Classifying Unit Tests by Side Effects and Cleanup Strategies

Generally, you can split unit tests into two additional categories (besides unit/integration or slow and fast), based on their side effects:

Tests with no side effects. They only read information from external sources, never modify anything, and can be run multiple times (even in parallel) without complications.

Tests with side effects. These are tests that write to a database, submit data to external systems, perform output operations on dependencies, etc.

The first type (read‑only tests) is easy to handle because they require no special maintenance. The second type (read/write tests) is more complex to maintain because you need to ensure that their operations are cleaned up immediately after the test finishes. There are two approaches:

Let all tests run, then clean up all test actions at the end of the test suite.

Clean up each test after it runs (recommended).

Cleaning up the side effects of each test is a better approach, because it allows you to run all tests in parallel or run individual tests separately (i.e., run a single test from your suite, then run it again, or a third time).

Being able to execute tests in parallel is a prerequisite for using dynamic test environments.

能够并行执行测试是使用动态测试环境的先决条件。

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