What Are Non-Functional Requirements?

What Are Non-Functional Requirements?

I want to wrap up our back-to-basics series on user stories with a way to account for non-functional requirements in your backlog.
Here in the U.S., we're in the midst of tax season. Our annual income taxes must be paid no later than April 18 this year. While thinking about—or gulp! actually paying—taxes is not very fun, it does provide a useful example of how to think about nonfunctional requirements.
A nonfunctional requirement is a requirement about how the system exists in the world, rather than about something users can do with that system.
Nonfunctional requirements are often about performance, capacity, maintainability, localizability, interoperability, usability, portability and so on. Because so many of these end with -ility, they are often called the -ilities of a system.
For example, a feature or functional requirement of a word processor might be that it has a spell checker. The spell checker is something the user can do with the word processor.
A nonfunctional requirement might be that the spell checker works in English, French, German and Spanish. This requirement is about how the spell check feature exists in the world; in this case, it exists in four languages.
If the development team speaks English, that version will be the one developed first. And the team will have a nonfunctional requirement to make the spell checker work in French, German and Spanish.
Whether this is thought of as one requirement for three additional languages or three requirements for one additional language each will not matter for my point in this tip.
The effort to produce these translated versions of the product can be thought of as a tax on the development team’s performance. Consider whatever speed the team was going at before. They must now go a little slower due to the need to produce those translations.
It doesn’t matter whether the translations happen in the sprint, a sprint behind or whenever. In the grand scheme, the team is going slower by the amount of effort it takes to produce the translations. That is, they are slower by the amount they are taxed.
The amount of these taxes can become quite significant over time: Support the three last releases of three browsers. That’s a tax. Work on the three latest Windows releases. That’s a tax. Perform at this level. That’s a tax. Future scalability to that number of users. That’s a tax.
I hope that through this metaphor about the costs of nonfunctional requirements, you can use it to think through the value of including each on your project.

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