Mike Cohn’s Test Automation Pyramid suggests that:

1. The bulk of tests should be at the Unit level (close to the production code)
2. The next largest groups of tests should be at the Service level (including the web services and business layer)
3. Only a few tests should be done at the User Interface level, as they are more brittle, take more time, and are often redundant (repeating tests that do not need repeating)

The lower the level, the faster the test.

It is tempting to do most tests at the UI level, but much of testing can be done deeper in the pyramid. Unit testing can also yield more precise information about where the defect is located in the source code.

Unit tests are small, modular tests that verify the logic of individual functions, without the need for integration of other functionality databases or user interfaces.

A "service" is something the application does in response to some input or set of inputs. The service level focuses on the functionality that the code provides (but without a user interface). Tests automated at this level can make calls to the products APIs or web services and/or the business logic to verify the integration of various individual functions.

For more information, see:

• The Forgotten Layer of the Test Automation Pyramid
• Hybrid Tests: Blurring the Lines of the Automation Pyramid
• TAU Business/Codeless Chapter 3: Developing for Test Automatability

-- SummerDale - 23 Jul 2020