Getting the noise out of test runs

By: Kim Herzig

Can you believe your test results? If a test runs green or red, is it a true pass or fail, or might it be a false positive or negative? Sorting out the noise of false alarms from correct test results is a nasty manual exercise and insidious waste of time. This experience report looks at a method to distinguish reliable from noisy test results.*

Testing code changes

Every day engineers change software systems by applying code changes to add new features, improve the product, or to fix known issues. However, their code changes increase the risk of introducing new issues or incompatibilities. To ensure that code changes do not lower product quality, developers typically test their code changes before merging them into the main code base.

It takes time to run tests

Testing all code changes applied to a code base for large software systems may be a time-consuming task. While test cases on unit level might run fast, higher-level tests, such as system and integration tests, usually take more time to complete. Thus, the more changes are applied the more tests need to be executed and the more time each code change spends in verification before being integrated into the final product. Testing processes tend to reduce code integration speed (code velocity) and slow down product development. Therefore, optimizing test processes is likely to positively affect speed and productivity of overall software development processes.

False test alarms

In theory, test cases either pass or fail, and if they fail, there may be code defects. In practice, running system and integrations tests (for example, Microsoft Windows or the Microsoft Dynamics business software suite) require complex test setups and infrastructures, which come with their own issues. Thus, system and integration tests may also fail due to test and infrastructure issues, for example, broken hardware prevents a test from retrieving a remote file. We call such test failures false test alarms.

The waste of manual investigation

As any test failure, false test alarms are reported to the engineers and require manual investigation, lowering development speed. However, false test alarms provide no insights into product quality but only harm the development process. Therefore, it is desirable to minimize or eliminate false test alarms, or at least, prevent them from disrupting the development process.

True test failures matter

At the same time, tests that fail due to code defects must remain enabled. These failures must be investigated because actual code defects may ship to customers.

The goal of this work is to develop a precise false test alarm classification model, which identifies false test alarms automatically.

Automatic classification of flaky tests

Our model analyzes reported and manually classified false test alarms. The Figure below illustrates a simple flow chart of how this is used in practice:

Classifying test alarms flow chart

Hypothesis: True failures lead to action

Using association rule mining, we detect frequently occurring patterns of individual test step failures that are unique for false test alarms. These mined association rules are then used to automatically classify newly reported test failures as false test alarms.

Distinguishing false alarms in practice

We evaluated our approach on a large set of false test alarms that were reported by system and integration test cases during development periods of Windows and Dynamics. We evaluated our classification model on two development processes and periods covering the development of Microsoft Windows and a 2-year development period of Microsoft Dynamics. Our classification model shows a mean precision above 0.85 for both products. This means that our classification model produces a false positive rate below 15%. Recall values for both products are low and range between 0.3 and 0.5.

Prune flaky tests rather than waste time on them

The model is an excellent tool to help engineers to prioritize test failures and to provide additional input for engineers to confirm the classification model’s decisions. Such a scenario has two important benefits:

  1. The model would reduce the risk to a minimum of code defects being wrongly classified as false alarms. Instead of suppressing the test failure, the failure still reaches engineers but warns them about the possibility of being a false test alarm. This workflow allows for human supervision of the classification system and may include a feedback loop that allows engineers to override classification results, in turn training the classification model further.
  2. At the same time, such an interactive model might help engineers prioritize their test failure inspection to focus on the failures most likely to matter.

The end goal is to be able to trust test results without having to waste time on inspection. Using a predictive model demonstrably improves the efficiency.


* Kim Herzig, Nachiappan Nagappan: Empirically Detecting False Test Alarms Using Association Rules. ICSE (2) 2015: 39-48

Kim Herzig Kim leads the analytical team on the Tools for Software Engineers team. The focuses lies on helping development teams to speed up their development process(es) while maintaining or increasing product quality and reliability.