What are Non-Functional Requirements?

What are non-functional requirements
Aran Davies
Blockchain Expert | Developer | Writer | Photographer

You have the functional requirements for your software development project. Does that end the job of creating the requirements document? No, since you need to document the non-functional requirements (NFRs).

What are NFRs and why should you focus on them? What are the various types of non-functional requirements and what are some examples of non-functional requirements? Which are the key factors to consider when you collect these software requirements?

Read on, as we explain these.

Functional vs non-functional requirements

Before delving into NFRs, let’s set the context. Let’s briefly recap the definitions of functional vs non-functional requirements.

What are the functional requirements?

When you develop a software system, you want to deliver system functionality. Business stakeholders want the proposed application to deliver specific system requirements.

The system should receive inputs and behave according to certain rules. Finally, it should deliver specific outputs to an end-user.

We call these requirements functional requirements. You will have a project manager, software architect, and business analysts working closely with the business stakeholder.

This team will gather information about user needs. Finally, business analysts will create a requirements document.

We also call this a functional specification document. You will manage these requirements and prevent scope creep. A few examples of functional requirements are as follows:

  • Transaction execution and validation rules;
  • User authentication rules;
  • User roles and access levels;
  • Reporting requirements;
  • Certification requirements.

Your project management practices for requirements management might change depending on the development methodology you use.

E.g., you will manage a product backlog with use cases and user stories in the case of an Agile project.

What are non-functional requirements?

While functional requirements define the functionality expected from a software system, non-functional requirements (NFRs) describe how well the system should function.

Unlike functional requirements, a non-functional requirement defines a quality attribute of a software system.

You can deliver an application to an end-user with the functional requirements alone. It will deliver functionality even if you don’t meet the NFRs.

However, a system quality might be low and an application might not deliver a good user experience.

While NFRs don’t concern themselves with user stories, they are important in software engineering. You need good requirements engineering skills to capture NFRs effectively. Gathering NFRs can be hard.

While they fulfill long-term user needs, there’s a high degree of subjectivity in NFRs. The architect needs to explain NFRs clearly to the development team.

The following are a few examples of NFRs:

  • Scalability;
  • Usability;
  • Maintainability;
  • Performance requirements;
  • Reliability requirements.

Examples of non-functional requirements

How do you understand the concept of non-functional requirements better?

Let’s understand it with the help of some key examples. The following are prominent examples of non-functional requirements (NFRs):

1. Scalability

Your proposed software needs to efficiently handle increasing as well as decreasing workloads. In other words, it should scale well. Scalability, a key NFR, corresponds to this.

Scalability isn’t the same as performance, although many use these terms interchangeably. Scalability refers to managing growing or reducing workloads effectively. 

You may need to document both vertical and horizontal scalability requirements. Vertical scalability means adding more capacity to one node, whereas, horizontal scalability refers to adding more nodes.

Defining scalability requirements can be hard work. You need a competent software architect with knowledge of industry benchmarks.

2. Accessibility

Many think of “accessibility” as making something usable by people with disabilities. In software engineering practices, the team has a broader meaning.

It means that you make your software usable to anyone. E.g., mobile users should be able to use a web app easily.

Making an app available to users with a slow network can be another example of accessibility. Think through your potential users early in the lifecycle, which will enable you to define this NFR.

3. Interoperability

A key NFR, interoperability, refers to the ability of a software system to share information with other systems. This non-functional requirement might cover sharing data with systems in external IT infrastructure.

You might need to plan for different kinds of interoperability. These are as follows:

  • Hardware interoperability;
  • Information interoperability;
  • Technical interoperability;
  • Business process interoperability.

4. Portability

You would like to build an application system that works irrespective of the operating system (OS) you use.

Moving the application between different operating systems should be easy. Portability refers to these requirements.

5. Performance requirements

Performance requirements describe attributes like response time and transaction throughput. For a web application, you might need to consider metrics like the time taken to load a webpage.

You need a software architect with a deep knowledge of performance benchmarks. Your application must be competitive in the target industry, therefore, it must deliver a performance better than the benchmark.

6. Security requirements

Application security carries plenty of importance and it’s becoming even more important. Among all NFRs, documenting security requirements can be quite hard.

You need to document information security regulatory requirements. That’s only one part though. The information security landscape is dynamic since hackers continue to become more organized.

You need the architect to work closely with information security specialists to document security requirements.

7. Compatibility

Your proposed application system should work on all key platforms. In today’s world, this has become a key requirement.

Take the example of mobile platforms like Android and iOS. They vary significantly, and you need to plan ahead to address compatibility requirements. E.g., you might need to undertake native development.

8. Data integrity

Data integrity is a key NFR, and it’s one of the hardest to define. Its importance increases manifold for strategic software systems.

Complex applications that integrate with multiple data sources might often have data integrity issues. Business stakeholders typically experience this too often, and they don’t want to see this repeating in new applications.

Defining data integrity rules and parameters can be an exhaustive exercise. 

9. Manageability

You need a software system that can be managed well. The manageability requirements are broad in nature. They include infrastructure, data, capacity, maintenance, governance, and several factors.

Your architect needs to work closely with business as well as operations teams to define manageability requirements.

10. Usability

You will certainly design a great user interface, and that’s included in your functional requirements. However, that alone doesn’t ensure usability.

The usability of software refers to the comprehensive user experience through the lifecycle of the application. It’s about the ease of understanding, learning, and operating it.

Document usability requirements carefully.

11. Recoverability

Despite the best design, architecture, and development, an application system will fail occasionally.

When this inevitably happens, the focus of the organization turns to recovering the application as quickly as possible.

Recoverability requirements refer to regaining the desired level of performance and data integrity after a failure.

12. Regulatory requirements

We talked about information security regulations, however, there can be many more regulatory requirements.

Examples are regulations related to privacy and transparency. If you are in a heavily regulated industry like banking or healthcare, you might have many regulatory requirements to meet.

Your architect should take a lead and discuss with the legal, compliance, finance, information security, and other relevant departments in your company. This will help you to gather all regulatory requirements.

13. Serviceability

Serviceability is a broad NFR, and it encompasses plenty of requirements. You need to build software that can be supported easily.

The application support team is often different from the development team, and the support team must find the application easy to support.

Maintainability and supportability requirements should cover the infrastructure, application, and interfaces.  

14. Testability

You will likely enhance/modify the software after you launch it. Each such enhancement needs to go through thorough testing.

However, you need to design and build the software in a manner that helps such a testing effort. This could require you to look at the architecture.

You might need to use a modular architecture, depending on your testability requirements.

Key factors to consider when gathering non-functional requirements

Keep the following key factors in mind when you gather non-functional requirements:

  • You need a competent team to gather NFRs since documenting NFRs tends to be harder than documenting functional requirements.
  • It’s important for you to onboard a competent project manager (PM), an experienced software architect, and a team of experienced business analysts (BAs). The architect needs to help the BAs and the PM to gather NFRs.
  • You need to implement a thorough review process. Reviewing NFRs can be hard, and you need experienced reviewers
  • You need your testing team to read the NFRs carefully. Testers need to plan for performance testing, security testing, stress testing, etc. NFRs form a key input for this.

You need formal approval for NFRs. This needs you to work closely with business and operations stakeholders. Use appropriate version control tools for approved requirements, and use a change-control process.

Final Thoughts

Are you ready to undertake a software development project? The initial stages of a software development lifecycle, including requirement gathering and specification building, are quite important.

Another important aspect is to take into account the constraints in achieving the defined non-functional requirements. These can be physical constraints like size, voltage, etc., operating constraints, usability constraints, etc.

You must invest in a professional product development team to build your software product successfully using the latest technologies and tools.

If you do not have such talent in your team, we advise you to partner with an experinced software development company.

DevTeam.Space can also help you here via its field-expert software developers and managers community.

Write to us your initial project specifications and one of our account managers will get back to you for more details on how we can help you build a next-level software solution.

Further Reading

Here are a few articles that might also interest you:

FAQs on Non-Functional Requirements

1. Is it harder to capture NFRs since they concern more with software quality?

Gathering NFRs and documenting them can be harder than gathering a functional requirement. NFRs are about quality attributes, and you might find it harder to define them. You need an experienced architect to lead the effort to capture NFRs. Some non-functional requirements examples are scalability, performance, recoverability, etc.

2. Should I use a requirements traceability matrix in my software development project? 

If you are executing a complex software development project, then we recommend you use a requirements traceability matrix. Your project management processes should include this. It will help you to cover all requirements during the development, test planning, and testing phases.

3. Should I first develop NFRs before deciding on a technology stack?

You should indeed develop non-functional requirements before choosing a technology stack. Take the example of scalability. Using a runtime environment like Node.js in the backend can help you develop scalable web apps. Therefore, defining NFRs can help you to choose a technology stack.


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