Functional and Non-functional Requirements in Software Development

When you start software development, the first thing you need to understand is what exactly you want to build. This understanding emerges during the discovery phase when you research the market, analyze competitors’ strengths and weaknesses, understand users’ needs, form project goals, decide on app functionality, and create a product roadmap. As a result, you get a clear vision of your product and the way to build it.

However, in most cases, you’re not the only person working on your project. You have dedicated developers, designers, QA engineers, and other specialists. And it’s important to ensure all your team members are on the same page about what to build.

That’s what functional and non-functional requirements are for.

In this article, we explain the difference between functional requirements and non-functional requirements, why they are important for successful software development, and how to form them. We also provide some examples and describe our own approach to requirement elicitation.

What are requirements in software development?

Functional and non-functional requirements form a significant part of the requirements usually created for a software project. To understand their role in software engineering, let’s first take a quick look at all types of requirements. According to A Guide to the Business Analysis Body of Knowledge (BABOK), there are four main types:

• Business requirements describe business goals and outcomes a product should help to achieve.
• User requirements describe a product from the user’s side, telling what functions software should perform to cover users’ needs and help to achieve business goals.
• Solution requirements describe what functionality a product should have and what qualities it should meet to be able to fulfill user requirements. Solution requirements are divided into functional and non-functional.
• Transition requirements describe temporary qualities and capabilities a product should have to be successfully updated. These requirements are needed only when implementing changes.

In other words, while business and user requirements make it clear why a client needs a product and how it will be used, solution requirements (in other words, functional and non-functional requirements) tell about what features to implement in a product and how they should perform.

All types of requirements are connected. You can’t create functional and non-functional requirements without understanding your software’s business and user needs, defining software functionality and the scope of work, performing functional decomposition, and prioritizing features. Only after these activities are completed, you can describe each feature in detail, creating solution requirements.

The requirements elicitation process usually starts during the project discovery phase, which is the initial phase of software development. Many companies, including Clockwise Software, run projects according to the Agile methodology and frequently implement the Scrum framework — a set of Agile project management practices and principles. In this case, requirements elicitation also continues during the delivery phase before each sprint.

A lot of specialists participate in the process of gathering and forming requirements. However, business analysts lead the process and know how to turn a client’s request into clear requirements documentation.

Business analysts deal with clients; discuss their needs, preferences, and expectations; create documentation; and communicate requirements to other team members, from UI/UX designers to developers and QA engineers.

Among all requirements, functional and non-functional play a significant role in successful software development. They not only give the entire development team an understanding of what to build but also help in making project estimations, reducing risks, and more. How? Let us explain.

Why is it important to form functional and non-functional requirements?

Requirements in software engineering serve as a bridge between the client and the development team. They convey the idea a client has in mind and help to:

• Ensure a clear software vision among all team members. Specified and documented system requirements help to avoid misunderstandings about what should be done among all team members, including UI/UX designers, frontend and backend developers, and QA engineers. When everyone knows the precise characteristics of the software they’re about to build, there is a higher chance that the development process will be smooth and efficient.
• Accurately estimate the project timeline and budget. Detailed information on required functionality and required quality standards allows your delivery team to understand the scope of work and estimate the time required for developing, testing, and implementing each feature. After evaluating requirements, it is possible to define realistic timelines and estimate the cost of software, which helps your team avoid missed deadlines and consider strategies to reduce software development costs.
• Reduce the risk of project failure. Requirements elicitation takes place before actual development starts, so you have a chance to evaluate the project’s technical feasibility, spot inconsistencies, choose the right technology stack, and decide on a software development team structure. It’s a great opportunity to ensure that all of the client’s requirements are taken into account and that it is actually possible to build the desired software.

As a result of forming functional and non-functional requirements, the team can work more efficiently, knowing what exactly the client expects, setting realistic deadlines, and finding ways to avoid potential risks.

We can say from our own experience that well-defined software requirements are essential for creating a great user experience and meeting clients’ expectations, and therefore ensuring project success. Vague requirements and poor communication with a team are among the top 10 issues that cause software development failure. This is why it's essential to gather and document functional and non-functional requirements at early project stages.

Now that you understand the role of requirements elicitation in software development, it’s time to answer the next question: What are functional and non-functional requirements?

Functional requirements

When you define business and user requirements and decide on the scope of work and prioritized features, you gain a general understanding of your software’s functionality. But to build that software, you need to describe each feature in detail. That’s what functional requirements are for.

Imagine you want to follow mobile app development trends like adding a face recognition feature or adding a chatbot for an AI SaaS product. How should you explain it to developers to ensure they fully understand your request? Here are functional requirement examples for these features:

• The mobile app must request a user’s permission to access the device’s camera to identify the user based on their facial features.
• With LLM implementation, the AI chatbot must perform predefined tasks or actions based on a user’s request after the user initiates interaction through text input.

In other words, functional requirements specify software features and how software behaves in response to certain user inputs. They prepare the foundation for development and serve as guidelines on the design and functionality of the software.

What do functional requirements define?

When creating functional requirements, you describe the system’s behavior under certain conditions, actions, or inputs. The range of functional requirements for each feature can vary. For example, you may need to describe:

• Technical details on processes a system uses to perform its functions
• Data processing and management
• Interactions with external interfaces and systems
• Authentication levels, etc.

Depending on your system’s features, you may need to create different types of functional requirements. Your goal is to describe each feature of your software in detail so your team will know what they need to develop to provide users with the desired functionality.

How to document functional requirements

Software development is a complex process, and each project has numerous requirements that define what functionality software should have. Of course, it’s impossible to keep all of these in mind during development. That’s why you need to document software requirements.

There are different types of functional requirement specification documents, and which you choose should mostly depend on the team’s preferences and the development methodology used — Waterfall or Agile. Here are some types of documentation you can create to describe functional requirements:

• Software requirements specification (SRS). This comprehensive document includes not only functional characteristics but also a product overview, background information, limitations, and non-functional requirements. Business analysts frequently create software requirements specifications for projects that are executed according to the Waterfall methodology, but you can use an SRS for Agile software development as well.

• User stories and acceptance criteria. Typically used in Agile project management, user stories coupled with acceptance criteria allow you to understand who will use your product, for what, and what features are important to allow users to complete their tasks. Here is a common structure for user stories:

  As a [user role], I want to [do something] so I can [reason].

  As a customer, I want to add items to my wish list so I can find them quickly later.

  Acceptance criteria, in turn, serve as rules for features, specifying how exactly a feature should perform to be accepted by a user:

  A user should see an “Add to wish list” button near an item when viewing it.

• Use cases and scenarios. This documentation is used to describe user flows. While user stories describe what software should do, use cases describe how software should do it and what action a user should take to get the desired result. A use case includes a description, pre- and post-interaction system states, and all possible methods of interaction.

There are also other types of documentation to specify requirements and ensure they are understood by the team and stakeholders. For example, we frequently represent requirements in the form of flow charts, diagrams, and software prototypes to agree on requirements with a client.

Non-functional requirements

App users have a simple goal: use a certain feature to achieve the desired outcome. That’s what we focus on when considering an app’s functionality: functional requirements. But there is another critical thing to determine to make your app successful: how exactly in-app features perform in use. This is what ensures a seamless user experience.

How quickly does a page load?

What user load should a system be able to handle?

To what standards should the app adhere to be secure for users?

When preparing for software development, you need to answer these and many other questions in the form of non-functional requirements. They bring context to how a system should perform to meet user expectations and therefore make it easy for an entire development team to understand what should be done. Defining non-functional requirements helps you avoid common product roadmapping mistakes and ensure that a system meets certain quality attributes and constraints.

Types of non-functional requirements

What are non-functional requirements? We can divide non-functional requirements into groups depending on how they describe system qualities and limitations from different perspectives. Let’s take a look at several types you may need for your project:

• Performance requirements (such as page loading time) specify how software should behave in different use cases and under different conditions.
• Usability requirements specify characteristics that make software easy to use for all types of users.
• Security requirements outline the legal and regulatory requirements a system should comply with to provide a secure user experience.
• Compatibility requirements outline what operating systems, hardware, and browsers software should be compatible with.
• Scalability requirements specify qualities software should have to perform well under increasing user demand.

Defining quality attributes and software constraints can be more difficult than defining functional requirements. There are numerous nuances you should take into account and document to make sure the team builds a product that meets the client’s expectations and guarantees a great user experience. Moreover, non-functional characteristics can be subjective or conflict with one another. For example, security requirements may conflict with usability requirements, as extensive security measures can slow down a user, impacting their experience in your app.

If you don’t have a technical background, you are unlikely to create clear non-functional requirements. That’s why it’s better to hire an experienced software development team. A business analyst, project manager, and software architect (or tech lead) can help you define how your system should perform to meet users’ expectations. Besides this, a dedicated team will help you organize all stages of startup development, ensuring a smooth and efficient development process.

How to document non-functional requirements

Documentation for non-functional requirements depends on the chosen software development methodology — Waterfall or Agile.

If you follow the Waterfall methodology, non-functional requirements can be specified in the software requirements specification (SRS) — a document we mentioned earlier that also includes functional requirements.

For an Agile project, you can either use the SRS or create separate documentation for functional and non-functional characteristics. We find separate documentation convenient for Agile projects, as we usually work with the Scrum framework that promotes flexibility and adaptability over complex documentation. Separate documentation allows us to adjust requirements more easily as a project progresses.

So, how should a Scrum team deal with non-functional requirements when documenting them separately from functional requirements? There are two methods you can use: non-functional requirements lists and technical stories. Here’s how they work:

• A non-functional requirements list is a simple but effective way to specify quality attributes of a system, where each list element defines certain conditions, qualities, and limitations a system should meet.

• Technical stories are similar to user stories but describe features not from the user’s point of view but from the system’s point of view. Let’s take as an example the following user story and create a technical story for it:

  As a user, I want to search in an app so I can find people by their usernames.

  The technical story for this example could be:

  Search results update in real time on the page as a user types their request in the search bar.

  In this case, the technical story specifies how the system must perform during and after user input. There can be dozens of technical stories for one feature that specify the smallest details of how exactly a system should perform under specific conditions.

It is also important to mention that according to the Agile methodology, requirements are created before each sprint. For example, say you want to develop a minimum viable product (MVP) first. A business analyst prioritizes features for the MVP and creates functional and non-functional requirements for one feature. The team starts the first sprint, working on this feature. For the next sprint, the business analyst creates requirements for another feature, and the process repeats until all MVP functionality is ready.

Clear documentation of both functional and non-functional requirements is essential for a clear understanding of project goals by the team. Whether you’re building a product in-house or choose a software outsourcing company to do it for you, make sure to provide your delivery team with detailed project guidelines in the form of requirements documentation.

Functional vs non-functional requirements: What’s the difference?

We’ve already said a lot about the characteristics of both types of requirements, but let’s define their key differences. Here is a comparison of functional vs non-functional requirements that will make it easier for you to spot the differences:

Here is an example of how functional requirements differ from non-functional.

Imagine you start developing an app based on the SaaS business model. Users will get access to your services on a subscription basis, so one of the features in your app should allow users to purchase and manage a subscription plan. Let’s define some functional and non-functional requirements for this feature:

• Functional:

  A user should be able to sign up for the service and manage their billing information in a subscription management system available from a sidebar navigation menu.

  This requirement adds clarity to your SaaS platform architecture and specifies what a user can do within the subscription system.

• Non-functional:

  A user’s personal information provided when starting a subscription must be encrypted according to security standards such as AES, DES, and RSA.

  This security requirement specifies how data provided by users in the subscription system should be protected to provide a secure user experience.

Functional and non-functional requirements complement each other. In the case described above, with well-defined functional and non-functional requirements, you will be able to meet users’ needs and provide users with a seamless and secure experience — and therefore succeed in your SaaS application development.

Functional and non-functional requirements elicitation: Clockwise Software approach

Each company has its own approach to requirements elicitation. At Clockwise Software, we believe that close cooperation with a client and Agile project management allow for the most efficient software development. Our approach includes the following steps:

1. Planning and preparation. We identify stakeholders, plan requirements elicitation activities, and choose techniques to gather requirements.
2. Gathering requirements. When we work on a project, a business analyst actively communicates with stakeholders, discussing their specific requirements and expectations. To identify stakeholders’ needs and expectations, we conduct one-to-one interviews, brainstorming sessions, and surveys. We can also prepare rough prototypes and show them to clients to identify misunderstandings and refine requirements if needed.
3. Documenting. We record all requirements in various formats. Starting with a work breakdown structure (WBS) and a business process diagram to define the scope of work, required functionality, and user workflows, we move to creating precise functional and non-functional requirements. The business analyst starts to create a product backlog that consists of user stories with acceptance criteria for functional requirements and technical stories for non-functional requirements.
4. Prioritizing. Our BA communicates with stakeholders to understand the importance of each feature and prioritize features. In this way, we organize our work to focus on critical features first.

As we follow the Agile methodology and work on product functionality in iterations (sprints), documentation isn’t created all at once but little by little before each sprint. Once a bunch of user stories are ready, our BA agrees on each one with the client and refines them to ensure they align with the client’s request and expectations. In this way, we get ready for a development phase.

Our main goal is to make sure requirements align with the client’s vision and goals. We continually validate with the client not only user stories for new features but already deployed functionality as well, ensuring the functionality developed according to requirements actually meets the client’s expectations. With this approach, we mitigate the risk of failure and avoid time and cost overruns.

If you have only an idea, our team can conduct other project discovery activities before eliciting requirements. We can help you research the market, define users’ needs and your business goals, validate your idea through a proof of concept (read more about what a proof of concept is in software development), and choose the right way to implement your idea.

Conclusion

All businesses are interested in how to find product–market fit. Great advice on this is to focus on users’ needs and expectations from using your app. In this way, you can understand what functionality your software should have. But another important thing is to explain your software vision to developers.

Functional and non-functional requirements help communicate to the development team what to build and what results you and your end users expect. This is done in the form of detailed documentation that describes each software feature and how it behaves under different conditions.

In the SaaS product lifecycle, requirements elicitation takes place at the very beginning, usually during the discovery phase. It requires cooperation from the product owner, business analyst, and the entire development team, along with solid technical knowledge. It’s best to involve specialists with relevant expertise in defining functional and non-functional product requirements.

If you’re looking for a team who can help you decide on functionality for your app, create clear requirements documentation, and build a successful product from scratch, reach out to Clockwise Software! We would like to hear about your idea and help with its implementation.

Sources

1. A Guide to the Business Analysis Body of Knowledge. The globally recognized standard for the practice of business analysis.
2. Systems and software engineering — Life cycle processes — Requirements engineering ISO/IEC/ IEEE 29148. An international standard that provides a unified approach to requirements engineering throughout the life cycle of systems and software.
3. The Narrative of User and Technical Stories. A discussion on the connection between user stories and technical stories and their role in describing technical aspects of software and desired functionality.
4. Requirements engineering issues causing software development outsourcing failure. A report on a study that explores 150 issues of requirements engineering for software development outsourcing.
5. Applied Software Project Management. A book with practical information on software project management tools, techniques, and principles, including a description of requirements elicitation practices and principles for project managers.