usability( in some way ) for example for a banking application a major non-functional requirement will be available the application should be available 24/7 with no downtime if possible.