Functional and Non-functional Requirements: Differences and Best Practices

The end goal of every project is the same: building a high-quality product that meets customer needs.

In order to build such a product, everyone in the process needs to understand the requirements clearly.

Ambiguous functional requirements can lead to a poorly defined scope, and ignoring non-functional requirements can greatly lower product quality.

The truth is that delivering high-quality software is a delicate process that takes time, skills, and proper communication between different stakeholders.

This article will look at the difference between functional and non-functional requirements, how they’re written, and the best practices to follow when writing your requirements.

Table of contents

• What Are Functional Requirements?
• What Are Non-functional Requirements?
• What is the Difference Between Functional and Non-functional Requirements?
• How Are Functional and Non-functional Requirements Written?
• Conclusion

What Are Functional Requirements?

Functional requirements form the behavior of the system. If someone is giving you functional requirements for a project, they’re giving you information on how the project’s product is going to work.

These requirements describe the interactions between a system and its environment.

Let’s say that you already have a product that’s out on the market, called Product 1.0. Now, your project is to work on Product 2.0.

The new features of the Product 2.0 would be functional requirements. The bugs that you will be fixing from Product 1.0 are also your functional requirements.

The new behavior you’re building into the product is also a functional requirement.

What Are Non-functional Requirements?

Non-functional requirements, also known as quality attributes, describe the general software characteristics.

The list of primary non-functional requirements includes:

• Usability: the ability of a user to easily interact with a product
• Reliability: the product should be secure enough to handle users’ sensitive information
• Performance: how quickly a product reacts to a user’s actions
• Error handling: how the product handles errors

For example, let’s say that your Laravel developers are building a ride-hailing app. One implicit expectation from the app is to load as soon as possible. When users land on your app, they shouldn’t wait 5 minutes for the app’s content to load.

What is the Difference Between Functional and Non-functional Requirements?

The major difference between functional and non-functional requirements in software engineering is that functional requirements explain what the system does, while non-functional requirements explain how the system works.

Functional requirements are easier to define as they’re driven by the business idea. They involve all the features of the project and the ways users engage with those features.

On the other hand, non-functional requirements are driven by experience. The best way to identify non-functional requirements is to analyze your product’s performance and improve overall user experience.

To illustrate the difference between functional and non-functional requirements, we’ll use Instagram as an example of how a development team might write its documentation.

Functional requirements:

• User stories: Instagram users want to upload image and video content, write captions, and publish updates. They also want to react to content published by other accounts.
• Use cases: When a user opens their Instagram, the first page is the feed. The user can interact with the content, leaving likes and comments.
• Additional functionality: Push notifications, automated location tagging, automated expiration of user stories, photo filters, search bars, etc.

Non-functional requirements:

• Performance: Content loading is a priority as users spend more time viewing content than uploading content.
• Usability: Users should be able to intuitively navigate between profiles and their subscriptions.
• Reliability and scalability: The system should work well in low-latency conditions, showing media content with the smallest delay possible.
• Safety: Instagram has detailed authentication protocols, photo upload, API integration, photo embedding, and encryption of direct messages.

Keeping these functional and non-functional requirement differentiations in mind, let’s look at how they are written.

How Are Functional and Non-functional Requirements Written?

Here are the most common methods used for writing functional or non-functional requirements:

Requirements Specification Document

A requirements specification document formalizes functional and non-functional requirements in software engineering. It describes functions and capabilities that the product must provide. It may also contain constraints and assumptions.

Though the documentation is usually text-based, visual elements are recommended to help non-technical stakeholders understand the scope.

A requirements specification document generally contains the following elements:

• Purpose (definitions, system overview, and background)
• Overall description (assumptions, constraints, business rules, and product vision)
• Specific requirements (system attributes, functional requirements, database requirements)

Use Cases

A use case document describes interactions between the system and the external users in a generalized, impartial way. It’s a written description of how users will perform tasks in the system and outlines the system’s behavior as it responds to requests. Each use case contains a sequence of steps, starting with a user’s goal and finishing when that goal is completed.

User Stories

User stories describe functionality from the perspective of the end-user, explaining exactly what the end-user wants the system to do. Agile teams organize user stories in a backlog, which is an ordered list of product functions.

For example, a user story may be:

My name is Jane and I want to invite my friends to join the app so we can experience it together.

Functional Decomposition

A functional decomposition document deconstructs complex functionalities by breaking them into structural components that help simplify the development process, and estimate project duration and costs.

For example,

Reserve a room -> Log in -> Select hotel and date -> Confirm reservation

Best Practices for Functional and Non-functional Requirements

Requirements should be clear and easy to understand.

Whether it’s a use case or a functional decomposition, requirements should be written in a way that is clear and easy to understand. Keep in mind that not all stakeholders understand technical language, and make the document as easy to understand as possible. Always choose the active voice over passive and keep your sentences short.

Requirements should be written accurately.

Make sure your requirements document is written accurately, using consistent terminology and formatting. Avoid adding confusing or irrelevant information.

Use requirement imperatives correctly.

Many requirements documents contain imperatives such as “shall,” “will,” and “should.” Some people will say “shall” is a requirement, while others think that “should” or “must” are requirements. Your company should outline what each of these means to them and how to use them correctly when writing requirements.

Don’t use weak phrases and subjective words.

For example, if you write “something bad happened to the system,” that will be interpreted differently by different people. If you do use language like this make sure that everyone shares the same context.

Conclusion

Understanding the difference between functional and non-functional requirements can be challenging but they create the foundation of a successful project.

Both functional and non-functional requirements are equally vital. A website can meet its functional requirements, but it can still be unusable if non-functional requirements like load speed are not in place.

FAQs

Q: What are functional and non-functional requirements?

The major difference between functional and non-functional requirements is that functional requirements explain what the system does, while non-functional requirements explain how the system works.

Q: What are examples of non-functional requirements?

• Usability
• Reliability
• Performance
• Error handling

Q: What are examples of functional requirements?

• Administrative functions
• Authentication
• External Interfaces
• Certification Requirements
• Reporting Requirements
• Historical Data