Question

What are unit tests, integration tests, smoke tests, and regression tests? What are the differences between them and which tools can I use for each of them?

For example, I use JUnit and NUnit for unit testing and integration testing. Are there any tools for the last two, smoke testing or regression testing?

Answer 1

• Unit test: Specify and test one point of the contract of single method of a class. This should have a very narrow and well defined scope. Complex dependencies and interactions to the outside world are stubbed or mocked.

• Integration test: Test the correct inter-operation of multiple subsystems. There is whole spectrum there, from testing integration between two classes, to testing integration with the production environment.

• Smoke test (aka sanity check): A simple integration test where we just check that when the system under test is invoked it returns normally and does not blow up.

  • Smoke testing is both an analogy with electronics, where the first test occurs when powering up a circuit (if it smokes, it’s bad!)…
  • … and, apparently, with plumbing, where a system of pipes is literally filled by smoke and then checked visually. If anything smokes, the system is leaky.

• Regression test: A test that was written when a bug was fixed. It ensures that this specific bug will not occur again. The full name is ‘non-regression test’. It can also be a test made prior to changing an application to make sure the application provides the same outcome.

To this, I will add:

• Acceptance test: Test that a feature or use case is correctly implemented. It is similar to an integration test, but with a focus on the use case to provide rather than on the components involved.

• System test: Tests a system as a black box. Dependencies on other systems are often mocked or stubbed during the test (otherwise it would be more of an integration test).

• Pre-flight check: Tests that are repeated in a production-like environment, to alleviate the ‘builds on my machine’ syndrome. Often this is realized by doing an acceptance or smoke test in a production like environment.