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Functional and Non-functional Requirement of a System

  • Last Updated : 12 Aug, 2020

Requirement simply means a thing that is needed or wanted. Requirement engineering is process of defining requirement, establishing, documenting it in a proper manner, and to keep quality of requirements of a customer from a system, and the limitations under which it operates and is developed. It is first activity of software engineering. Requirements are something that is needed to satisfy whether by designing, product, or process of a software system. Requirements can be classified as :



  1. User requirements :
    User requirement simply means needs of users that should be fulfilled by software system. It is documented in a User Requirement Document (URD). Overall statements are generally written in natural language plus a description of the services system provides and its operational constraints. User requirement is good if it is clear and short, results in increasing overall quality, increases productivity, is traceable, etc.
  2. System Requirements :
    System requirement simply means needs of system to run smoothly and efficiently. It is a structured document that gives a detailed description of system functions, services, and operational constraints. It requires many hardware and software resources. If these hardware and software resources are not or less available, then it may result in system failure or causes problems during performance. Between client and contractor, it is written as a contract to define all requirements that are needed to be implemented to increases productivity.
  3. Software specification :
    It is a detailed description of software system requirements with the help of which designing and implementation can be done to develop software. For software developers, software specification is usually written that makes it easier for the developer to understand overall requirement of software.




Two main types of requirements of a system :

  1. Functional requirements :
    Functional requirements are mandatory which means it is compulsory and needed to be fulfilled. They generally describe and define features of end product of software system and simply focuses on what the end product does.

    These are the requirements that a system should accomplish or do like calculations, data manipulations, etc. It is very easy to find out functional requirements and captured in use cases.

    A system should provide a statement of service which describes how system reacts to inputs provided and should be clear and how a system reacts in a particular situation. Functional requirements are type of requirements that depends upon type of software as different software has a different functional requirement, system on which software is used as it heavily affects functions of software and users to fulfill their requirements. Functional requirement of users is high-level abstract statements. it generally describes of what system should whenever required but system functions should be described in detail by functional system requirements.


  2. Non-Functional Requirements :
    Non-functional requirements are not mandatory which means that they are not compulsory to be fulfilled. The non-functional requirements define system properties and system performance. Different properties of a system are there which can be Reliability, response time, maintainability, availability, storage requirements.

    It simply focuses on how the end product works and it is not very easy and hard to find out non-functional requirements and captured as a quality attribute. Testing includes performance, stress, security testing, etc. Non-functional requirements are more disapproved and if the non-functional requirements are not fulfilled then complete system is of no use.

    Example –
    Product requirement, organizational requirement, external requirement, capacity or storage requirement, resources requirement, overall performance requirement time, etc.


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