Monorepo CI/CD Strategies

Monorepos bring great benefits like code sharing and simplified dependency management. However, they also introduce unique challenges for Continuous Integration/Continuous Deployment (CI/CD) pipelines.

We need smart strategies to keep builds and tests fast and efficient!

In a monorepo, many independent projects often reside within a single repository. If your CI/CD system rebuilds and re-tests every single project whenever any code changes, it becomes incredibly slow and wasteful.

This 'build everything' approach negates many of the efficiency gains of a monorepo.

The core principle for efficient monorepo CI/CD is selective builds. This means your CI system should only build, test, and deploy the projects that were actually impacted by a recent code change.

This drastically reduces the workload and speeds up feedback.

Monorepo management tools like Nx or Turborepo are designed for this. They analyze your repository to build a project graph, mapping dependencies between projects.

When you commit changes, these tools can use the graph to precisely identify which projects (and their dependents) are 'affected' by those changes.

You can implement basic change detection with Git commands. For example, to check if changes occurred within a specific application's directory since the main branch:

Even with selective builds, the remaining affected projects might still have a large suite of tests. Parallel testing allows you to run these tests simultaneously across multiple machines or processes.

This dramatically cuts down the total test execution time, providing faster feedback to developers.

There are several ways to implement parallel testing:

• Test Sharding: Tests are split into fixed groups (shards), and each shard is run on a separate CI agent.
• Dynamic Distribution: More advanced tools dynamically assign tests to available agents, often prioritizing slower tests or distributing them based on historical run times.

To further optimize, monorepo tools can cache the output of previous builds and test runs. If a project hasn't changed, and its dependencies are also unchanged, its cached output can be instantly reused.

Remote execution takes this further, offloading builds and tests to powerful cloud machines, delivering results back to your local machine or CI/CD environment.

An efficient monorepo CI/CD pipeline often follows a 'fan-out, fan-in' pattern:

• Fan-out: The initial stage identifies affected projects, then triggers a multitude of parallel jobs, one or more per affected project.
• Fan-in: A later stage collects and aggregates the results from all parallel jobs for final reporting, deployment, or further processing.

Which of the following are key benefits of implementing selective builds and parallel testing in a monorepo CI/CD pipeline?

We've explored how to design efficient CI/CD pipelines for monorepos. Key strategies include:

• Selective Builds: Only processing projects affected by changes.
• Parallel Testing: Running tests concurrently to save time.
• Caching: Reusing previously built or tested artifacts.

By implementing these, you can drastically improve feedback speed and reduce operational costs in large monorepo environments.