Requirements are the cornerstone of every software project – no exceptions.
So, nailing them is an absolute must if you want your project to succeed.
Without well-defined non-functional requirements, even the most feature-rich software will fail to meet expectations.
Here, we’ll dig deeper into non-functional requirements – what they are, why they’re important, how to write them, and more.
Let’s dive in!
Table of Contents
What are non-functional requirements? A closer look
Non-functional requirements (NFRs) define the qualities of a system. In other words, they’re not about what a system does but how it performs.
Here’s an example – a functional requirement might be that an app should allow users to log in, but a non-functional requirement defines how quickly, securely, or reliably that login happens.
Think of a car. Its ability to drive (functional) is essential, but factors like fuel efficiency, safety, and comfort (non-functional) will determine its overall success.
NFRs do the same thing for software. They ensure the software you build meets user expectations and business goals.
And clear and well-written requirements are key to building a successful product – 37% of projects fail because of unclear or wrong requirements.
Getting them right will help you avoid confusion, uncertainty, and unnecessary delays during development.
And that’s why they’re so important.
Functional vs. non-functional requirements: key differences
While functional and non-functional requirements sound similar, they are very different.
Functional requirements define what a system does. They describe the features and functionalities that allow users to achieve their goals.
This could be something like:
A search bar that retrieves relevant results
A payment gateway for processing transactions
A login system with email and password authentication
Non-functional requirements, on the other hand, focus on how the system performs. They establish benchmarks for performance, scalability, security, and more.
Here’s what they might look like:
Ensuring the search bar returns results within 2 seconds.
Guaranteeing 99.95% uptime for the payment gateway.
Encrypting login credentials using AES-256 encryption.
So, in the simplest terms, functional requirements describe what your software does while non-functional requirements describe how it does it.
Here’s a more detailed comparison:
Functional vs. non-functional requirements: comparison
Category
Functional requirements
Non-functional requirements
Focus
What the software does
How the software performs
Scope
Task-specific and goal-oriented
System-wide, focused on specific characteristics
Measurement
Measured by testing individual features (pass/fail)
Measured by specific metrics, like speed and reliability
User perspective
Directly impact how users interact with your product
Indirectly impacts user satisfaction and experience
Documentation
Usually detailed through user stories and use cases
Typically defined in service-level agreements (SLAs) or technical specs
Frequency of changes
Likely to evolve as user needs change
More stable
Testing
Verified by functional testing, e.g. unit tests, integration tests
Verified by non-functional testing, e.g. load tests, security tests
Impact on development
Essential for core system operations
Makes core operations effective and efficient
But, the most important takeaway is that functional and non-functional requirements are two halves of the same whole.
Combined, they help you create software that’s functional, reliable, and delivers value.
And that’s why they’re key to successful development.
Examples of non-functional requirements
Now that we’ve defined them, we’ll go ahead and give you some specific examples of non-functional requirements.
To make things easier, we’ll use a fintech app as an example.
Here are some examples of non-functional requirements you might write:
The app must be able to process 1000 transactions per second with an average response time of less than 2 seconds.
During peak usage hours, the system must maintain a latency of under 1.5 seconds.
The app should scale to accommodate a 10x increase in daily active users during high-demand periods.
All user data must be encrypted using AES-256 encryption during transmission and storage.
The app must support multi-factor authentication (MFA) for all user logins.
Transactions must comply with PCI DSS standards to ensure secure handling of payment information.
The app must guarantee 99.99% uptime, allowing for no more than 52.56 minutes of downtime annually.
In case of a server failure, failover mechanisms must ensure the system recovers within 30 seconds.
New users must complete the registration process in under 3 minutes, with clear guidance and error handling.
Accessibility features, like screen readers and voice commands, must meet WCAG 2.2. standards.
The system must allow deployment of bug fixes within 12 hours of identifying critical issues.
The app must comply with GDPR to ensure data privacy for European users.
It must meet AML (anti-money laundering) and KYC (Know-your-customer) regulations to prevent financial crimes.
The app must operate seamlessly on iOS, Android, and web browsers with consistent performance.
The system should provide 24/7 availability, with downtime restricted to scheduled maintenance only.
Non-functional requirements are more than just technical details and specifications.
They’re key to building dependable, user-centric software that lasts.
Next, we’ll cover (some of) the different types of non-functional requirements.
Types of non-functional requirements
Non-functional requirements can be grouped into distinct types, each targeting a specific aspect of the system’s behavior.
Now, there are a lot of different types of non-functional requirements out there and we can’t cover them all.
But, we’ll cover the most important ones below.
We’ll use a fitness app as an example so they’re easier to understand.
Performance requirements
Performance requirements focus on the speed and efficiency of a system under various conditions.
If you’re building a fitness app, a performance requirement could be: “The app must sync real-time workout data from wearable devices within 3 seconds of completion.”
Scalability requirements
Scalability requirements address the system’s ability to handle growth, whether it’s more users or data.
For a fitness app, a scalability requirement might be: “The backend must handle a 500% increase in logged workouts during promotional campaigns without downtime.”
Security requirements
Security requirements protect data and systems from unauthorized access and vulnerabilities.
A security requirement for a fitness app could be: “All stored user health data must be encrypted using AES-256 encryption.”
Reliability requirements
Reliability requirements are all about making sure the system always remains operational and dependable.
For a fitness app, a reliability requirement could be: “If the server crashes, users should be able to access cached workout logs offline.”
Usability requirements
Usability requirements ensure the app is intuitive and engaging for all users.
A usability requirement might be something like: “Users must be able to log a new workout session in under 2 minutes, even without prior experience using the app.
Maintainability requirements
Maintainability requirements focus on ensuring the system is easy to update, modify, or fix over time.
For a fitness app, a maintainability requirement could be: “Bug fixes for critical issues must be deployable within 24 hours of identification.”
Compliance requirements
Compliance requirements are about making sure your software follows relevant legal, regulatory, and industry standards.
This could be: “All nutritional advice must align with USDA guidelines to ensure accuracy.”
Portability requirements
Portability requirements ensure the system works well on different platforms and in different environments.
In our fitness app example, a portability requirement could be: “The app must operate smoothly on both Android and iOS devices, syncing data across all logged-in platforms.”
Availability requirements
Availability requirements are about making sure users can access the app whenever they need it.
For a fitness app, this could be: “Users must have access to workout plans and logs even during server maintenance through offline caching.”
Next, we’ll discuss the best practices you should follow to write great non-functional requirements.
Best practices for writing non-functional requirements
Poorly written non-functional requirements lead to misunderstandings, delays, or software that fails to meet user expectations.
How you write them can mean the difference between product success and failure – so, you can’t afford to get it wrong.
Here are best practices you should follow to ensure your requirements are clear, measurable, and actionable:
Write specific and measurable requirements
Vague requirements lead to ambiguity and poor implementation. Make sure that every requirement is specific and easily quantifiable.
So, instead of saying “The system should perform well”, write: “The system must process 10,000 requests per second with a response time under 2 seconds.”
Align requirements with business goals
Your non-functional requirements should reflect your priorities and long-term objectives.
For example, if you have an e-commerce app, prioritize uptime and scalability during holiday sales.
Write: “”The system must scale to handle a 200% increase in traffic during Black Friday.”
Write them in plain language
Avoid (too much) technical jargon. Requirements should be clear to everyone, from developers to business executives.
So, instead of writing, “Achieve a latency threshold of 50ms,” write: “The system should respond to user actions within 50 milliseconds.”
Use standardized templates
Consistency will make your requirements easier to read and implement. Use templates or frameworks to structure them.
Here’s an example:
Requirement: The app must handle 1,000 concurrent users during peak hours.
Metric: Simultaneous users.
Threshold: 1,000 users.
Measurement method: Load testing.
Reusable templates will also help you standardize documentation across different projects.
Prioritize critical requirements
Not all requirements are created equal. Focus on those that have the biggest impact on your software’s core functionality or user experience.
For example, you should prioritize uptime over aesthetic interface requirements, especially if you’re building something like a banking app.
Validate non-functional requirements through testing
Every non-functional requirement should be testable.
If you can’t verify or measure it, it’s not a good requirement.
Consider trade-offs
Improving one non-functional requirement can affect others. For example, improving security might impact performance.
You need to make sure you find the right balance. Discuss trade-offs with stakeholders and document the rationale for your decisions.
Regularly review and refine your requirements
As your product evolves, you’ll need to adjust your requirements. Regular reviews will keep them relevant.
Schedule periodic reviews to make sure they’re aligned with the latest business and technical changes.
Non-functional requirements: FAQs
Functional requirements describe and define your product’s features and functionalities, i.e. the specific operations it needs to perform to fulfill its intended purpose.
Non-functional requirements, on the other hand, describe specific metrics it needs to hit, like performance and security metrics.
In other words, functional requirements cover what your product does while non-functional requirements cover how it does it.
Non-functional requirements are measured using specific metrics tied to performance, reliability, security, and so on.
For example, performance might be measured by response times under load, such as “handling 10,000 users with under 2 seconds latency.”
In short, you need to set specific benchmarks in order to successfully meet your requirements.
Both are equally important but serve different purposes. Functional requirements enable features, while NFRs ensure those features work well.
For example,a banking app must allow fund transfers (functional), but it must also process them securely and quickly (non-functional).
Treat functional and non-functional requirements as complementary and not as competing priorities.
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As a solution architect, Kristina makes sure no detail goes overlooked and every solution fits our clients' needs perfectly. Thanks to her knack for software architecture and deep love for all things tech, she can translate even the most complex business requirements into feasible software solutions.
Outside of work, you'll find her enjoying board games with friends and planning her next unforgettable trip. Her ideal workspace? Somewhere where summer never ends, like the Seychelles. It doesn't get much better than that, right?
