The Difference Between Functional and Non-Functional Software Requirements

You are thinking about buying a coffee machine. You want it to make espresso, americano, and cappuccino as well as froth milk whenever you need it. You also would like it to be red, work quietly, and make a cup of coffee within 10 seconds. Consider this as a simple explanation of functional and non-functional requirements.

Requirement analysis is crucial for project success. It helps Business Analysts and Project Managers define the needs and conditions to be met, set the right goals, and create necessary product documentation.

Types of Requirements in Software Engineering

Any project starts with an idea and a range of criteria to be met. The requirements outline the features a product should have in order to address the needs of both parties — business and end-user.

First, all the requirements can be divided into three main categories that describe the business, user, and system guidelines.

Business requirements describe the high-level goals of the company and stakeholders’ vision of what the software should do in terms of their business.

User requirements define what users can do with the system, their needs, and a list of activities they expect to perform.

System requirements are the building blocks that are used to create a software solution in practice. They can be functional or non-functional.

Clearly defined requirements help the development team and client move in the same direction and achieve initial goals. Generally, they are captured in requirement documentation that enables covering the following moments:

• Setting the right goals
• Defining team roles
• Estimating project time more accurately
• Foreseeing and avoiding possible pitfalls
• Eliminating miscommunication

It is crucial to pay attention to detailed requirements beforehand as further adjustments during the development process often cause project delays and cost increase. On top of that, there is even a risk of project failure if the key points are determined imprecisely.

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Understanding the definition of requirements in software engineering plays an essential role. First, it helps deeply analyze project needs and identify the right scope of work.That being so, it results in cost optimization as your custom business software product will include only the most significant features.

Furthermore, knowing the differences between functional and non-functional requirements will empower you to support customers with a better experience.

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What Are Functional Requirements?

To put it in simple words, functional requirements (FRs) describe the basic software system behavior. It points out what the application should do and what should not to work in a trouble-free manner.

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When we talk about the features and functions the product must have to meet user needs, we imply functional requirements. They include the actions users take to interact with the system as well as specific operations the system should perform. Usually, these operations are data input and output, calculations, or business flows.

Functional requirements greatly impact and even define the entire engineering process. They are covered in a software requirement specification (SRS) that provides detailed descriptions of the product functions and capabilities. This document enables considering key moments of the project, addressing the needs, and ensuring a smooth and efficient development process.

Commonly, SRS documentation contains such important parts as:

• Purpose of the software solution
• Overall description of the system
• Specific requirements
• Use cases
• Work breakdown structure
• And other

Everything may sound easier in theory than in practice. If it is still puzzling for you to determine the FRs, then let’s move to the list of realistic examples. They will allow you to have a better idea of what kind of features and functionalities you should identify in the first place.

Examples of Functional Software Requirements

Whether it comes to web app development or a mobile product, there is a variety of functional requirements that apply to both cases. The examples include:

• Authentication
• Levels of authorization
• System reports
• Administrative functions
• Certification requirements
• Audit trail
• Transaction corrections
• Interactions with external systems

If we delve even deeper, then more detailed examples would be the following:

• A system should charge the customer for the service when a certain action is completed
• A system should be able to create an account, modify and delete it
• A system should send an email or pop-up notification when a certain condition is met

What Are Non-Functional Requirements?

Contrary to functional requirements, non-functional requirements (NFRs) define the quality attributes of the software system. They outline how the system should perform and the ways the product should behave.

Even if, for some reason, these requirements are neglected, the system will still provide its basic functionality. Then you might ask why they are important at all? A short answer is non-functional requirements immensely impact the user experience, which is something you would not want to ignore.

Learn why NFRs are One of the Top Considerations for Realistic Software Project Estimation

Non-functional requirements are essential to ensure the overall usability and efficiency of the software solution. They highlight particular conditions for the system to operate under, rather than certain behavior.

While underestimating them leads to poor user experience and inability to meet their expectations, overestimating causes the rise in software development costs. Thus, it is vital to define the most significant ones to deliver a successful product and stay on budget. Brainstorm ideas with all stakeholders, classify NFRs according to execution and evolution qualities, and engage with your development team.

Examples of Non-Functional Software Requirements

Now, let’s look through some examples of NFRs to help you facilitate their identification in your project. Here they are:

• Execution qualities examples
  • Usability
  • Security
  • Reliability
  • Regulatory compliance
• Evolution qualities examples
  • Scalability
  • Maintainability
  • Testability
  • Extensibility
  • Stability

Below you can find more specific practical examples of non-functional requirements:

• A website should load in 2 seconds
• An e-commerce platform should be able to handle 500,000 users simultaneously
• A banking app should be available for users 24/7 without downtime
• An abandoned cart email should be sent 3 hours after the user leaves the cart

Functional vs Non-Functional Software Requirements

All the information covered above serves as a guideline to pave the way for a top-notch software solution. We have summarized the key differences between functional and non-functional requirements so you could have a brief look at them once again.

So Does It Make a Difference?

To make a long story short, functional requirements describe what features and functions a software system must have to enable flawless and uninterrupted operation. They are derived from the business idea and responsible for the basic system behavior.

On the other hand, non-functional requirements help determine how the system should work and how to provide users with a seamless experience. At this stage, consider the performance, convenience, and usability of your future product.

Although functional and non-functional requirements in software engineering sound easy to reveal, they often leave companies perplexed. Understanding the differences between them is essential for a successful outcome. Thus, having a software development partner by your side will help cast light on the subject, optimize project time and costs, avoid major pitfalls, and ultimately deliver a robust product.

Velvetech is staffed with experienced teams of BAs, PMs, and developers who aspire to know more about your project idea. Our expertise goes far beyond project estimations and ensures custom software solutions meet the most challenging requirements. Share with us your vision and we will help with its realization.

About the author

Shelly Parker

With a background in business analysis and a broad interest in the software development field, Shelly covers the most meaningful IT news, digital trends, and events.