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Flaky Tests at Google and How We Mitigate Them

by John Micco

At Google, we run a very large corpus of tests continuously to validate our code submissions. Everyone from developers to project managers rely on the results of these tests to make decisions about whether the system is ready for deployment or whether code changes are OK to submit. Productivity for developers at Google relies on the ability of the tests to find real problems with the code being changed or developed in a timely and reliable fashion.

Tests are run before submission (pre-submit testing) which gates submission and verifies that changes are acceptable, and again after submission (post-submit testing) to decide whether the project is ready to be released. In both cases, all of the tests for a particular project must report a passing result before submitting code or releasing a project.

Unfortunately, across our entire corpus of tests, we see a continual rate of about 1.5% of all test runs reporting a "flaky" result. We define a "flaky" test result as a test that exhibits both a passing and a failing result with the same code. There are many root causes why tests return flaky results, including concurrency, relying on non-deterministic or undefined behaviors, flaky third party code, infrastructure problems, etc. We have invested a lot of effort in removing flakiness from tests, but overall the insertion rate is about the same as the fix rate, meaning we are stuck with a certain rate of tests that provide value, but occasionally produce a flaky result. Almost 16% of our tests have some level of flakiness associated with them! This is a staggering number; it means that more than 1 in 7 of the tests written by our world-class engineers occasionally fail in a way not caused by changes to the code or tests.

When doing post-submit testing, our Continuous Integration (CI) system identifies when a passing test transitions to failing, so that we can investigate the code submission that caused the failure. What we find in practice is that about 84% of the transitions we observe from pass to fail involve a flaky test! This causes extra repetitive work to determine whether a new failure is a flaky result or a legitimate failure. It is quite common to ignore legitimate failures in flaky tests due to the high number of false-positives. At the very least, build monitors typically wait for additional CI cycles to run this test again to determine whether or not the test has been broken by a submission adding to the delay of identifying real problems and increasing the pool of changes that could contribute.

In addition to the cost of build monitoring, consider that the average project contains 1000 or so individual tests. To release a project, we require that all these tests pass with the latest code changes. If 1.5% of test results are flaky, 15 tests will likely fail, requiring expensive investigation by a build cop or developer. In some cases, developers dismiss a failing result as flaky only to later realize that it was a legitimate failure caused by the code. It is human nature to ignore alarms when there is a history of false signals coming from a system. For example, see this article about airline pilots ignoring an alarm on 737s. The same phenomenon occurs with pre-submit testing. The same 15 or so failing tests block submission and introduce costly delays into the core development process. Ignoring legitimate failures at this stage results in the submission of broken code.

We have several mitigation strategies for flaky tests during presubmit testing, including the ability to re-run only failing tests, and an option to re-run tests automatically when they fail. We even have a way to denote a test as flaky - causing it to report a failure only if it fails 3 times in a row. This reduces false positives, but encourages developers to ignore flakiness in their own tests unless their tests start failing 3 times in a row, which is hardly a perfect solution.
Imagine a 15 minute integration test marked as flaky that is broken by my code submission. The breakage will not be discovered until 3 executions of the test complete, or 45 minutes, after which it will need to be determined if the test is broken (and needs to be fixed) or if the test just flaked three times in a row.

Other mitigation strategies include:
  • A tool that monitors the flakiness of tests and if the flakiness is too high, it automatically quarantines the test. Quarantining removes the test from the critical path and files a bug for developers to reduce the flakiness. This prevents it from becoming a problem for developers, but could easily mask a real race condition or some other bug in the code being tested.
  • Another tool detects changes in the flakiness level of tests and works to identify the change that caused the test to change the level of flakiness.

In summary, test flakiness is an important problem, and Google is continuing to invest in detecting, mitigating, tracking, and fixing test flakiness throughout our code base. For example:
  • We have a new team dedicated to providing accurate and timely information about test flakiness to help developers and build monitors so that they know whether they are being harmed by test flakiness.
  • As we analyze the data from flaky test executions, we are seeing promising correlations with features that should enable us to identify a flaky result accurately without re-running the test.

By continually advancing the state of the art for teams at Google, we aim to remove the friction caused by test flakiness from the core developer workflows.

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