Functional Requirement vs. Non-Functional Requirement

Attribute	Functional Requirement	Non-Functional Requirement
Definition	Specifies what the system should do	Specifies how the system should perform
Focus	Functionality	Performance, usability, reliability, etc.
Measurable	Yes	Yes
Implementation	Directly implemented in the system	Guidelines for system attributes

Definition

Functional requirements are specifications that define what a system should do. They describe the system's behavior under specific conditions. Non-functional requirements, on the other hand, define how a system should perform. They focus on the quality attributes of the system, such as performance, reliability, and usability.

Scope

Functional requirements are typically more concrete and specific compared to non-functional requirements. They outline the features and functionalities that the system must have to meet the user's needs. Non-functional requirements, on the other hand, are more abstract and subjective. They address the overall user experience and system performance.

Measurability

Functional requirements are usually easier to measure and test compared to non-functional requirements. Since functional requirements are specific actions or behaviors that can be observed and verified, they can be easily validated through testing. Non-functional requirements, on the other hand, are often more difficult to measure objectively. They are often evaluated subjectively based on user feedback and experience.

Dependencies

Functional requirements are typically independent of each other. Each requirement can be implemented and tested separately without relying on other requirements. Non-functional requirements, on the other hand, are often interdependent. The performance of one non-functional requirement may impact the performance of other non-functional requirements, making it challenging to address them in isolation.

Prioritization

Functional requirements are usually prioritized based on the user's needs and the system's core functionalities. They are essential for the system to function correctly and meet the user's requirements. Non-functional requirements, on the other hand, are often prioritized based on the system's performance goals and constraints. They are critical for ensuring the system's overall quality and usability.

Flexibility

Functional requirements are typically less flexible compared to non-functional requirements. Once defined, functional requirements are difficult to change without impacting the system's core functionalities. Non-functional requirements, on the other hand, are often more flexible and can be adjusted to meet changing performance goals and user expectations.

Documentation

Functional requirements are usually documented in detail to ensure that the system meets the user's needs and expectations. They are often specified using use cases, user stories, or functional specifications. Non-functional requirements, on the other hand, are often documented less formally. They may be included in the system architecture or design documents as performance goals or constraints.

Impact on Design

Functional requirements have a direct impact on the system's design and architecture. They drive the development of specific features and functionalities that must be implemented in the system. Non-functional requirements, on the other hand, influence the system's overall design and performance. They guide decisions related to scalability, security, and usability.

Validation

Functional requirements are typically validated through testing to ensure that the system behaves as expected under different conditions. Test cases are designed to verify that each functional requirement is implemented correctly. Non-functional requirements, on the other hand, are validated through performance testing and user feedback. They are evaluated based on the system's responsiveness, reliability, and usability.

Conclusion

In conclusion, functional requirements and non-functional requirements play distinct roles in defining and designing a system. Functional requirements focus on what the system should do, while non-functional requirements focus on how the system should perform. Both types of requirements are essential for ensuring that the system meets the user's needs and performs effectively. By understanding the differences between functional and non-functional requirements, project teams can better prioritize, design, and validate their systems to deliver high-quality products.