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20 March 2024 - 6 minutes

Functional vs Non-Functional Requirements: Understanding the Core Differences and Examples

Functional requirements are mandatory for the system to work, while non-functional requirements are not. But there is more to the difference between the two.

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If you're diving into the tech world, get ready to be a lifelong learner. But hey, that's a good thing! Why? Because learning opens up doors to new opportunities. So, if you're here chances are you're eager to grasp some fresh tech-related knowledge.

This time, let's focus on the gap between functional and non-functional requirements. Below you will find all the relevant information you need to tackle future work challenges like a pro. 

Enough talk, let's get down to business.

What Are Software Development Requirements?

Let's start with the basics: what do we mean by product requirements in web development and why are they essential? When we're working on creating a website or any other software product, requirements refer to everything the system needs to function properly, as well as any potential issues we might encounter along the way.

To ensure that the requirements are thorough, we need to address a few key questions:

  • What are the client's business goals? Understanding the client's business goals is crucial as it determines the purpose behind the product creation. These goals can range from expanding audience reach, entering new markets, or validating an idea. The level of complexity required for the product hinges on these objectives. Sometimes, a minimally viable product with basic features suffices, while other times, a comprehensive, multi-functional product adapted to various languages is necessary.

  • Who are the people we're targeting with this product, and how will they use it? Understanding the target audience's demographics, activity field, and tech awareness helps determine the product's complexity. For example, when developing a fitness app aimed at health-conscious people aged 25-40, we can provide features such as workout tracking and nutritional recommendations. Knowing how people will use it - whether on the go or during focused workouts - influences interface design and performance optimization.

  • What will the end product look like? This question aims to define the visual design and functionality of the product being developed. For example, if we're talking about a fitness app, the final product might have a sleek interface with sections for workout tracking, goal setting, and nutritional advice. The idea is to create an easy-to-use tool that empowers individuals to improve their fitness by providing them with personalized workout plans, progress tracking, and nutritional insights.

  • What technology will we use to make it happen? This can mean making choices about programming languages, frameworks, and database systems to ensure that the application's functionality and performance meet user needs and industry standards.

  • What problems might arise, and how can we avoid or solve them? It refers to the fact that the team needs to anticipate and plan for potential problems that may arise with the product, such as technical failures, compatibility issues, maintenance issues due to excessive user base growth, user dissatisfaction, or something else. We need to come up with a strategy to prevent or solve these problems, such as thorough testing, intuitive design, and responsive customer support.

Here's something interesting: did you know that almost 40% of projects fail just because of poor requirements? Keep this in mind. Besides, web development trends for 2024 continue to emphasize the importance of thorough requirements. 

To make your project successful, it's really important for you and your project team to not just gather requirements right, but also to know the difference between two main types of requirements: functional and non-functional. 

Functional and Non-Functional Requirements
Before we zoom in on the core difference between these two, let's make sure we're on the same page about what they mean.

Functional requirements

Functional requirements are like a checklist for a system as they specify what the system must do. Let’s take a fitness app as an example. In this case, functional requirements would outline tasks such as:

  • Creating user profiles

  • Logging workouts

  • Tracking progress

  • Displaying workout details

  • Sending notifications about upcoming workouts

If these requirements aren't met, your software won't function properly. Imagine you purchased an online course, but after you made the payment, it didn't show up in your email inbox right away, or maybe it didn't show up at all. This is because one of the functional requirements was for the course to be emailed to you after payment, but it didn't happen the way it was supposed to.

Here's what the requirements we’re talking about cover:

  • Authentication 

  • Input requirements

  • Output requirements

  • Processing 

  • Notifications

  • Performance

  • Error Handling

  • Reporting

  • Admin Tools

Alright, let's switch gears and delve into the second category of software requirements.

Non-functional requirements

These aren't just about the features themselves; they say how those features should work and be built. Let's look at the fitness app as an example. Non-functional requirements ensure the app loads quickly, has an easy-to-use interface, works well on different devices, and can handle many users without crashing. For instance, the app should load within 3 seconds on average and no single action should take longer than 1 second.

These requirements are all about:

  • How easy it is to use

  • How well it runs

  • How reliable it is

Simply put, non-functional requirements determine exactly how the system should operate: its speed, what's going on behind the scenes, and so on. For instance, how swiftly you should receive a training course after payment, or how fast you can access your personal account and its features.

Now that we've understood what those requirements are, let's discover their main difference.

Functional vs. Non-Functional Requirements

When we compare these two, here's what we see:

  • Functional requirements are essential; the system can’t run unless these are met.

  • Non-functional requirements, on the other hand, aren't mandatory; the system can still work even if these aren't fully met.

Now, let's explore the other distinctions below.

Functional Requirements:

  • What the system does

  • Outline system capabilities

  • Can be directly tested

  • Can be easily defined

  • Deals with features

  • System-oriented

  • Cover user registration, data processing

  • User requirements are prioritized.

Non-Functional Requirements:

  • How the system does it

  • Focus on performance, security, etc.

  • Not directly testable

  • Hard to be defined

  • Deals with attributes

  • User-oriented

  • Cover response time, security

  • User expectations and needs are prioritized.

How to Set Up Software Requirements

You may wonder how to collect these requirements. Well, there are several methods, but the most common ones are as follows:

  • Software Requirements Specification (SRS) document

  • User stories

  • Use cases

SRS document

Typically, software requirements are captured in what's called an SRS document, where the team outlines all the system's features in detail. The quality of this document affects how much development will cost and whether it will meet the customer's expectations.

To write an SRS document, you first need to:

  • Understand the product's premise, purpose, and value, along with the essential terminology and references.

  • Outline the general product features, describe features, set code standards, define business logic, and identify implementation constraints.

  • Specify detailed features, clarify how the system functions, describe input data requirements, outline database features, and so on.

  • Address software quality, performance, and security requirements.

Use cases

This document outlines how the system interacts with the user (UX). By the way, if you want to become a UX/UI Designer, download a course program to gain new skills and knowledge. 

Use cases describe every situation in which a user can use one or another system feature. This doc includes characters - different user groups defined by the system - and system features that enhance user experience and help achieve goals.

For instance, in a fitness application, a use case scenario might be: "When a user selects 'Step Tracking' from the menu, the app shows their daily progress."

User stories

Unlike use cases, which concentrate on user-software interactions, user stories prioritize the user experience, enabling us to view the system through the user's eyes. These are short descriptions of what users want from the system and what the system needs to do to meet those wants. 

The structure of a user story usually looks like this: user role (who), goal (what they want to do), reason (why they want to do it).

For example, if someone uses a booking service to plan a vacation, they'd expect to find a search bar where they can type in what they're looking for, like a place to stay. Then, they'd want to check if their booking is confirmed or if they need to wait for more information.

Similarly, in a fitness app, a user might say, "I want to keep track of how many steps I take each day to see how active I am."

Examples of Functional vs Non-functional Requirements: 

Now, let's look at some real-world examples to understand how these software requirements work in practice. Take Airbnb, for example.

Let's break down the functional requirements for Airbnb:

  • Effortless sign-up and log-in: users must be able to easily create accounts using email or social media credentials.

  • Simplified search: the platform should provide a straightforward interface for users to find apartments, with filters for location, dates, price range, property type, and amenities.

  • Comprehensive listings: listings should include high-quality photos, detailed descriptions, availability calendars, cancellation policies, and accurate pricing.

  • Direct communication: Airbnb's messaging feature should enable direct communication between guests and hosts.

  • Secure booking: the payment system must ensure secure transactions, supporting multiple payment methods while protecting users' financial information with encryption and fraud prevention measures.

  • Share experiences: guests should be able to leave detailed reviews and ratings for properties and hosts.

Let's outline the non-functional requirements for Airbnb:

  • Fast and reliable performance: Airbnb's platform needs to respond quickly, ensuring smooth browsing, searching, and booking experiences, especially during busy times.

  • Data security: Airbnb must prioritize safeguarding users' personal and financial data by adhering to industry standards like PCI DSS and GDPR.

  • User-friendly design: the Airbnb app should be easy to navigate, with a clear layout and responsive design.

  • Device compatibility: Airbnb should make sure that users can access the platform from any device.

  • Compliance and trust: Airbnb needs to implement rigorous verification procedures for hosts and listings, ensuring compliance with local regulations.

Want more examples? Here are some from other areas.

Functional requirements:

  • Users must be able to schedule appointments with healthcare providers using the system.

  • The system must update inventory levels in real-time as items are bought and sold.

  • Users should be able to easily transfer funds between their accounts.

  • The system needs to provide personalized suggestions based on the user's browsing and purchase history.

Non-functional requirements:

  • Web pages should take no longer than 3 seconds to load.

  • The system must comply with GDPR to safeguard user data.

  • The streaming service should handle up to 1 million concurrent users during peak times.

  • The mobile app must be easy to use and maintain an average rating of at least 4 stars.

Functional and non-functional requirements are essential for project success. Starting with them helps you know where you're headed and what's needed. On the flip side, if you don't document requirements well, you risk failure. And not just for this project - your team's reputation is on the line too.

Think about it: the services and projects we rely on every day are successful for a reason. That's why it's vital to get the difference between functional and non-functional requirements right from the start.

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