What is the difference between functional and non-functional requirements in software development?

Functional and non-functional requirements are two categories of requirements that are essential in the process of software development and system design. They serve different purposes and address distinct aspects of a system.

Functional Requirements:

• Definition: Functional requirements define what a system should do. It describes the core functionalities and features that the system must have to meet the basic needs of the users.

  A functional requirement defines a specific action, task, or behavior that the system must be able to perform. It details the inputs, outputs, processing steps, and expected outcomes of each function. Functional requirements are essential for ensuring that the system delivers the intended value to its users.

• Examples:

  • User authentication and authorization.
  • Data input and validation.
  • Report generation.
  • Search functionality.
  • Payment processing.

• Characteristics:

  • What the system should do: Functional requirements describe the system's expected behavior and functionality.
  • User interactions: Typically, functional requirements focus on the user's interactions with the system and the system's responses to those interactions.
  • Measurable and testable: Functional requirements are usually specific and can be tested to ensure that the system meets the specified criteria.

• Documentation: Functional requirements are often documented in a Functional Requirements Specification (FRS) or a Use Case document.

Non-functional Requirements:

• Definition: Non-functional requirements define how a system should do what it's supposed to do. It describes the qualities and characteristics of the system, such as performance, usability, security, and reliability.

• Examples:

  • Performance (e.g., response time, throughput).
  • Reliability and availability.
  • Scalability.
  • Usability.
  • Security.
  • Maintainability.

• Characteristics:

  • How the system should perform: Non-functional requirements focus on aspects like performance, reliability, and security.
  • Constraints: Non-functional requirements often set constraints on the system's design and implementation.
  • Quality attributes: They define the overall quality and characteristics of the system.

• Documentation: Non-functional requirements are documented in a separate document, often called the Non-Functional Requirements Specification (NFRS) or simply as part of the system requirements document.

In summary, functional requirements describe what the system should do in terms of specific features and functions, while non-functional requirements define the qualities and constraints that the system must adhere to, impacting its overall performance and behavior. Both types of requirements are crucial for ensuring the successful development and operation of a system.