There is a trend that many companies and open-source projects are moving from multi-repo into monorepo. Monorepo is not new, Google and Facebook have been using it for years. So why mass adoption now? I believe it depends a lot on the maturity of build systems and tools.
Monorepo is a software development strategy where code for many projects is stored in the same repository, the repository is large in content size and number of files, commits, or branches.
You literally can throw any kinds of projects into one monorepo, loosely connected or tightly coupled, one or multiple languages, one or multiple dependency management tools.
Monorepo has many benefits like single source of truth (unified versions,simplified dependency managers), large-scale refactoring (atomic changes), better collaboration (less boundaries, wide code visibility, clear tree structure, flexible ownership), and fluid continuous integration and delivery.
However monorepo does increase the codebase complexity, build time, and effort invested in both code health and tooling.
These tools are great for enterprise-level projects but seem a bit overkill for individuals or small teams, following are main characteristics:
Google is famous for its enormous monorepo, early Google employees decided to work with a shared codebase managed through a centralized source control system. This approach has served Google well for more than 16 years, and today the vast majority of Google’s software assets continues to be stored in a single, shared repository, managed by Bazel build system.
Facebook developed Buck and improved Mercurial to suit its main source repository is enormous—many times larger than even the Linux kernel with thousands of commits a week across hundreds of thousands of files.
Twitter uses a custom build of Git which includes patches enhancing it to better deal with very large repositories in the range of multiple gigabytes.
Notable package managers like Yarn, Npm, or Pnpm have built-in feature called workspaces aim to make working with monorepo easy. At minimum, you’ll be able to keep multiple related packages all together in a single repository, and test changes in an integrated environment, without continually re-linking.
Lerna operates in a higher level than above package managers’ workspaces but not close to a build system when it has no support for build languages, caching, action directed graphs, or incremental builds.
Rush helps to ensure that installs and builds are completely deterministic. If you define custom commands or options, they are strictly validated and documented as part of Rush’s command-line help.
Nx is a set of extensible dev tools for monorepo, has first-class support for many frontend and backend technologies like Angular, React, and Node.
Monorepo is a software development strategy where code for many projects is stored in the same repository. Keep in mind monolithic source control does not necessarily result in monolithic software. And microservices can play well with monorepo.
Working with monorepo gets complicated over time when its size grows. Use monorepos to increase velocity during early stages of product development.