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Functional and Non-Functional Requirements

  • Functional Requirements - a description of the facility or feature required. Functional requirements deal with what the system should do or provide for users. They include description of the required functions, outlines of associated reports or online queries, and details of data to be held in the system.
  • Non-Functional Requirements - a description and, where possible, target values of associated non-functional requirements. Non-functional requirements detail constraints, targets or control mechanisms for the new system. They describe how, how well or to what standard a function should be provided. For example, levels of required service such as response times; security and access requirements; technical constraints; required interfacing with users' and other systems; and project constraints such as implementation on the organisation's hardware/software platform. Service level requirements are measures of the quality of service required, and are crucial to capacity planning and physical design. Identify realistic, measurable target values for each service level. These include service hours, service availability, responsiveness, throughput and reliability. Security includes defining priority and frequency of backup of data, recovery, fallback and contingency planning and access restrictions. Access restrictions should deal with what data needs protected; what data should be restricted to a particular user role; and level of restriction required, eg physical, password, view only. Non-functional requirements may cover the system as a whole or relate to specific functional requirements.

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