Unit tests are good when there are clear and durable units, but what many people seem to mean by a unit test is one that exercises the internal functions of a p

Unit tests are good when there are clear and durable units, but what many people seem to mean by a unit test is one that exercises the internal functions of a package, or the API of an internal package that evolves as fast as your product. Stable interfaces that the user cares about (e.g. CLI, public API, RPCs) make for the highest-value tests.