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Understanding Flaky Tests: A Quick Guide for Developers

By Josh MarinacciJanuary 16, 2024

Flaky tests in development are a prevalent challenge, causing headaches for developers and teams striving for efficient and reliable CI/CD pipelines. This article delves into the world of flaky tests, common causes, and solutions to help you and your team tackle this pervasive issue.

What are Flaky Tests?

Flaky tests are automated tests that exhibit inconsistent behavior by yielding varied results under the same conditions. Unlike stable tests, which consistently pass or fail, flaky tests unpredictably sway between passing and failing, despite no changes in your code or testing environment. 

This inconsistency is distinct from general test unreliability, which might encompass broader issues like connectivity interruptions, resource constraints, or CI/CD platform instabilities. Flaky tests, in contrast, usually stem from issues within the test logic itself, such as flawed assumptions or logical errors. While both unreliability and flakiness can disrupt the testing process, flaky tests are unique in that they point directly to problems within the test design. Addressing these flaky tests is crucial, as they not only undermine the reliability of the testing process but also impact the overall efficiency of your software development cycles.

Common Causes of Flaky Tests

Flaky tests often stem from a variety of issues within the testing framework and application environment. Understanding the root causes of flaky tests is crucial for effectively addressing them and ensuring the stability of your testing process. Here are some primary factors contributing to test flakiness:

  1. Environment Factors: Variations in your system resources, network issues, or external service dependencies can lead to flaky tests.

  2. Timing and Synchronization Issues: Tests failing due to timing problems, such as race conditions or delays in the application state, are common culprits.

  3. Inconsistent Test Data: Flaky tests often occur when test data is not controlled or consistent, leading to varied outcomes in different runs.

  4. Non-Deterministic Application Behavior: Aspects like asynchronous operations, randomness, or concurrent processes can make tests flaky.

Detecting Flaky Tests

The process of detecting flaky tests in your suite is complex and requires a multifaceted approach, each with its own set of trade-offs. Here are some key methods and their associated costs:

  1. Statistical Analysis of Test Execution History: This high-level analysis is effective for spotting trends in test failures and inconsistencies. However, it may not provide a detailed view of the current unreliability of individual tests. It offers a macro perspective, which could miss specific, ongoing issues in test reliability.

  2. Running Tests Multiple Times in the Same State: Repeating tests can offer a more granular insight into their flakiness by observing outcome variability. The cost here is significant in terms of time and resources, as it multiplies the load on CI systems and prolongs the overall testing duration. 

  3. Implementing Continuous Monitoring Systems: Continuous monitoring offers real-time insights into test runs and failure rates, aiding early detection of flakiness. The downside is the potential for data overload, where critical issues might be lost in the volume of information gathered. 

By understanding these costs and balancing them against the benefits, your team can choose the most effective strategies for detecting flaky tests in your development environment.

Strategies to Mitigate Flakiness

Tackling the challenge of flaky tests requires your team to go beyond mere detection and correction. It's essential to cultivate a team culture towards test reliability and build robust strategies in the software development process. These strategies should not only address the symptoms of flakiness but also target their underlying causes, ensuring long-term stability in your testing environment. Here are some practical strategies to mitigate flakiness:

  1. Refine Test Isolation Techniques: Develop a framework where each test can operate in a self-contained environment. This reduces dependencies on external factors and other tests, ensuring that the outcomes are solely based on the test's logic. 

  2. Standardize the Test Environment: Create a uniform testing environment that mimics your production setup as closely as possible. Use containerization technologies like Docker to encapsulate your testing environment.

  3. Optimize Retry Mechanisms with Analytics: While retry mechanisms can be useful, they should be applied judiciously. Analyze test results to understand the patterns and root causes of failures. Use retries strategically for cases where transient environmental issues are known to cause flakiness. Avoid using retries as a blanket solution for all test failures.

  4. Implement Synchronization Techniques: Beyond basic waits and timeouts, utilize intelligent synchronization methods in your tests. Develop strategies to dynamically wait for conditions within your application, such as specific UI states, database states, or backend responses, before proceeding with test execution. This approach minimizes timing-related flakiness.

Maintaining a Flaky Test-Free Suite

Establishing a flaky test-free test suite is an ongoing process. It's not just about fixing immediate issues, but also about creating a sustainable environment where tests continuously evolve to meet the dynamic nature of software development. Here are a few key practices that are essential in building and maintaining a reliable test suite:

  1. Regular Test Maintenance: Regularly review and update test cases to ensure relevance and accuracy.

  2. Collaboration and Communication: Foster a culture of open dialogue between developers and testers to collectively tackle flakiness.

  3. Documenting Test Environments: Keep a detailed record of test environments and configurations for consistency and reproducibility.

  4. Leverage CI/CD Integration: Utilize Continuous Integration and Delivery pipelines for frequent testing and early detection of flakiness.


In conclusion, managing flaky tests is an integral part of maintaining a robust and efficient development process. By understanding their causes, employing effective detection strategies, and implementing best practices to reduce and eliminate flakiness, developers can significantly enhance the reliability and efficiency of their testing processes.

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