Before we go into the topic of how to craft effective, fast, reliable, automated, and maintainable test cases, let's have a look at what assets to test. There are tests on a code layer as well as end-to-end tests. Code layer tests are based on the project's source code and are usually executed during development and build time, whereas end-to-end tests, of all kinds, operate on a running application.

Depending on the test scopes, which we will get to know in the next section, there are different layers of tests, whether tests operate on classes, multiple components, enterprise applications, or whole environments. In all cases the test object needs to be isolated from external concerns. The nature of tests is that they verify certain behavior under specific conditions. The environment surrounding the test object, such as the test cases as well as used components, have to interact with the test object accordingly. The test case will therefore control the test object. This not only includes tests at code level, but also end-to-end tests with external systems being simulated and mocked away.

Most importantly, software tests should verify business behavior. The specified use cases all have to perform certain logic that has to be tested before production deployment. Software tests should therefore verify that the application fulfills the business requirements. Special and corner cases need to be covered as well as negative tests.

For example, testing authentication functionality not only needs to verify that a user can log in with the correct credentials, but also that they can not log in using wrong the credentials. A corner case of this example would be to verify that the authentication component notifies a user whose password is about to expire as soon as he logs in successfully.

Besides business behavior, technical aspects and cross-cutting components also need to be tested. Accessed databases and external systems and the form of the communication is required to be verified on both ends in order to guarantee a working team. These concerns are best tested in end-to-end tests.

In all cases the test object should not be modified during the test, but work in the way as it will in production. This is crucial for crafting reliable tests that will not alter their behavior later on. For code level tests, this only requires that the contents of all involved components are the same. For end-to-end tests, this includes the whole enterprise application as well as the installation and configuration of the application's runtime.