What is a Monorepo and Why Should You Care?

In today’s fast-paced software development landscape, choosing the right repository strategy can significantly impact your team’s efficiency and project success. One approach that’s gaining traction, especially among large tech companies, is the monorepo. But what exactly is a monorepo, and why should you consider adopting it for your projects? Let’s dive into the world of monorepos and explore their benefits, drawbacks, and why they might be the right fit for your organization.

Decoding the Monorepo: A Single Source of Truth

At its core, a monorepo (short for monolithic repository) is a unified version-controlled code repository that houses multiple projects. These projects, while often related, can be logically independent and managed by different teams within an organization. Think of it as a digital library where all your company’s books, manuscripts, and research papers are stored under one roof, even if they belong to different departments or authors.

While the projects within a monorepo can vary widely, ranging from applications and libraries to tools and configurations, the defining characteristic is their co-location in a single repository. This contrasts with the traditional approach of multirepos, where each project resides in its own separate repository.

Some of the most prominent tech giants, including Google, Microsoft, Facebook (Meta), and Twitter (X), are known for their massive monorepos. Google’s monorepo, for instance, is rumored to be one of the largest code repositories in existence, boasting tens of thousands of commits daily and exceeding 80 terabytes in size. These examples highlight the scalability and potential of monorepos for even the most complex and demanding software development environments.

It’s important to distinguish monorepos from monolithic architecture. While the terms sound similar, they represent different concepts. Monolithic architecture refers to a self-contained application design where all components are tightly coupled and deployed as a single unit. A monorepo, on the other hand, is a repository strategy that can be used with various architectures, including monolithic and microservices. You can have a monorepo containing a monolithic application, or a monorepo hosting multiple microservices – the repository structure is independent of the application’s architecture.

Monorepo vs. Multirepo: Understanding the Fork in the Road

The alternative to a monorepo is the multirepo approach. In a multirepo setup, each project, component, or service is housed in its own dedicated, version-controlled repository. This is often the natural starting point for new projects – it feels clean, organized, and compartmentalized. Creating a new repository is simple and provides an immediate sense of isolation and independence.

The transition from a multirepo to a monorepo involves consolidating your projects into a single, shared repository. While the initial step might seem as simple as moving files into a new directory and initializing Git, the real work lies in the subsequent refactoring and integration.

Transitioning from a Multirepo to a Monorepo Structure

Visual representation of consolidating multiple repositories into a single monorepo, highlighting the unification of projects under one version control system.

It’s crucial to understand that multirepos are not synonymous with microservices. You can implement microservices with either a monorepo or multirepo strategy. In fact, many organizations successfully combine monorepos with microservices architectures, leveraging the benefits of both approaches. A monorepo can effectively manage numerous microservices, provided you establish a robust CI/CD pipeline tailored for deployment within a monorepo environment.

Why Embrace the Monorepo? Unveiling the Advantages

Choosing between a monorepo and multirepo is a strategic decision that significantly impacts your development workflows and team dynamics. While the choice might initially seem trivial, the implications are far-reaching. Let’s explore the compelling advantages that monorepos offer:

Enhanced Visibility and Collaboration: A monorepo fosters transparency by making everyone’s code accessible to the entire organization. This heightened visibility promotes collaboration and cross-team contributions. A developer working on one project can readily understand and even contribute to other projects, leading to faster problem-solving and innovation. Imagine a scenario where a developer from a different team identifies and fixes a bug in your code that you were unaware of – this level of proactive collaboration is a hallmark of monorepo environments.
Simplified Dependency Management: Sharing dependencies becomes significantly easier within a monorepo. With all modules residing in the same repository, the need for complex package managers is reduced. Dependencies can be managed internally, promoting code reuse and consistency across projects. This streamlined dependency management eliminates the complexities and potential conflicts associated with external package dependencies often encountered in multirepo setups.
Single Source of Truth and Dependency Harmony: A monorepo ensures that there’s only one version of each dependency in use across all projects. This eliminates versioning conflicts and the dreaded “dependency hell” – situations where incompatible dependency versions cause build failures and runtime errors. By enforcing a single version of truth, monorepos promote stability and predictability in your codebase.
Code Consistency and Enforced Standards: Maintaining consistent code quality standards and a unified coding style is much easier when all your codebase is centralized in one place. Monorepos facilitate the implementation of linters, formatters, and other code quality tools that can be applied consistently across all projects. This uniformity improves code readability, maintainability, and reduces the cognitive load for developers working across different parts of the codebase.
Shared Timeline and Proactive Communication: Breaking changes in APIs or shared libraries become immediately apparent in a monorepo. This forces teams to communicate and coordinate proactively when introducing such changes. Everyone is incentivized to stay updated with the latest changes and adapt accordingly, fostering a culture of shared responsibility and continuous integration. This shared timeline minimizes integration headaches and ensures smoother transitions when evolving shared components.
Atomic Commits for Large-Scale Refactoring: Monorepos enable atomic commits, which are crucial for large-scale refactoring efforts. A developer can update multiple packages or projects in a single commit, ensuring that changes are applied consistently and atomically across the entire codebase. This simplifies complex refactoring tasks and reduces the risk of introducing inconsistencies during large-scale code modifications.
Implicit Continuous Integration (CI): Continuous integration is inherently facilitated by a monorepo structure. Since all code is already unified in one repository, integrating changes becomes a natural part of the workflow. Every commit to the monorepo triggers the CI process, ensuring early detection of integration issues and promoting a culture of continuous feedback.
Unified CI/CD and Build Processes: You can leverage the same CI/CD pipeline and build process for every project within the monorepo. This standardization simplifies your DevOps infrastructure, reduces configuration overhead, and promotes consistency in your deployment workflows. A unified CI/CD process streamlines releases and makes it easier to manage deployments across multiple projects.

Navigating the Monorepo Maze: Addressing the Challenges

As monorepos grow in size and complexity, they can present certain challenges that need to be addressed effectively. Version control systems, build systems, and CI/CD pipelines can encounter scaling limitations. These potential drawbacks might lead some organizations to reconsider the multirepo approach. Let’s examine the common challenges associated with monorepos:

Performance Bottlenecks: Large monorepos can strain version control tools and IDEs. Commands like git blame might become slow, IDEs can lag, and overall developer productivity can suffer. Testing the entire repository on every commit can become computationally expensive and time-consuming, impacting development velocity.
Broken Main/Master Branch Impact: A broken main or master branch in a monorepo can disrupt the entire development workflow, affecting every team working within the repository. While this can be seen as a disadvantage, it also serves as a strong incentive to maintain robust testing practices and keep the codebase healthy and stable. The shared impact of a broken master branch can foster a culture of quality and vigilance.
Steeper Learning Curve for Newcomers: Onboarding new developers can be more challenging in a large monorepo, especially if the repository spans numerous tightly coupled projects. Understanding the overall structure, dependencies, and interrelationships within a vast monorepo can take time and effort. Proper documentation, onboarding processes, and mentorship are crucial to mitigate this learning curve.
Large Data Volumes and Repository Size: Monorepos can accumulate substantial volumes of data and commit history over time, leading to large repository sizes. This can impact cloning times, storage requirements, and the overall efficiency of version control operations. Effective repository management practices, such as shallow cloning and Git LFS (Large File Storage), become increasingly important for handling large monorepos.
Ownership and Access Control Complexity: Managing code ownership and access control can be more complex in a monorepo, as traditional version control systems like Git don’t inherently provide directory-level permissions. Implementing mechanisms to enforce ownership and restrict access to specific parts of the monorepo often requires custom tooling or workarounds like CODEOWNERS files.
Noisy Code Reviews and Notifications: In a large monorepo with numerous active projects, code review notifications can become overwhelming. Generic notification settings in platforms like GitHub might not be well-suited for the high volume of pull requests and code reviews in a busy monorepo environment. Customized notification filtering and routing mechanisms can be necessary to manage code review workflows effectively.

It’s important to note that most of these challenges are technical in nature. Organizations that have successfully adopted monorepos have often addressed these issues by investing in tooling, developing integrations, and creating custom solutions tailored to their specific needs.

Beyond Technology: The Cultural Dimension of Monorepos

Choosing a repository strategy is not solely a technical decision; it’s also deeply intertwined with how teams communicate and collaborate within an organization. As Conway’s Law aptly states, organizational structure and communication patterns significantly influence the systems they design:

“Organizations which design systems … are constrained to produce designs which are copies of the communication structures of these organizations.” – Melvin E. Conway

Multirepos, while offering autonomy to individual teams, can inadvertently create silos and hinder cross-team collaboration. They can encourage a narrow focus on individual project boundaries, potentially overlooking the broader organizational context and opportunities for synergy.

Conversely, a monorepo acts as a central hub, a virtual “town square” where developers, engineers, testers, and business analysts converge. Monorepos encourage conversations, break down silos, and foster a sense of shared ownership and collective responsibility. By promoting visibility and interaction, monorepos can facilitate a more collaborative and integrated development culture.

Monorepo Culture in Practice: Real-World Examples

Monorepos are not a new concept. For decades, open-source projects like FreeBSD have successfully utilized monorepos (initially with CVS and later Subversion) for development and package distribution. Many other open-source projects have also embraced monorepos, demonstrating their viability for collaborative software development.

However, the critical question is whether commercial software development can also benefit from monorepo layouts. To gain insights, let’s examine the experiences of several companies that have adopted monorepos:

Segment: From Multirepos to Monorepo and Back to Velocity

Segment, a customer data infrastructure company, initially adopted a mix of microservices and multirepos. As their customer base and the number of data forwarding destinations grew rapidly, managing over 140 repositories with increasingly divergent dependencies became a significant operational burden. Maintaining, testing, and deploying this fragmented ecosystem consumed excessive engineering resources, hindering development velocity.

Segment’s solution was consolidation. They migrated all services and dependencies into a single monorepo. While the transition was challenging, requiring reconciliation of shared libraries and comprehensive testing, the end result was a significant reduction in complexity and improved maintainability. This shift to a monorepo directly translated to increased development velocity and faster innovation.

Airbnb: Scaling Globally with Microservices and Monorepos

Airbnb, facing exponential growth, initially relied on a monolithic Ruby on Rails application (“the monorail”). As the codebase expanded, managing releases and merging changes became increasingly difficult. To address these scaling challenges, Airbnb transitioned to microservices and adopted a two-monorepo strategy – one for the frontend and one for the backend.

These monorepos encompass hundreds of microservices, documentation, infrastructure-as-code (Terraform, Kubernetes), and maintenance tools. Airbnb’s monorepo approach streamlined dependency management, facilitated cross-service changes with atomic commits, and enabled unified tooling and deployment processes, supporting their global scaling efforts.

Uber: A Round Trip – Monorepo, Multirepo, and Back to Monorepo

Uber initially adopted a monorepo approach for their Android client development. However, as the company and codebase grew, they encountered performance issues with IDEs, Git operations, and build times. Broken master branches became frequent, hindering development. To alleviate these problems, Uber temporarily switched to a multirepo strategy.

While multirepos initially improved performance and provided teams with greater autonomy, they also led to increased silos, inconsistent coding styles, and dependency management challenges. Integrating individual components into a unified product became more complex. Eventually, Uber returned to a monorepo approach, investing heavily in tooling, IDE optimizations, merge queues, and differential builds to overcome the challenges of scale and regain the benefits of a unified codebase.

Pinterest: Full Speed Ahead to Monorepo Consolidation

Pinterest is currently undergoing a multi-year migration to consolidate over 1300 repositories into just four monorepos. Their motivation stems from the challenges of code reuse, inconsistent development workflows, and variable build process quality in a multirepo environment.

Pinterest is pairing their monorepo adoption with trunk-based development practices, emphasizing short-lived branches and frequent merges to the main branch. They aim to achieve a consistent development workflow, streamline release processes through automation and standardization, and reduce boilerplate code, enabling developers to focus on core product development.

Investing in the Monorepo Ecosystem: Essential Tooling

A key takeaway from these real-world examples is the critical role of tooling in enabling effective monorepo management. Rethinking build and test processes is paramount. Instead of rebuilding the entire repository on every change, intelligent build systems that understand project structure and dependencies are essential. These systems can optimize build and test times by focusing only on the affected parts of the codebase.

Fortunately, many large tech companies have open-sourced their internal build systems, making them accessible to the wider development community:

Bazel (Google): A powerful, open-source build system designed for speed and correctness, ideal for large monorepos.
Buck (Meta): Another open-source build system, optimized for performance and scalability in monorepo environments.

Monorepos are also gaining increasing traction in the JavaScript ecosystem, as evidenced by projects like:

Lerna: A popular tool for managing JavaScript monorepos, streamlining dependency management and publishing.
Nx: A set of extensible dev tools for monorepos, providing features for building, testing, and deploying JavaScript and TypeScript projects.

Scaling Version Control for Monorepos:

Version control performance is another critical aspect of monorepo management. Tools and techniques for scaling repositories include:

Git Partial Clone and Shallow Clone: Git features that allow for cloning only a subset of the repository history and files, reducing clone times and storage requirements.
GVFS (Git Virtual File System) (Microsoft): A virtualized file system for Git that optimizes performance for extremely large repositories by lazily downloading files as needed.

Best Practices for Monorepo Success

Drawing upon the experiences of companies that have successfully implemented monorepos, we can outline a set of best practices for effective monorepo management:

Establish a Clear Directory Structure: Define a consistent and well-organized directory structure for your monorepo to facilitate code discovery and navigation.
Maintain Branch Hygiene: Adopt branching strategies that promote small, short-lived branches and encourage trunk-based development practices to minimize merge conflicts.
Pin Dependencies Consistently: Use pinned dependencies for all projects within the monorepo to ensure consistent dependency versions and facilitate coordinated dependency upgrades.
Leverage Git Optimizations: Utilize Git features like shallow clone and filter-branch to manage large repository sizes and improve performance.
Adopt a Smart Build System: Invest in a build system like Bazel or Buck to optimize build and test times in your monorepo.
Implement Code Ownership Mechanisms: Utilize CODEOWNERS files or similar mechanisms to define code ownership and manage access control within the monorepo.
Utilize Cloud-Based CI/CD: Leverage cloud CI/CD platforms like Semaphore to handle the scaling and performance demands of testing and deploying applications from a monorepo.

Should You Take the Monorepo Plunge?

The decision to adopt a monorepo is not a one-size-fits-all answer. There’s no universal solution that applies to every organization and project. Some companies might initially choose a monorepo, later switch to multirepos, or even opt for a hybrid approach. It’s important to remember that transitioning from a monorepo to multirepos is generally easier than the reverse migration.

Ultimately, the most crucial factor is to consider your organizational culture and communication patterns. Technology is an enabler, but the way your teams work together and communicate is paramount. Choose the repository strategy that best aligns with your desired work culture and facilitates effective collaboration.

Read next: Learn how to run monorepos at scale in Semaphore CI/CD.