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Difference between functional and non-functional requirements

In this article, we will discuss two important terms used in software engineering that are functional requirements and non-functional requirements, along with the comparison between them. Understanding the difference between both terms helps to ensure that the delivered product meets the expectations of the client.

functional vs non-functional requirements

So, without more delay, let's start the topic.

Functional Requirements

Functional requirements define a function that a system or system element must be qualified to perform and must be documented in different forms. The functional requirements describe the behavior of the system as it correlates to the system's functionality.

Functional requirements should be written in a simple language, so that it is easily understandable. The examples of functional requirements are authentication, business rules, audit tracking, certification requirements, transaction corrections, etc.

These requirements allow us to verify whether the application provides all functionalities mentioned in the application's functional requirements. They support tasks, activities, user goals for easier project management.

There are a number of ways to prepare functional requirements. The most common way is that they are documented in the text form. Other formats of preparing the functional requirements are use cases, models, prototypes, user stories, and diagrams.

Non-functional requirements

Non-functional requirements are not related to the software's functional aspect. They can be the necessities that specify the criteria that can be used to decide the operation instead of specific behaviors of the system. Basic non-functional requirements are - usability, reliability, security, storage, cost, flexibility, configuration, performance, legal or regulatory requirements, etc.

They are divided into two main categories:

Execution qualities like security and usability, which are observable at run time.

Evolution qualities like testability, maintainability, extensibility, and scalability that embodied in the static structure of the software system.

Non-functional requirements specify the software's quality attribute. These requirements define the general characteristics, behavior of the system, and features that affect the experience of the user. They ensure a better user experience, minimizes the cost factor. Non-functional requirements ensure that the software system must follow the legal and adherence rules. The impact of the non-functional requirements is not on the functionality of the system, but they impact how it will perform. For a well-performing product, atleast some of the non-functional requirements should be met.

Functional requirements v/s Non-functional requirements

functional vs non-functional requirements

Now, let's see the comparison chart between the functional and non-functional requirements.

Functional Requirements Non-functional requirements
Functional requirements help to understand the functions of the system. They help to understand the system's performance.
Functional requirements are mandatory. While non-functional requirements are not mandatory.
They are easy to define. They are hard to define.
They describe what the product does. They describe the working of product.
It concentrates on the user's requirement. It concentrates on the expectation and experience of the user.
It helps us to verify the software's functionality. It helps us to verify the software's performance.
These requirements are specified by the user. These requirements are specified by the software developers, architects, and technical persons.
There is functional testing such as API testing, system, integration, etc. There is non-functional testing such as usability, performance, stress, security, etc.
Examples of the functional requirements are -
Authentication of a user on trying to log in to the system.
Examples of the non-functional requirements are -
The background color of the screens should be light blue.
These requirements are important to system operation. These are not always the important requirements, they may be desirable.
Completion of Functional requirements allows the system to perform, irrespective of meeting the non-functional requirements. While system will not work only with non-functional requirements.

So, that's all about the article. Hope the article is informative, helpful, and knowledgeable to you in order to take information about functional requirements, non-functional requirements, and their comparison.

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