Testing levels

Depending on the size of the SUT and the scenario in which it is exercised, testing can be carried out at different levels. In this book, we classify the different testing levels in four phases:

• Unit testing: Here, individual program units are tested. Unit testing should focus on the functionality of objects or methods.
• Integration testing: Here, units are combined to create composite components. Integration testing should focus on testing components, interfaces.
• System testing: Here, all of the components are integrated and the system is tested as a whole.
• Acceptance testing: Here, consumers decide whether or not the system is ready to be deployed in the consumer environment. It can be seen as a high-level functional testing performed at system level by final users or customers.

The first three levels (unit, integration, and system) are typically carried out during the development phases of the software life cycle. These tests are typically performed by different roles of software engineers (that is, programmers, testers, QA team, and so on). The objective of these tests is the verification of the system. On the other side, the fourth level (acceptance) is a type of user testing, in which potential or real users are usually involved (validation). The following picture provides a graphical description of these concepts: