Fail as early as possible
Your application should identify failures as soon as they happen. That means problems should become expected failures close to where that input was received, which brings a host of benefits in terms of user experience, debugging, and resilience. You’re aiming to avoid unexpected failures that propagate through the system.
Failing early in the release cycle
Failing early applies both to the project overall and within your application. In terms of the project, your goal is to catch bugs as soon after they are written as possible. In practice, that means writing unit tests alongside the code that constantly check the main functionality and error cases. Those will have limitations: being written by the developer, they don’t have a second pair of eyes, so any misreading of the specification will be written into both the code and the tests. And since they are unit tests, there is a class of integration and system problems they won’t be...
