What is a monorepo? A practical guide for developers5 min read

A monorepo is a single repository for multiple projects, such as an app, its backend, and shared libraries. It helps teams make related changes together, but it also calls for careful build and access controls.

Here is how a monorepo compares with multiple repositories, when it helps, and what its trade-offs bring to the table.

What is a monorepo?

A monorepo is one version-controlled repository that contains multiple projects, such as a frontend app, backend service, and shared libraries. A multi-repo setup keeps each project in a separate repository. A monorepo makes cross-project changes and code sharing easier, but builds, permissions, and repository size need deliberate management.

For example, if you work on a web app with a backend service, frontend app, and some shared libraries, a monorepo would house all of these projects in one central place. This setup makes it easier to share code, manage dependencies, and maintain consistency across projects, which is especially helpful for big teams.

How a monorepo works

• Single Repository: All projects are stored together in a single version-controlled repository.
• Shared Code and Dependencies: Easily share code and dependencies between projects.
• Atomic cross-project changes: A single commit can update an app and a library together; individual packages can still have their own release versions.
• Coordinated tooling: Shared conventions help teams standardize builds and tests, while deployment pipelines can remain independent.

Example monorepo structure

project/
├── apps/
│   ├── web/
│   └── api/
├── packages/
│   └── ui/
└── package.json

Here apps/web can import components from packages/ui. A single commit can update both, while apps/web and apps/api still deploy independently. Define which projects depend on each package so CI can test the affected projects, not every app on every change. See Nx’s guide to reducing CI waste for one implementation.

Why consider a monorepo?

Monorepos have become popular because they bring a lot of practical benefits, especially for teams working on complex systems or for organizations with lots of interdependent projects. Let’s look at some of the big advantages:

Simplified dependency management

With everything in one place, it’s easier to manage dependencies. Shared libraries, for instance, can be updated in one spot and used across multiple projects without complicated dependency juggling.

Code reusability and consistency

A monorepo makes it easy to reuse components, libraries, and modules across projects, so you don’t end up reinventing the wheel. This reusability also fosters consistency, reducing the chance of bugs due to mismatched code or dependencies.

Better collaboration and transparency

When all code is in one place, developers can more easily see what other teams are working on. This setup improves collaboration and can break down silos, making it easier to share ideas and align on goals.

Unified versioning and atomic changes

A single commit can update a shared component and its consumers together. Teams can still release individual apps or packages independently; the repository layout does not require one shared release version.

Streamlined CI/CD pipelines

Monorepos can share CI configuration without running every job for every change. Use affected-project detection and caching to test and build only what a change touches; deploy each app or service according to its own release needs.

Monorepo vs. multi-repo: what’s the difference?

In a monorepo setup, all code is in a single place, while with a multi-repo setup, each project has its own repository. Here’s a quick look at the differences:

Challenges of using a monorepo

Monorepos aren’t perfect and come with their own set of challenges. Here’s what you should keep in mind:

Scalability and performance

As monorepos grow, they can become difficult to manage. Large codebases can lead to slower performance, so it’s important to use tools that can handle a big repository and help maintain its organization.

Complex CI/CD requirements

Monorepos require more advanced CI/CD configurations. Without the right setup, you could end up re-running tests and builds unnecessarily, which eats up resources. Advanced tools like Bazel, Nx, and Lerna are often used to help manage these complexities.

Tool compatibility

Not all development tools are built for monorepos. Teams might need to invest in new tools or custom scripts to support specific needs, like selective builds or dependency management.

Learning curve for developers

Switching to a monorepo requires some training. Developers need to understand how to work within a shared environment and follow best practices to keep the monorepo organized.

Getting started with a monorepo

Thinking of trying out a monorepo? Here are some tips to make the transition smoother:

1. Plan Your Structure: Organize projects within the repo in a way that makes sense and keeps things clean.
2. Pick the Right Tools: For larger monorepos, tools like Bazel, Nx, or Lerna can help keep builds and tests fast and efficient.
3. Optimize CI/CD: Set up selective builds and tests so your pipeline only processes the parts of the code that have changed.
4. Document Best Practices: Make sure developers know the do’s and don’ts for working in a monorepo to avoid clutter and confusion.

Companies using monorepos

Some of the biggest tech companies are using monorepos:

• Google: Google uses a massive monorepo to store code for nearly all its products, using a custom version control system called Piper.
• Facebook: Facebook manages its backend and frontend code in a monorepo, which helps its teams deploy updates quickly across different services.
• Microsoft: Microsoft uses a monorepo structure for large codebases like Windows and Office to improve collaboration and consistency.

A monorepo is a powerful way to organize code, especially when multiple teams work on related projects. It makes it easier to share code, manage dependencies, and keep projects aligned. But to make the most of a monorepo, you’ll need solid tools and good practices to handle growth and complexity.