How to Structure Code Repositories: Multi, Mono, or Organic?
One of the most important decisions when starting a new software project is how to structure the codebase. Should all code live in a single monolithic repository? Should it be split across multiple repositories by service or component? Or perhaps the repository structure should evolve organically over time?
As a software architect, it‘s crucial to understand the trade-offs between these approaches and choose the one that best fits your team and project. The wrong choice can lead to collaboration friction, slow development velocity, and maintenance headaches.
In this article, we‘ll dive deep into the world of code repositories. We‘ll explore the spectrum of options from monorepo to multirepo, understand the pros and cons of each approach, and discuss key factors to consider when making the decision. By the end, you‘ll have a solid framework for thinking critically about repository structure and adapting it to your team‘s needs. Let‘s get started!
Understanding the Spectrum of Repository Structures
Before we can weigh the trade-offs, let‘s clearly define the three main repository structures:
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Monorepo: With a monorepo, all code for an organization or project lives in a single massive repository. There are no repo boundaries between services, libraries, or tools – everything is developed and versioned together in one place. Examples of prominent open-source monorepos include the Babel compiler and React web framework.
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Multirepo: On the other end of the spectrum, we have multirepos where each service, library or component lives in its own dedicated repository. Boundaries between repos are typically drawn along service or modular lines. Multirepos are the most common structure and examples are everywhere – Spring Boot, Django, the list goes on.
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Organic: An organic repository structure evolves over time based on the changing needs of the project. It often starts as a monorepo in the early stages of a project when everything is tightly integrated. As the codebase and team grows, logical sub-projects are split out into their own repositories. Microservices are a good example – an app may begin as a monolith and gradually split out services into dedicated repos.
With these definitions in mind, let‘s look at the advantages and disadvantages of each approach.
Advantages of Monorepos
Simplified Organization
Monorepos provide a single unified view of all code in an organization. There‘s no confusion or debate about where new code should go – everything lives together in one place. This makes it easy to navigate, search, and discover existing code.
Shared Code and Collaboration
Since all code lives together, it‘s easy to share and reuse code across projects without the overhead of publishing packages. Developers can easily make atomic changes across multiple projects and libraries in a single commit. Collaboration is natural since all developers work out of the same repository.
Simplified Dependency Management
Monorepos simplify dependency management by eliminating the need for complex versioning of internal dependencies. Projects automatically pick up changes to their dependencies since everything is developed and versioned together. There are fewer version conflicts to manage.
Atomic Changes
With a monorepo, it‘s easier to make broad atomic changes that touch multiple parts of the codebase. Refactoring, API changes, and cross-cutting concerns can be done in a single commit. This is particularly valuable for frameworks and libraries that have many downstreams within an organization.
Tooling Reuse
Since all code lives in one place, monorepos enable reuse of tooling configuration like linters, formatters, and build scripts. There‘s less duplication of config files and it‘s easy to enforce consistent conventions across the entire organization.
Disadvantages of Monorepos
Repo Size and Scale
The main drawback of monorepos is that they can grow very large over time. As the size and history grows, operations like cloning and fetching changes get slower. Large diffs from mega-commits can be hard to code review. Git doesn‘t scale that well to massive repos.
Access Control Complexity
With all code in one place, granular access control becomes more challenging. Teams may want certain code or history to be private. Controlling visibility and permissions across one massive repo can require complex custom tooling.
CI/CD Overhead
Validation and testing in massive monorepos can be complex and time-consuming. By default the entire repo is built and tested on every change. Determining which projects are impacted by a change and only testing those is difficult. CI/CD pipelines need to be optimized with tools like build partitioning and caching.
Tight Coupling
While simplified dependencies are a benefit, the downside is that projects in a monorepo can become tightly coupled over time. Since it‘s easy to reach into other projects, unnecessary dependencies can accumulate leading to higher coupling and maintenance overhead.
Flexibility Constraints
Monorepos can constrain flexibility in terms of project-specific tooling, release cadence, and programming languages. Since everything lives together, there can be pressure to standardize on lowest common denominator tooling and conventions rather than using the best tool for each job.
Advantages of Multirepos
Repository Isolation
Multirepos provide the strongest boundaries and isolation between projects. Each project has its own repository with full control over tooling, dependencies, and release cadence. Teams have autonomy to use the best tools for the job without impacting the rest of the org.
Least Privilege Access
With multirepos, it‘s easier to define granular access permissions for each project. Sensitive projects can be locked down without having to resort to complex path-based access control within a monorepo. Giving teams least-privilege access to only the repos they need is more manageable.
Focused CI/CD
Each repository has its own focused CI/CD pipeline tailored to its specific needs. Builds and tests run only when the code for that repository changes. Pipelines are simpler and faster since they don‘t have to account for the entire organization‘s codebase.
Gradual Adoption and Migration
Multirepos make it easier to incrementally adopt new technologies, tools, and languages. Teams can experiment in isolated repositories without forcing org-wide changes. Migrating between frameworks, build tools, and module structures is more straightforward.
Disadvantages of Multirepos
Dependency Management Complexity
The biggest challenge with multirepos is managing dependencies between them. Pining library versions, publishing changes, and updating downstreams can be a painful process. Diamond dependency problems are more prevalent. Tools like Lerna and Rush can help somewhat but add additional complexity.
Cross-Repository Changes
Making changes that span repositories is more difficult with multirepos. Coordinating releases, tracking issues, and managing PRs across repos creates overhead. Refactoring APIs that span repositories is more complex and time-consuming.
Code Duplication
Without easy code sharing mechanisms, multirepos tend to accumulate duplicated utility code, scripts, and configuration. There‘s a greater tendency to reinvent wheels across repos due to lower visibility. More discipline is required to identify and extract shared libraries.
Discoverability
As the number of repositories grows, it can become difficult to know what code exists and where to find it. Discoverable documentation, consistent naming conventions, and good search tooling is a must. Some companies address this with service catalogs and developer portals for discovering projects.
Organic Repositories: A Balanced Approach
For many projects, the right approach to repo structure is somewhere between the extremes of monorepo and multirepo. An organic structure evolves over time to balance the trade-offs and adapt to the project‘s needs. Here are some strategies for taking an organic approach:
Start with a Monorepo
In the early stages of a project when everything is changing rapidly and code is tightly integrated, starting with a monorepo is often the right choice. Optimize for collaboration, code reuse, and fast iteration. Avoid premature splitting of the codebase.
Split Out Repos as Needed
As distinct components, services, and libraries emerge, consider splitting them out into dedicated repositories. Good candidates for splitting include:
- Microservices: Services exposed via an API that can be developed and deployed independently
- Standalone Libraries: Reusable libraries that have a well-defined API consumed by multiple projects
- High-churn Modules: Modules that change frequently and have distinct testing/release needs from the rest of the codebase
- Experimental Projects: Exploratory projects that are higher-risk and change rapidly
When deciding to split something into a separate repo, carefully weigh the benefits against the overhead of separate CI/CD pipelines, versioning, and cross-repo changes. Resist the urge to prematurely split – wait until module boundaries and APIs are relatively stable.
Use Virtual Monorepos
For very large codebases that are impractical to develop in a single Git repository, consider using a virtual monorepo. This involves splitting the codebase across multiple "physical" Git repositories while still treating it as a cohesive monorepo from a tooling and workflow standpoint.
Tools like Git Subtree and Git Submodules can be used to compose multiple repositories into a unified directory structure. Other tools like Bazel and Pants provide a unified build system across multiple repos.
Virtual monorepos can be a good compromise when you need the benefits of a monorepo but are hitting scaling limits of a single massive repository. They do introduce additional tooling complexity to setup and maintain over time.
Choosing the Right Structure for Your Project
As you can see, there is no one-size-fits-all answer to repository structure. The right choice depends on your specific project needs and constraints. Here are some key factors to consider:
- Team size and structure: Multirepos tend to work better for larger teams with distinct responsibilities and ownership areas. Monorepos are often simpler for smaller teams.
- Service architecture: If you have a microservices architecture with many independent services, multirepos can provide good boundaries. More monolithic architectures are a better fit for monorepos.
- Release cadence: If different parts of your codebase have different release cycles and versioning needs, multirepos can provide the right isolation. More tightly coordinated releases are easier to manage in a monorepo.
- Language and tooling homogeneity: Monorepos work well when all projects use a similar language, framework, and tools. Multirepos provide flexibility to use different stacks across projects.
- Dependency structure: Monorepos handle tightly interdependent projects with many cross-cutting changes well. Multirepos are a better fit when projects are more loosely coupled.
- Security and compliance needs: Multirepos may be needed to meet strict security and compliance requirements around code access and isolation.
Take time to understand your project‘s needs and constraints across these dimensions. Discuss the trade-offs with your team. Don‘t be afraid to evolve your repo structure over time as needs change – that‘s the beauty of an organic approach. The key is to be intentional about your structure and adapt it as you learn.
Conclusion
Choosing the right repository structure is a critical decision that can have a big impact on your team‘s velocity and happiness. By understanding the spectrum of options and trade-offs, you can pick the approach that best fits your project‘s needs.
Monorepos optimize for code reuse, atomic changes, and simplified dependency management. Multirepos provide strong boundaries, least privilege access, and flexibility to use different tools. Organic repos evolve over time to balance the trade-offs.
Remember, the decision isn‘t set in stone! Take an iterative approach to finding the right structure for your team. Start with a monorepo and split out projects if needed. Use virtual monorepos for very large codebases. Adapt your structure as you learn.
The most important thing is to be intentional about your repo structure and continuously evolve it in response to pain points and friction. With the right structure in place, your team can collaborate effectively and focus on rapidly delivering value to your customers.
What repo structure has worked well for your team? Let me know in the comments!