Continuous Delivery: Reliable Software Releases through Build, Test, and Deployment Automation

by David Farley

Commit testing is the first stage in the deployment pipeline. It is vital to the process that commit tests run quickly. The commit stage is the point at which developers are sitting waiting for a pass before moving on. Every 30 seconds added to this stage are costly. In addition to the outright performance of commit stage testing, commit tests are the primary defense against inadvertent changes to the system. The more these tests are tied to the specifics of the implementation, the worse they are at performing that role. The problem is that when you need to refactor the implementation of some aspect of your system, you want the test to protect you. If the tests are too tightly linked to the specifics of the implementation, you will find that making a small change in implementation results in a bigger change in the tests that surround it. Instead of defending the behavior of the system, and so facilitating necessary change, tests that are too tightly coupled to the specifics of the implementation will inhibit change. If you are forced to make significant changes to tests for relatively small changes in implementation, the tests are not performing effectively their role of executable specifications of behavior.