What is a test-driven development (TDD) test pyramid and how is it used?

The test-driven development (TDD) test pyramid is a concept that helps in structuring and prioritizing the different types of tests in software development. It consists of three layers or levels of tests: unit tests, integration tests, and end-to-end tests.

1. Unit Tests: These are the foundation of the test pyramid and focus on testing individual units or components of the software in isolation. Unit tests are typically written by developers and aim to verify the correctness of small, independent pieces of code. They are fast to execute and provide quick feedback on the behavior of individual functions or methods.

2. Integration Tests: The middle layer of the pyramid involves integration tests, which verify the interaction and integration between different components or modules of the software. These tests ensure that the units work together correctly and that the integration points are functioning as expected. Integration tests are slower than unit tests as they involve multiple components, but they are still relatively fast compared to end-to-end tests.

3. End-to-End Tests: The top layer of the pyramid consists of end-to-end tests, also known as system tests or acceptance tests. These tests simulate real user scenarios and cover the entire software system, including all the integrated components. End-to-end tests validate the overall behavior and functionality of the software from the user's perspective. They are slower to execute and often involve external dependencies such as databases or network connections.

The TDD test pyramid is used as a guideline for test coverage and helps in determining the appropriate balance between different types of tests. The pyramid suggests that the majority of tests should be unit tests, followed by a smaller number of integration tests, and even fewer end-to-end tests. This approach ensures that most of the testing effort is focused on the lower levels, where issues can be identified and fixed more easily and quickly. By following the test pyramid, developers can achieve faster feedback cycles, better code quality, and more reliable software.