What is functional and non functional requirement? -


i read many sites. don’t understand difference between functional , non-functional requirements in context of designing software system.

could please give example both? , give each brief definition.

a functional requirement describes what software system should do, while non-functional requirements place constraints on how system so.

let me elaborate.

an example of functional requirement be:

  • a system must send email whenever condition met (e.g. order placed, customer signs up, etc).

a related non-functional requirement system may be:

  • emails should sent latency of no greater 12 hours such activity.

the functional requirement describing behavior of system relates system's functionality. non-functional requirement elaborates performance characteristic of system.

typically non-functional requirements fall areas such as:

  • accessibility
  • capacity, current , 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 available @ wikipedia's entry non-functional requirements.

non-functional requirements defined in terms of metrics (something can measured system) make them more tangible. non-functional requirements may describe aspects of system don't relate it's execution, rather it's evolution on time (e.g. maintainability, extensibility, documentation, etc).


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