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Functional and Non-Functional Requirements of Software System | Software Engineering

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functional and non-functional requirementsFunctional and Non-Functional Requirements of Software System,
Software Engineering Notes | Sixth Semester,
BSc.CSIT | Tribhuvan University (TU)

Functional requirements
The functional requirements for a system describes what the system should do(defines a function of a system or its component). Functional requirements may be calculations, technical details, data manipulation and processing and other specific functionality that define what a system is supposed to accomplish. These requirements depend on the type of software being developed, the expected users of the software and the general approach taken by the organization when writing requirements. When expressed as user requirements, the requirements described in a fairly abstract way. However, functional system requirements describe the system function in detail, its inputs, expectations, behavior and outputs.

Functional requirements for the software system may be expressed in a number of ways. For example, Functional requirements for a library system, used by students to order books and documents from other libraries could be following points;

  • The user shall be able to search either all of the initial set of databases or select a subset from it.
  • The system shall provide appropriate viewers for the user to read documents in the document store.
  • Every order shall be allocated a unique identifier (ORDER_ID) which the user shall be able to copy to the account’s permanent storage area

Non-functional requirements
Non-functional requirements are not directly concerned with the specific functions delivered by the system. It is a requirement that specifies criteria that can be used to judge the operation of a system, rather than specific behaviors. It defines system properties and constraints like, reliability, response time and storage requirements. Constraints are I/O device capability, system representations, etc.

It is concerned with specifying system performance, security, availability, and other emergent properties. This means they are often more critical than individual functional requirements. These requirements are not just concerned with the software system to be developed. Some non-functional requirements may constrain (restrict) the process that should be used to develop the system. Process requirements may also be specified mandating a particular CASE system, programming language or development method. Non-functional requirements may be more critical than functional requirements, so if these are not met, the system could be useless also.

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