There are a number of potential advantages to a monorepo over individual repositories:
Ease of code reuse - Similar functionality or communication protocols can be abstracted into shared libraries and directly included by projects, without the need of a dependency package manager.
Simplified dependency management - In a multiple repository environment where multiple projects depend on a third-party dependency, that dependency might be downloaded or built multiple times. In a monorepo the build can be easily optimized, as referenced dependencies all exist in the same codebase.
Atomic commits - When projects that work together are contained in separate repositories, releases need to sync which versions of one project work with the other. And in large enough projects, managing compatible versions between dependencies can become dependency hell. In a monorepo this problem can be negated, since developers may change multiple projects atomically.
Large-scale code refactoring - Since developers have access to the entire project, refactors can ensure that every piece of the project continues to function after a refactor.
Collaboration across teams - In a monorepo that uses source dependencies, teams can improve projects being worked on by other teams. This leads to flexible code ownership.
Limitations and disadvantages
Loss of version information - Although not required, some monorepo builds use one version number across all projects in the repository. This leads to a loss of per-project semantic versioning.
Lack of per-project security - With split repositories, access to a repository can be granted based upon need. A monorepo allows read access to all software in the project, possibly presenting new security issues.
More storage needed - With split repositories, you can fetch only the project you are interested in. With a monorepo, you might need to fetch all projects. Note that this depends on the versioning system. This is not an issue if you use e.g. SVN in which you can download any part of the repo.
Scalability challenges
Companies with large projects have come across hurdles with monorepos, specifically concerning build tools and version control systems. Google's monorepo, speculated to be the largest in the world, meets the classification of an ultra-large-scale system and must handle tens of thousands of contributions every day in a repository over 80 terabytes large.
Companies using or switching to existing version control software found that software could not efficiently handle the amount of data required for a large monorepo. Facebook and Microsoft chose to contribute to or fork existing version control software Mercurial and Git respectively, while Google eventually created their own version control system. For more than ten years, Google had relied on Perforce hosted on a single machine. In 2005 Google's build servers could get locked up to 10 minutes at a time. Google improved this to 30 seconds - 1 minute in 2010. Due to scaling issues, Google eventually developed its own in-house distributed version control system dubbed Piper. Facebook ran into performance issues with the version control system Mercurial and made upstream contributions to the client, and in January 2014 made it faster than a competing solution in Git. In March 2014 Microsoft announced that it switched over to using Git for its monorepo. In the transition, Microsoft made substantial upstream contributions to the Git client to remove unnecessary file access and improve handling of large files with Virtual File System for Git.
Scaling build software
Few build tools work well in a monorepo, and flows where builds and continuous integration testing of the entire repository are performed upon check-in will cause performance problems. Directed graph builds systems like Buck, Bazel, Pants and Please solve this by compartmentalizing builds and tests to the active area of development. Twitter began development of Pants in 2011, as both Facebook's Buck and Google's Bazel were closed-source at the time. Twitter open-sourced Pants in 2012 under the Apache 2.0 License. Please, a Go-based build system, was developed in 2016 by Thought Machine who was also inspired by Google's Bazel and dissatisfied with Facebook's Buck. Bazel, Buck, Pants and Please, all use the same build language, a domain-specific language based on Python. Some specialized monorepo build tools such as Lerna, solve fetching of duplicate dependencies, but lack any directed graph capabilities.
