Functional vs Non-Functional Requirements: The Definitive Guide

Knowing exactly what features and functionalities a customer wants in the app is quite challenging for a software development team.

To avoid any misunderstandings, a customer and a software development team need to define project requirements: both functional and non-functional requirements for the future application.

In this article, we explain the difference between the two types of requirements and share the best practices on how they are gathered.

What are Functional Requirements in Software Engineering?

In software development, functional requirements determine the functions an entire application or just one of its components should perform.

A function consists of three steps: data input – system behavior – data output. It can calculate, manipulate data, carry out business processes, establish user interaction, or do any other tasks.

In other words, a functional requirement is WHAT an application must or must not do after some data input.

Functional requirements are important as they show software developers how the system is intended to behave. If the system doesn’t meet functional requirements it means that it doesn’t work properly.

What Are Non-Functional Requirements?

Non-functional requirements determine the performance standards and quality attributes of software, e.g. system usability, effectiveness, security, scalability, etc.

While functional requirements determine what the system does, non-functional requirements describe HOW the system does it.

For example, a web application must handle more than 15 million users without any decline in its performance, or a website must not load more than 3 seconds.

If an app doesn’t meet non-functional requirements, it continues to perform its basic functions, however, it won’t be able to provide a great user experience.

Non-functional requirements are important as they help software developers define the system abilities and constraints that are essential for developing high-quality software. Therefore, non-functional requirements are as important as functional requirements for successful product adoption.

Why Is It Important to Know the Difference Between Functional and Non-Functional Requirements?

Well-defined functional and non-functional requirements help software developers build a product that exactly corresponds to the client’s needs. However, is it really necessary to know the difference between functional and non-functional requirements?

The main reason to know the difference between functional and non-functional requirements is that they define the scope of work for a project. Software developers need to keep up with this scope to develop an application within its timeframe and budget.

If a work scope constantly changes, the development team has to extend the deadlines and the development costs increase. This may lead to adverse consequences for a project.

The importance of distinguishing between the two types of requirements is paramount when creating an MVP.

A development team and a customer should discuss which features and functionalities to implement in the app first. A customer may have his own vision of the project and its requirements.

If a customer decides that they want to remove or modify some feature, it’s essential to realize what type of requirement it is.

Most of the time, software developers can simply change the non-functional requirements while functional requirements will demand more work and profound changes.

When a customer and a software development provider know the difference between functional and non-functional requirements, it helps them to more precisely define the scope of work, more accurately range the requirements by importance, optimize the project costs, and better meet the customer’s needs.

How are Functional and Non-Functional Requirements Gathered?

Ideally, before turning to a software development company, customers should already have all the functional and non-functional requirements at hand. Therefore, they need to prepare them in advance on their own or ask a 3d party provider.

Let’s have a look at what each type of requirement includes.

The functional requirements can be divided into three groups:

• Business requirements – describe the project goals and expectations, the benefits the project can bring, possible project constraints, and its scope;
• User requirements – involve the user needs and what activities a user will be able to perform in the system;
• System requirements – include system actions, specifications of software and hardware, and so on.

The non-functional requirements fall under various categories, including:

• Usability – defines how easily a user can interact with the app’s interface, e.g. the screen color, buttons size, etc.;
• Availability – ensures that the app will work stably for a certain time, e.g. rare downtimes throughout the year 24/7;
• Reliability – defines that the app will work in a certain environment or for a specific period of time without any failures;
• Recoverability – ensures that the app can recover all the data after the system failure or restore the system to certain parameters;
• Scalability – determines that the app will continue functioning properly after its size or volume changes;
• Performance – assesses how fast the app is;
• Supportability – defines if the app is easy to support and maintain throughout its life cycle and what kind of support it requires e.g. an in-house team or remote support;
• Security – defines how secure the app should be, for example, FinTech and banking apps should meet international and regional security standards;
• Capacity – assesses the amount of data or services the app can handle.

The Impact of Poorly Defined Requirements

Without proper requirements, a software project can easily go out of control and die under delays, overbudgets, and even total failures. With no clear notion of what the system must perform and how, it’s very easy for a team to lose track of the project scope.

This often results in scope creep, which involves disorderly changes piling up, causing missed deadlines and increased costs.

This means if the solution doesn’t appropriately take usability and performance into consideration, it is hard to make people accustomed to it.

Security also might become an issue; a system may suffer from vulnerabilities of cyber threats or data breaches because of undefined requirements on security.

And let’s not forget about scalability—if that software is not built to adapt to future growth, it can lag or completely crash when new users are suddenly onboard.

Due to these very headaches, the teams are recommended to take the necessary time out to elicit and validate the requirements with stakeholder communication never turned off.

It’s a whole lot easier when everyone is on the same page from the start to stay on track, keep costs in check, and actually build a product that meets business and user needs.

How to Prioritize Functional and Non-Functional Requirements

Not all requirements carry the same weight. Some are essential for the core functionality of the system, while others enhance usability, security, or performance.

To ensure a flawless development process, teams should categorize and prioritize these requirements based on business goals, user needs, and technical feasibility.

One common approach is the MoSCoW method, which categorizes requirements into four groups:

• Must-have: Critical for the software to function as intended.
• Should-have: Important but not vital; can be deferred if necessary.
• Could-have: Desirable but not essential; often considered if resources allow.
• Won’t-have: Not a priority for this development cycle but may be revisited in the future.

Functional requirements typically fall into the “must-have” category, as they define what the system must do. Non-functional requirements, on the other hand, may range from “must-have” (e.g., security standards for a banking app) to “could-have” (e.g., minor UI enhancements).

Prioritization also depends on the project’s scope and timeline. For example, in an MVP (Minimum Viable Product), the focus is on core functional requirements, while some non-functional requirements, such as advanced scalability or supportability, may be addressed in later stages.

Common Challenges in Defining Requirements

Even with a structured approach, defining functional and non-functional requirements can be challenging. Some common obstacles include:

• Changing client expectations: Customers may not always have a clear vision of what they want, leading to scope creep and project delays.
• Miscommunication between stakeholders and developers: Technical jargon can create confusion, making it essential to document requirements in a clear, structured manner.
• Balancing performance and cost: Some non-functional requirements, such as high availability or extensive security measures, can increase development costs. Teams must find a balance between meeting these requirements and staying within budget.
• Ensuring scalability and future-proofing: As businesses grow, their software needs may evolve. Anticipating future scalability requirements can prevent costly rework later.

To overcome these challenges, it’s essential to engage all stakeholders early in the process, use clear documentation, and maintain openness in the development approach.

Examples and Best Practices

There is a wide range of other formats that can help make up the project requirements. Let’s have a look at the most effective ones.

User Stories

It is a common practice to formulate the requirements in the form of user stories. User stories are the requirements conveyed by a user. They usually take the form of several simple sentences which have the same pattern:

As a (user), I want to (goal) so that (reason).

A user story example: As a project manager, I want to understand the software development team’s progress so that I can report on the outcomes to the CEO and project stakeholders.

Software developers usually compile requirements using user stories when they want to communicate ideas on product features and functionality to non-technical members.

Use Cases

Use cases have a broader scope than user stories. They include types of users and all the possible actions a user can do in an app. Unlike user stories that describe the end purpose of a feature, use cases involve the flow of steps which lead to the purpose.

For example, if you want to create an application for logistics and supply chain management, the number of roles software developers should think about are: sellers, buyers, suppliers, managers, dispatchers, and many others.

The actions for these roles can look like these:

• Both seller and buyer can view the product route in the app;
• All the dispatchers can track products in warehouses;
• All the users can view the delivery time on items in their accounts, etc.

Software Requirements Specification Document

Software Requirements Specification (SRS) is the document that a software development team relies on when creating an app. It includes all the customers’ needs and wants translated into a clear for the development team language – a detailed description of all the product functions and features.

The main sections that are usually included in an SRS document are:

• Introduction with the product purpose, glossary of terms, references to specific literature and materials related to the app development;
• Description contains a detailed description of product features, coding standards, data exchange policies, and others;
• The system features section which explains how the app’s features should function;
• External interface requirements define the hardware, software, or databases the app has to interact with;
• Non-functional requirements – all the performance standards, an app’s quality attributes, and security requirements.

The creation of an SRS, user cases, and user stories are essential for effective application development.

However, there are other documents that are equally important for a successful project launch and development.

Conclusion

With custom software services companies can build any type of app for their effective business development. However, for an app to really meet its business needs, it should have detailed functional and non-functional requirements.

To form functional and non-functional requirements, you can search for help from your software development company, or third-party businesses, or do it on your own.

Well-elaborated requirements ensure that your software development partner will fully understand how to develop a digital solution that would entirely meet your expectations and satisfy your business needs.

FAQ: Understanding Functional and Non-Functional Requirements in Software Engineering

What is the difference between functional and non-functional requirements?

Understanding the difference between functional and non-functional requirements helps make software that works as expected. Functional requirements specify what the system must do, for example, processing payments or administering user accounts. Non-functional requirements describe how a system does it. It covers speed, security, and system usability. Both types are important because functional requirements need to be met for the software to work, and non-functional requirements are essential for guaranteeing that users won’t leave a software application frustrated.

Why is it important to have clearly defined requirements at the start of a project?

Clearly defined requirements at the start help avoid many problems, such as missed deadlines, unforeseen costs, and software that doesn’t go in line with user wants. A poor definition of requirements can also lead to confusion and last-minute changes. Writing functional requirements and at least some non-functional requirements early on proves the system is built correctly from the beginning.

What happens if functional and non-functional requirements are not properly defined?

If functional requirements and non-functional requirements are unclear, the software might not work properly or seem confusing. Without specific requirements, the system interacts poorly with users, lacks security, or fails to extend as more people get onboard. Functional requirements specify the features users want, while non-functional requirements explain how the system must perform. Both types of requirements are critical to confirming the system meets expectations and runs as demanded.

How are functional and non-functional requirements documented?

Usually, developers use a functional requirements document (FRD). It details how a system should work, including system interactions, user roles, and expected behaviors. The FRD can include non-functional requirements as well or put them in a different section, covering system usability, safety, and different system access levels. Many companies do this work day-in-day-out because software must meet both functional and non-functional requirements before seeing the market.