Designing Data-Intensive Applications, 2nd Edition

Chapter 2. Defining Nonfunctional Requirements

The Internet was done so well that most people think of it as a natural resource like the Pacific Ocean, rather than something that was man-made. When was the last time a technology with a scale like that was so error-free?

Alan Kay, in interview with Dr. Dobb’s Journal (2012)

If you are building an application, you will be driven by a list of requirements. At the top of your list is most likely the functionality that the application must offer: what screens and what buttons you need, and what each operation is supposed to do in order to fulfill the purpose of your software. These are your functional requirements.

In addition, you probably have nonfunctional requirements: for example, the app should be fast, reliable, secure, legally compliant, and easy to maintain. These requirements might not be explicitly written down, because they may seem somewhat obvious, but they are just as important as the app’s functionality; an app that is unbearably slow or unreliable might as well not exist.

Many nonfunctional requirements, such as security, fall outside the scope of this book. But we will consider a few, and this chapter will help you articulate them for your own systems. In particular, we will look at the following:

• Defining and measuring the performance of a system

• What it means for a service to be reliable—namely, continuing to work correctly, even when things go wrong

• Allowing a system to be scalable by having efficient ...