Everything You Need To Know About Functional and Non-Functional Requirements

The app development process is quite similar to building a house. You really need to know what you want in order to make your dream come true. You basically need to have a clear and detailed vision of your project.

In the app development industry, these are called functional and non-functional requirements, and in today’s article, we are going to take a look at what they are, and how to write functional and non-functional requirements. So, let’s dive right in.

What Are Functional Requirements?

Functional requirements are used to define how a system function should behave, such as manipulating data and handling user interaction, calculation, etc. In other words, functional requirements are represented by features that make the system work as intended. If these requirements are not met, the system would simply not work.

In order to get a better grasp of how this works, let’s take a look at an example. So a functional requirement would be the following: “When a visitor creates an account, the server will send a welcome email.”

What Are Non-Functional Requirements? 

So we saw that functional requirements basically tell the system what to do. Well, the non-functional requirements guide the system on how to actually do it. One key difference here is that if the non-functional requirements are not met, the system will work regardless.

So why are they so important then? Well, the requirements define features that impact the user experience. The better the non-functional requirements are defined, the easier is the system to use.

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Most of the time, functional and non-functional requirements go hand in hand. As we’ve previously said, if a functional requirement tells the system what to do, the non-functional one will show how to actually do it.

So, let’s stick with the example we mentioned above. The non-functional requirement related to it would be: “When sending a welcome email, the server must send it within 10 minutes after the registration.”

How Are They Written? 

There are plenty of ways to write functional and non-functional requirements. However, one of the most common ones is through the requirement specification document. Basically, this document describes the functionally of the system and its capabilities.

A specification document must contain the following sections: the purpose of the project; an overall description, where you would explain the product vision, business rules, etc; specific requirements; use cases, where you would describe the interaction between the system and its users; the type of users who will interact with your product, system functional requirements, etc.

Another way to write these requirements would be through user stories, meaning that the functionality of the system would describe from the user’s point of view.

In short, imagine that you’re a user and describe how you would like the system to actually work. Now, user stories are written in a specific way where you need to highlight three main parts, such as: “As a (type of user), I want (goal), so that (reason).”