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Functional vs Non-Functional Requirements

In software engineering, requirements define what a system should accomplish and how it should behave. These are broadly categorized into functional requirements and non-functional requirements, each serving a distinct purpose in system design and development.

Functional requirements describe the specific behaviors, tasks, and functions the system must perform. They directly define the features that allow users and other systems to interact with the software. For example, in an e-commerce application, functional requirements include the ability to create user accounts, add products to a cart, process payments, and generate order confirmations. These requirements answer the question: “What should the system do?” and form the foundation for functional testing methods such as unit testing, integration testing, and user acceptance testing.

On the other hand, non-functional requirements (NFRs) define the quality attributes and constraints of the system rather than its specific behaviors. They describe how the system should operate, focusing on performance, reliability, scalability, security, usability, and maintainability. For instance, the same e-commerce system may require that product pages load within two seconds, the site remains available 99.9% of the time, and sensitive customer data be encrypted. These requirements answer the question: “How well should the system perform its functions?” and are validated through performance testing, stress testing, and security testing.

The key difference is that functional requirements focus on what the system does, while non-functional requirements emphasize how well it does it. Both are equally important: without functional requirements, the system has no purpose; without non-functional requirements, the system may function but fail to meet user expectations for speed, reliability, or security. Together, they ensure that the system is not only capable of performing necessary tasks but also meets quality standards essential for user satisfaction and long-term success.