Functional and Non-functional Requirements: Key Differences

Functional and non-functional requirements in software engineering are the essential criteria for determining if a new product meets the client’s expectations. However, having clear requirements for the system is crucial not only for assessing the project’s final results. They also play an important role in the software development process. In particular, the precise description of the future software solution helps programmers stay on track and build a product that solves real business problems.

Yet, each type of software requirements has its specifics, and it’s important to consider them when creating tasks for a development team. This article will explain how functional and non-functional requirements differ and provide you with some examples.

Classification of requirements

In general, there are several types of product requirements. Each of them belongs to a different level of the specification hierarchy and exists for a particular purpose. Here are the main groups of product requirements.

• Business requirements. Basically, they outline what a client tries to achieve by building a product. For example, it can be the optimization of the sales process or automation of certain operations.
• Stakeholder requirements. This type of requirements shows what different categories of stakeholders (e.g., business owners, end-users) expect from a product. For example, it can be syncing an email service and sales pipeline, generating reports, etc.
• Solution requirements. These requirements constitute technical criteria that a product must meet. They can be divided into:
  • functional requirements (describe product features), and
  • non-functional requirements (describe product properties)

Besides, some projects can also have other categories of product requirements. For instance, organizations often create transition requirements to make the migration from legacy to new systems smoother.

What are the functional requirements?

As the name suggests, functional requirements outline the functions that a product must perform (i.e., what it should do). We usually think of such functions as product features. Simply put, it’s a detailed description of how a system should behave and respond to specific user actions or other conditions. For instance, if we apply functional requirements to the way a bicycle works, we can say that pedals should move when you push on them, making the wheels go forward.

Functional requirements are most commonly captured in the form of use cases or user stories. These terms are sometimes used interchangeably, but they actually mean slightly different things. Use cases focus more on a system and what it has to do to meet user needs. On the other hand, user stories show product functions from the user perspective, specifying user roles and particular goals they want to achieve. Here is how both options look like.

Use case

Most use cases are constructed of the following elements:

• Description. A general statement of what a system should do.
• Actors. Users who will interact with a system.
• Preconditions. These are the conditions that must exist to trigger the use case.
• Basic flow. It points out the sequence of tasks that a system will perform if nothing goes wrong.
• Alternative flows. They describe the variations of a basic flow that will be executed if something will go wrong.
• Postconditions. These are the conditions that must become true after the use case runs.

User story

As a rule, user stories consist of the goal a user wants to achieve and acceptance criteria. Most teams use the following template:

Here is an example:

The description of the user story is always followed by acceptance criteria. Such criteria specify the particular conditions that a product must meet so a user can accept it. For instance, “a registration form should contain a field for inserting an email address”, “the field for inserting an email must show “error” when an email address is invalid”, and so on.

Examples of functional requirements

Any product feature was, at some point, a product functional requirement. But let’s take a look at some typical examples so you can better understand what we’re talking about:

• User authorization and authentication
• Adding products to a cart
• Check-out and payment processing
• Report generation
• Tracking customer interactions with a website
• Adding new leads to a sales pipeline
• Grouping customers
• Allowing users to view the newsfeed
• Etc.

The format in which these requirements are written can differ depending on the common practice of a development team and the project’s specifics. Complex products may require writing use cases and user stories, or some combination of both formats.

What are the non-functional requirements?

Non-functional requirements are a set of quality attributes that a product must possess, and that describe how it should behave. They do not specify what specific tasks a system should perform. Instead, non-functional requirements provide a list of standards that, if met, ensure the effective work of a product. Since such standards are more abstract than functional requirements, it’s usually more difficult to determine, measure, and test them.

Some people believe that non-functional requirements are not mandatory since the core product functionality cannot be affected if they are not present. But it’s only partially true. When a certain non-functional requirement isn’t met, a system will still work. However, its functions can lose their value for users.

For instance, if the load speed is too low, a user can just cancel the operation and decide not to enter a web page. As a result, a product will not help a business achieve the goals for which it was originally built. That’s why correctly determining non-functional requirements is as crucial as writing functional requirements.

Examples of non-functional requirements

When it comes to building a software system, the number of non-functional requirements is naturally lower than the number of functional requirements. But there still can be quite a few of them. Let’s take a look at the most common examples.

Usability

The usability requirement defines if a product in general and its core functions are easy to use for a target audience. It mainly focuses on a user interface and can be assessed based on several criteria, such as:

• Clarity. The app architecture has to reflect the specifics of its targeted business domain (e.g., taxi services, food delivery industry, e-learning)
• Scalability. As mentioned, a good mobile app architecture allows you to add new features and scale up a product easily.
• Readability. The code must be easy to read to other developers — even those who were not involved in the project.
• Testability. If a mobile app architecture is built right, testing a product will take less time and resources, while the effectiveness of the QA efforts will remain at a high level.
• Efficiency. A mobile app that is based on solid architecture offers smooth performance and can cope with high loads.
• Maintainability. A well-designed mobile app architecture ensures that the product will be maintainable, and the costs needed for its support aren’t too high.
• Optimization. If you create a good architecture for a mobile development project, it’ll save time and money required to bring a product to life.
• Security. The mobile application architecture should allow for data encryption and create a secure environment for data transmission.

Yet, the above list isn’t exhaustive. If you want to create a high-end mobile solution, it’ll need a unique approach. Seeing a big picture is great, but you shouldn’t forget about the details either.

The bottom line

Gathering functional and non-functional requirements is a mandatory step in every software development project. Functional requirements help a team and client determine what exactly a system should do, while non-functional requirements aim to outline the core product’s quality attributes. Although writing both types of requirements takes some time and resources, it always pays off in the end. When programmers have clearly defined guidelines on what has to be built, it’s easier for them to create results that meet the client’s expectations.