21 Mar 2024 · Software Engineering

    Addressing Flaky Tests in Legacy Codebases: Challenges and Solutions

    9 min read

    The ever-evolving landscape of software development demands constant adaptation and modernization. Legacy codebases, however, hold a vital place within many organizations, serving as the backbone of essential functionalities. While their longevity ensures stability, their age often presents challenges, and one particularly disruptive issue is the presence of flaky tests.

    These tests, prone to intermittent failures without code changes, wreak havoc on development workflows. Imagine the frustration of a seemingly green build suddenly turning red on your next run, casting doubt on the validity of your changes and hampering progress. This uncertainty is only amplified within legacy codebases, where inherent complexities pose unique hurdles in addressing flakiness. Here’s a concise diagram to visually depict our discussion:

    flaky legacy

    This article delves into the critical issue of flaky tests within legacy codebases. We’ll begin by demystifying the term and exploring its detrimental effects on development efficiency and code quality. Then, we’ll delve into the specific challenges posed by older code structures and dependencies. Finally, we’ll equip you with a roadmap of practical solutions, empowering you to tackle these roadblocks and restore trust in your test suite.

    Technical Challenges & Solutions

    Beyond the process hurdles, flaky tests often expose underlying technical issues within the legacy codebase itself. In this section, we’ll dissect these technical challenges and explore solutions to ensure your tests are reliable and effective.


    This section discusses the technical challenges posed by flaky tests in legacy codebases.

    Difficulty identifying root causes due to complex code and dependencies: Legacy codebases often evolve, accumulating layers of complexity and dependencies. This tangled web makes it challenging to pinpoint the root causes of flaky tests. Untangling the intricacies of legacy code to isolate issues requires a deep understanding of the system’s architecture and historical context.

    Integration issues with existing testing frameworks: Legacy codebases may rely on outdated or incompatible testing frameworks, exacerbating the problem of flakiness. Integrating modern testing tools and practices into the existing infrastructure can be met with resistance and compatibility issues, hindering efforts to address flaky tests effectively.

    Limited observability and debugging capabilities: Legacy systems frequently lack robust observability and debugging capabilities, making it arduous to diagnose and troubleshoot flaky tests. Without comprehensive logging, monitoring, and debugging tools, developers may struggle to gain insights into test failures and identify patterns of flakiness.

    Potential impact on existing functionality during test refactoring: Refactoring tests to improve reliability can inadvertently disrupt existing functionality in legacy codebases. The interconnected nature of legacy systems means that modifications to one part of the codebase can have unintended consequences elsewhere. Balancing the need to refactor tests with the risk of introducing regressions requires careful planning and testing strategies.


    In this section, we’ll discuss solutions for overcoming the technical challenges encountered in legacy codebases with flaky tests.

    Utilizing modern testing tools and frameworks designed for flaky test detection and mitigation: Adopting advanced testing frameworks like JUnit with the @Flaky annotation (Java) or pytest with the --rerun flag (Python) can significantly improve the reliability of testing processes in legacy codebases. These frameworks often offer features like test retries, assertion retries, and statistical analysis to identify and mitigate flakiness effectively. Visual testing tools like Applitools or Cypress can also be valuable for detecting UI inconsistencies that might contribute to flakiness in web applications.

    Implementing dependency management strategies to isolate tests and identify external factors: Implementing robust dependency management strategies can help isolate tests from external factors that contribute to flakiness in legacy codebases. By managing dependencies carefully and minimizing external influences on test execution, developers can create a more stable and predictable testing environment, reducing the likelihood of flaky tests.

    Leveraging logging and monitoring tools for better observability and debugging: Integrating logging and monitoring tools into the testing infrastructure provides developers with valuable insights into test execution and failure patterns. By capturing detailed logs and metrics during test runs, developers can diagnose flaky tests more effectively and identify underlying issues that contribute to instability in legacy codebases.

    Refactoring tests incrementally with clear documentation and version control: Refactoring tests incrementally allows developers to improve test reliability gradually without introducing disruptive changes to the existing codebase. By documenting changes thoroughly and using version control systems to track modifications, developers can ensure transparency and accountability throughout the refactoring process, minimizing the risk of unintended consequences on existing functionality.

    Process Challenges & Solutions

    Beyond the technical hurdles, flaky tests introduce complexities in our development workflow. This section will unveil the challenges we face in managing, tracking, and efficiently resolving these issues. We’ll then equip you with solutions to streamline the process and conquer these flaky foes.


    This section comprehensively outlines the process challenges associated with flaky tests in legacy codebases.

    Lack of ownership or accountability for legacy tests: In many organizations, legacy tests may lack clear ownership or accountability, leading to neglect and inconsistency in maintenance efforts. Without designated individuals or teams responsible for managing and improving legacy tests, issues such as flakiness may persist indefinitely.

    Resistance to change from developers familiar with the existing codebase: Developers familiar with the intricacies of a legacy codebase may resist changes to testing practices or frameworks, fearing disruptions to their workflow or uncertainty about the impact on existing functionality. Overcoming this resistance requires effective communication, education about the benefits of addressing flaky tests, and collaboration to devise solutions that mitigate risks.

    Time constraints and competing priorities: Software development teams often face time constraints and competing priorities, making it challenging to allocate sufficient resources to address flaky tests in legacy codebases. In a fast-paced environment where deadlines loom, new features take precedence, investing time and effort into test maintenance and improvement may be deprioritized, perpetuating the cycle of flakiness.


    In this section, we’ll discuss solutions for overcoming the process challenges encountered in legacy codebases with flaky tests.

    Establishing clear ownership and responsibility for test quality within the team: Assigning clear ownership and accountability for test quality within the development team ensures that flaky tests are actively monitored, managed, and resolved. By designating individuals or teams responsible for maintaining test suites and addressing flakiness, organizations can ensure that testing efforts remain consistent and proactive.

    Promoting a culture of test automation and continuous improvement: Fostering a culture of test automation and continuous improvement encourages developers to prioritize testing practices and invest in automation tools and frameworks. By emphasizing the importance of test reliability and encouraging collaboration and knowledge sharing among team members, organizations can cultivate an environment where flaky tests are identified and addressed promptly.

    Prioritizing flaky test fixes based on impact and feasibility: Prioritizing flaky test fixes based on their impact on software quality and feasibility of resolution allows development teams to allocate resources effectively and focus on addressing the most critical issues first. By conducting impact assessments and feasibility analyses, teams can identify high-priority flaky tests that require immediate attention and develop targeted strategies for resolution, minimizing disruption to development workflows.

    Case Studies: Taming Flaky Legacy Tests in Action

    The following case studies illustrate how real-world teams successfully addressed flakiness in their legacy codebases. These examples showcase different approaches that can be adapted to various testing scenarios and legacy system challenges. Let’s delve into these specific examples to see how strategic planning and proactive measures can conquer flakiness in your legacy codebase.

    Case Study 1: E-commerce Platform Streamlines Testing An e-commerce platform faced a growing problem with flaky tests in its legacy codebase. These tests caused frequent build failures, hindering development velocity. The team implemented a two-pronged approach:

    • Test Refactoring with Ownership: They established clear test ownership. Each developer became responsible for maintaining the unit tests associated with their code modules. This fostered accountability and encouraged developers to write robust, maintainable tests.
    • Mocking External Dependencies: Legacy tests often relied on external dependencies like databases or external services, causing flakiness. The team implemented mocking frameworks to isolate tests from these dependencies, ensuring consistent testing environments.

    The outcome? Build failures caused by flaky tests dropped by 70%. This not only accelerated development but also improved developer confidence in the test suite.

    Case Study 2: Financial Services Company Enhances Test Automation

    A financial services company struggled with a large suite of manually executed regression tests for its legacy core banking system. These tests were time-consuming, prone to human error, and unreliable. The team embarked on a test automation journey:

    • Prioritization and Automation: The team prioritized critical user journeys and functionalities, focusing automation efforts on these areas first. They utilized automation frameworks to convert manual tests into automated scripts.
    • Flaky Test Detection and Flake Analysis: Tools were implemented to automatically detect flaky tests. Analysis of these tests revealed issues like timing dependencies and external resource contention.

    By automating critical test cases and identifying flaky tests, the company significantly reduced regression testing time and improved test suite reliability. Additionally, the insights from flake analysis helped developers fix underlying code issues, leading to a more robust system overall.


    I remember the frustration of dealing with flaky tests in a legacy codebase at my previous job. Our e-commerce platform, built years ago, had a growing suite of UI tests that were becoming increasingly unreliable. Every build felt like a coin toss – would the tests pass, or would a random failure bring the whole process to a halt? It was a major bottleneck for development.

    We tackled the problem head-on, implementing strategies like the Page Object Model for UI tests and mocking external services. Slowly but surely, the flakiness subsided. Tests became dependable, builds went smoothly, and our confidence in the codebase grew significantly. This experience, along with countless others in the industry, underscores the importance of addressing flaky tests in legacy systems.

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