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Software Engineering | Classification of Software Requirements

  • Difficulty Level : Basic
  • Last Updated : 29 Oct, 2021

According to IEEE standard 729, a requirement is defined as follows: 


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  • A condition or capability needed by a user to solve a problem or achieve an objective
  • A condition or capability that must be met or possessed by a system or system component to satisfy a contract, standard, specification or other formally imposed documents
  • A documented representation of a condition or capability as in 1 and 2.

A software requirement can be of 3 types: 

  • Functional requirements
  • Non-functional requirements
  • Domain requirements


Software Requirement

Functional Requirements: These are the requirements that the end user specifically demands as basic facilities that the system should offer. All these functionalities need to be necessarily incorporated into the system as a part of the contract. These are represented or stated in the form of input to be given to the system, the operation performed and the output expected. They are basically the requirements stated by the user which one can see directly in the final product, unlike the non-functional requirements. 

For example, in a hospital management system, a doctor should be able to retrieve the information of his patients. Each high-level functional requirement may involve several interactions or dialogues between the system and the outside world. In order to accurately describe the functional requirements, all scenarios must be enumerated. 

There are many ways of expressing functional requirements e.g., natural language, a structured or formatted language with no rigorous syntax and formal specification language with proper syntax. 

Non-functional requirements: These are basically the quality constraints that the system must satisfy according to the project contract. The priority or extent to which these factors are implemented varies from one project to other. They are also called non-behavioral requirements. 
They basically deal with issues like: 

  • Portability
  • Security
  • Maintainability
  • Reliability
  • Scalability
  • Performance
  • Reusability
  • Flexibility

NFR’s are classified into following types: 

  • Interface constraints
  • Performance constraints: response time, security, storage space, etc.
  • Operating constraints
  • Life cycle constraints: maintainability, portability, etc.
  • Economic constraints

The process of specifying non-functional requirements requires the knowledge of the functionality of the system, as well as the knowledge of the context within which the system will operate. 

Domain requirements: Domain requirements are the requirements which are characteristic of a particular category or domain of projects. The basic functions that a system of a specific domain must necessarily exhibit come under this category. For instance, in an academic software that maintains records of a school or college, the functionality of being able to access the list of faculty and list of students of each grade is a domain requirement. These requirements are therefore identified from that domain model and are not user specific.

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