Change based testing in a Yarn-managed monorepo

Change-Based Testing in a Yarn-Based Monorepo

In the world of software development, especially when working with large projects, efficiency is key. One approach to manage large codebases effectively is by using a monorepo, where multiple projects are stored in a single repository. Yarn, a popular package manager for JavaScript, provides powerful tools to manage such monorepos. One of the standout features is the ability to run commands across multiple workspaces with precision and efficiency. In this blog post, we'll explore how to leverage Yarn's capabilities for change-based testing, focusing on a specific command: yarn workspaces foreach --since --recursive --topological --exclude root -vv exec eslint src.

Understanding the Command

Before diving into the details, let's break down what this command does:

• yarn workspaces foreach: This is the base command to run a specified operation across all workspaces defined in your monorepo.
• --since: Runs the command only in workspaces that have changes since the last commit (or a specified point in history).
• --recursive: Applies the command recursively to all dependent workspaces.
• --topological: Ensures the command runs in topological order, meaning dependencies are processed before their dependents.
• --exclude root: Excludes the root workspace from the operation.
• -vv: Sets verbosity to very verbose, providing detailed logging.
• exec eslint src: Executes the ESLint command on the src directory within each applicable workspace.

The Need for Change-Based Testing

In large monorepos, running tests or linters across all projects every time can be time-consuming and inefficient. Change-based testing addresses this by focusing only on the parts of the codebase that have changed. This not only saves time but also speeds up the feedback loop, allowing developers to catch and fix issues faster.

Benefits of Change-Based Testing with Yarn

• Efficiency: By running tests and linters only on changed workspaces, you reduce the amount of computational resources required.
• Speed: Faster test runs mean quicker feedback, which is crucial for maintaining high development velocity.
• Focus: Developers can concentrate on relevant issues rather than being overwhelmed by unrelated test failures.

Implementing Change-Based Testing

Let's walk through how you can implement change-based testing in a Yarn-based monorepo using the command we've discussed.

1. Setting Up Your Monorepo

Ensure your monorepo is correctly set up with Yarn workspaces. Your package.json should include the following configuration:

{
  "private": true,
  "workspaces": ["packages/*"]
}

2. Adding ESLint

Make sure each workspace has ESLint installed and configured. You can add ESLint to a workspace by running:

yarn workspace <workspace-name> add eslint

3. Running the Command

Use the following command to lint only the changed workspaces:

yarn workspaces foreach --since --recursive --topological --exclude root -vv exec eslint src

How It Works

• Detect Changes: The --since flag ensures that only workspaces with changes since the last commit are considered.
• Respect Dependencies: The --recursive and --topological flags ensure that any dependencies are processed first, maintaining the integrity of your workspace relationships.
• Exclude Unnecessary Workspaces: The --exclude root flag skips the root workspace, which typically doesn’t need linting.
• Detailed Logging: The -vv flag provides verbose output, which helps in debugging and understanding the command execution flow.

Real-World Example

Consider a monorepo with the following structure:

/monorepo
  /packages
    /package-a
      /src
      /tests
    /package-b
      /src
      /tests

If you’ve made changes only to package-a, running the command will lint only the src directory within package-a, ignoring package-b entirely unless it has dependencies affected by package-a.

Conclusion

Change-based testing in a Yarn-based monorepo is a powerful strategy to maintain code quality efficiently. By leveraging the yarn workspaces foreach command with appropriate flags, you can ensure that only relevant parts of your codebase are tested, saving time and resources. This approach not only enhances productivity but also aligns with modern development practices, promoting faster and more focused feedback loops.

Embrace change-based testing in your Yarn monorepo today, and experience the benefits of a more efficient and streamlined development process.