Properties of tests you need to know - Repeatable and Deterministic

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In software development, tests are most effective when they are reliable and consistent. A core principle of good testing is that tests should produce the same results consistently when executed multiple times with the same inputs. Non-deterministic tests—tests whose outcomes vary across runs—undermine confidence in the testing process and can lead to wasted time debugging issues unrelated to the actual code being tested. This is why tests must be repeatable and deterministic.

What Are Repeatable and Deterministic Tests?

Repeatable Tests: Tests are repeatable if they yield the same results every time they are run, given the same inputs and system state.

Deterministic Tests: Tests are deterministic if their outcomes depend solely on the code being tested and its inputs, rather than external factors like time, random numbers, or system configuration.

Why Repeatability and Determinism Matter

  1. Build Confidence: Reliable tests provide consistent feedback, helping developers trust the results.

  2. Facilitate Debugging: Deterministic tests make it easier to pinpoint the cause of failures because their behavior is predictable.

  3. Support Automation: Repeatable tests are essential for Continuous Integration/Continuous Deployment (CI/CD) pipelines, ensuring consistent results across environments.

  4. Save Time: Non-deterministic tests can cause false positives or negatives, leading to wasted effort chasing issues that do not exist.

Common Causes of Non-Deterministic Tests

  1. Dependency on External Systems: Tests that rely on databases, APIs, or third-party services can fail due to network issues or service unavailability.

  2. Use of Random Values: Randomized inputs or conditions can produce different outcomes across runs.

  3. Time Sensitivity: Tests that depend on the system clock or time zones can fail unexpectedly.

  4. Shared State: Tests that modify or depend on global/shared state can interfere with each other, leading to inconsistent results.

  5. Environment Variability: Differences in machine configurations, such as file paths or environment variables, can cause failures.

Examples of Non-Deterministic Tests

Test Dependent on External System:

@Test
public void testFetchDataFromApi() {
    String result = apiClient.fetchData("http://example.com/resource");
    assertEquals("ExpectedData", result);
}

This test may fail if the API is down, slow, or returns unexpected data.

Test Using Random Values:

@Test
public void testGenerateRandomNumber() {
    int randomNumber = randomNumberGenerator.generate();
    assertTrue(randomNumber >= 1 && randomNumber <= 10);
}

This test’s result changes with each run and does not validate consistent behavior.

How to Ensure Repeatable and Deterministic Tests

  1. Mock External Dependencies: Replace calls to external systems with mocks or stubs that return predictable results.

    @Test
public void testFetchDataWithMock() {
    ApiClient mockApiClient = mock(ApiClient.class);
    when(mockApiClient.fetchData(anyString())).thenReturn("ExpectedData");

    String result = mockApiClient.fetchData("http://example.com/resource");
    assertEquals("ExpectedData", result);
}

  2. Control Randomness: Use fixed seeds for random number generators to produce consistent results.

    @Test
public void testGenerateRandomNumberWithSeed() {
    Random random = new Random(12345);
    int randomNumber = random.nextInt(10) + 1;
    assertEquals(3, randomNumber);
}

  3. Avoid Time-Based Dependencies: Mock time-dependent code to ensure predictable results.

    @Test
public void testTimeBasedLogic() {
    Clock fixedClock = Clock.fixed(Instant.parse("2024-01-01T00:00:00Z"), ZoneId.of("UTC"));
    LocalDateTime now = LocalDateTime.now(fixedClock);

    assertEquals(LocalDateTime.of(2024, 1, 1, 0, 0), now);
}

  4. Reset State Between Tests: Ensure each test starts with a clean state to avoid interference from shared state.

    @BeforeEach
public void resetState() {
    database.clear();
    cache.clear();
}

  5. Isolate the Environment: Use consistent test environments and avoid reliance on machine-specific configurations.

Benefits of Repeatable and Deterministic Tests

  • Faster Debugging: Failures point directly to the issue in the code.

  • Reliable CI/CD Pipelines: Automated tests succeed consistently, avoiding flaky builds.

  • Developer Confidence: Engineers can trust tests as a reliable safety net.

Conclusion

Repeatable and deterministic tests are the foundation of effective testing practices. By eliminating reliance on external systems, controlling randomness, and isolating test environments, you can create tests that are reliable, predictable, and invaluable in maintaining code quality. Commit to making your tests repeatable and deterministic, and your entire development process will benefit.

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