What is Non-Functional Requirement in Software Engineering Types and Examples

What is a Non-Functional Requirement in Software Engineering? Types and Examples

What is a Non-Functional Requirement

A non-functional requirement is a requirement that is not related to functional software. It is also called quality attribute or supplementary specification or simply specs. More specifically, non-functional requirement describes an aspect of the functionality of a system that is desirable but not essential, although it may have a significant impact on the overall quality of the system

Non-functional requirements are impossible to verify automatically by computer but are likely to be analyzed by a human auditor. Examples of non-functional requirements are safety, security, maintainability, testability, usability, etc.

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Example of Non-Functional Requirement

Examples of Non-Functional Requirements in Software Engineering Reliability is the ability of a system or component to perform its required functions under stated conditions for a specified period of time. It is an assessment of the quality of service. Reliability requirements are usually stated informally as part of the quality objective,  e.g A website should handle 20 million users without affecting its performance.

Performance can be measured in different ways depending on what is being considered: there are seven commonly used criteria by which performance may be judged: Speed (fast), Capacity (large volume), Efficiency (low cost per unit volume), Accuracy (little error or distortion), Usability (easy to use), Adaptability (flexible) and Maintainability (easy to repair). Although many performance objectives can be quantified, not all can be; it depends on the criteria used for measurement.

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Types of Non-Functional Requirements

  • Usability  requirement
  • Serviceability requirement
  • Recoverability requirement
  • Security requirement
  • Data integrity requirement
  • Capacity requirement
  • Availability requirement
  • Scalability requirement
  • Interoperability requirement
  • Reliability requirement 
  • Maintainability requirement
  • Regulatory requirement
  • Environmental requirement

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Functional vs Non Functional Requirements

A functional requirement is a general term used in software engineering to define the needs of the client to be delivered by the software product. These are the requirements that will not change as a result of any changes to the product or service being developed. Non-functional requirements are other requirements that are not related to the functionality of the product or service being developed. They are related to other aspects of the product, such as performance, scalability, usability, reliability, maintainability, and portability. 

Functional requirements are set out in terms of what the system should do, leaving implementation details open for programmers.  But non-functional requirements are concerned with what the system must do, leaving no room for compromise. The systems should deliver data that is consistent and accurate. Non-functional requirements are other requirements that are not directly related to the functionality of the product or service being developed. They are related to other aspects of the product, such as performance, scalability, usability, reliability, maintainability, and portability.

Advantages of Non-Functional Requirement

  • Non-functional requirements can lead to a better understanding of a software project’s purpose and the team’s objectives
  • Non-functional requirements focus more on business needs rather than technical details
  • Functional requirement focuses on functional specifications of the module/systems which help in designing the modules/systems more functionally
  • They are more likely to be deliverable than functional requirements
  • They aid communication between customer and project team
  • They earn customer’s trust
  • They drive the development process toward important milestones
  • They help prioritize functional requirements

Disadvantages of Non-functional requirement

  • Non-functional requirements aren’t tested
  • Tests can’t be executed against it
  • Time is wasted moving between design and development teams.
  • A non-functional requirement is hard to be quantified and checked
  • Tendency to miss the non-functional requirements
  • May contradict the functional requirement
  • Requirements engineering is less concerned with non-functional

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