Functional Requirements vs. Non-Functional Requirements

Attribute	Functional Requirements	Non-Functional Requirements
Definition	Specify what the system should do	Specify how the system should perform
Measurable	Can be tested and verified	May be subjective and difficult to measure
Implementation	Directly impact the system's functionality	Support the functional requirements
Priority	Essential for system operation	Enhance the system's performance, usability, etc.

Introduction

When it comes to software development, requirements play a crucial role in defining what needs to be built. Functional requirements and non-functional requirements are two key types of requirements that are used to specify the behavior and characteristics of a software system. While both types of requirements are essential for the successful development of a software product, they serve different purposes and have distinct attributes.

Functional Requirements

Functional requirements describe the specific functions or features that a software system must perform. These requirements are typically expressed in terms of actions that the system must take in response to certain inputs or events. Functional requirements are often considered the "what" of a system, outlining the specific tasks that the software must be able to accomplish to meet the needs of its users.

One key attribute of functional requirements is that they are typically verifiable. This means that it should be possible to test whether the software meets the specified functional requirements by executing test cases that validate the expected behavior of the system. Verifiability is important because it ensures that the software development team and stakeholders have a clear understanding of what needs to be built and can assess whether the system meets the desired functionality.

Another important attribute of functional requirements is that they are often prioritized based on the needs of the users. Functional requirements are typically categorized based on their importance to the overall system, with some requirements considered essential for the system to function correctly, while others may be considered nice-to-have features that can be implemented if time and resources allow.

Functional requirements are also typically documented using techniques such as use cases, user stories, or functional specifications. These documents provide a detailed description of the expected behavior of the system, including how users will interact with the software and the specific outcomes that should be achieved when certain actions are performed.

In summary, functional requirements define the specific functions and features that a software system must perform, are verifiable, prioritized based on user needs, and are typically documented using techniques such as use cases or user stories.

Non-Functional Requirements

Non-functional requirements, on the other hand, describe the qualities or characteristics that a software system must possess. These requirements are often related to aspects such as performance, security, usability, reliability, and scalability. Non-functional requirements are sometimes referred to as "quality attributes" because they define the overall quality of the software system beyond just its functional capabilities.

One key attribute of non-functional requirements is that they are often more subjective and difficult to measure than functional requirements. While functional requirements can be verified through testing, non-functional requirements may require more qualitative assessments to determine whether the system meets the desired qualities. For example, determining whether a system is "user-friendly" or "secure" may involve subjective evaluations by users or experts.

Another important attribute of non-functional requirements is that they are often cross-cutting concerns that apply to the system as a whole. Unlike functional requirements, which are typically specific to individual features or functions, non-functional requirements apply to the entire system and impact its overall performance and usability. For example, a requirement for the system to be able to handle a certain number of concurrent users would be a non-functional requirement that affects the system as a whole.

Non-functional requirements are also typically more stable and enduring than functional requirements. While functional requirements may change over the course of a project as new features are added or requirements are refined, non-functional requirements are often more static and provide a consistent framework for evaluating the system's quality throughout its lifecycle.

In summary, non-functional requirements describe the qualities or characteristics that a software system must possess, are often subjective and difficult to measure, apply to the system as a whole, and are typically more stable and enduring than functional requirements.

Conclusion

Functional requirements and non-functional requirements are both essential components of the requirements specification for a software system. While functional requirements define the specific functions and features that the system must perform, non-functional requirements describe the qualities and characteristics that the system must possess. Understanding the attributes of both types of requirements is crucial for ensuring that a software system meets the needs of its users and performs effectively in a variety of contexts.