The Art of Agile Development, 2nd Edition

Fast, Reliable Tests

AUDIENCE

Programmers

Our tests don’t get in our way.

Teams who embrace test-driven development accumulate thousands of tests. The more tests you have, the more important speed and reliability become. With TDD, you run the tests as often as one or two times every minute. They must be fast, and they must produce the same answer every time. If they don’t, you won’t be able to get feedback within 1-5 seconds, and that’s crucial for the TDD loop to work effectively. You’ll stop running the tests as frequently, which means you won’t catch errors as quickly, which will slow you down.

You can work around the problem by programming your watch script to run only a subset of tests, but eventually, slow tests will start causing problems during integration, too. Instead of getting feedback within five minutes, it will take tens of minutes, or even hours. To add insult to injury, the tests will often fail randomly, requiring you to start the long process all over again, adding friction and causing people to ignore genuine failures.

Fast, reliable tests are a game changer. They take practice and good design, but once you know their secrets, they’re easier and faster to write than slow, flaky tests. Here’s how.

Rely on Narrow Unit Tests

Broad tests are written to cover large parts of your software: for example, they might launch a web browser, navigate to a URL, click buttons and enter data, then check that the browser shows the expected result. They’re sometimes called ...