Functional vs Non-Functional Requirements

Functional Requirement:

Functional requirements define what a software system should do. It defines a function of a software system. Functional requirement describes the functions software must perform. A function is nothing but inputs, its behavior, and outputs. It can be a calculation, data manipulation, business process, user interaction, or any other specific functionality which defines what function a system is likely to perform.

Functional requirements describe what specifically needs to be implemented in a software system or product, what actions should be performed by users about this development. Functional requirements in software engineering help us to capture the intended behavior of the system. This behavior may be expressed as functions, services or tasks or which system is required to perform.

We can describe functional requirements as the behavior of the system as it connects to the system’s functionality. Authentication, audit tracking, certification requirements, etc are examples of functional requirements.

Non-Functional Requirement:

A non-functional requirement defines the quality attribute of a software system. Some of the basic non-functional requirements are reliability, security, usability, scalability, maintainability, storage, cost, configuration, performance, etc.

The non-functional requirements primarily include various attributes of product quality properties – the requirements that determine the qualitative characteristics of development (software, information system) such as reliability, scalability, product performance, etc.

Non-functional requirements describe exactly how the created system or software product works, what properties and characteristics a particular development has.

A non-functional requirement is essential to ensure the usability and effectiveness of the entire software system. Failing to meet non-functional requirements can result in systems that fail to satisfy user needs.

 

Difference between Functional and Non-Functional Requirements:

Functional Requirements	Non-Functional Requirements
Functional requirements help to understand the functions of the system.	Non-functional requirements help to understand the system's performance.
A functional requirement defines a system or its component.	A non-functional requirement defines the quality attribute of a software system.
Helps us verify the functionality of the software.	Helps us to verify the performance of the software.
Functional requirements are mandatory.	Non-functional requirements are not mandatory.
Functional requirements allow us to measure the functionality of the software.	It allows us to check the performance of the system.
The functional requirements mentioned the activities a software system should perform.	It sets some rules and regulations for a software system on how it should fulfil the essential need.
These requirements are established by the user.	These requirements are established by technical persons, such as e.g. software developers, architects, and more.
These are easy to define.	These are hard to define.
Focuses on user requirement	Concentrates on the user's expectation and experience.
Functional requirements can exist without a non-functional requirement.	Non-functional requirements cannot exist without functional requirement.