Monorepo
In version control systems, a monorepo ("mono" meaning 'single' and "repo" being short for 'repository') is a software development strategy where code for many projects is stored in the same repository. This software engineering practice dates back to at least the early 2000s, when it was known as a 'shared codebase'. A related concept is the monolith, but while a monolith combines its sub-projects into one large project, a monorepo may contain independent projects. Google, Facebook, Microsoft, Uber, Airbnb, and Twitter all employ very large monorepos with varying strategies to scale build systems and version control software with  a large volume of code and daily changes. Advantages. There are a number of potential advantages to a monorepo over individual repositories: 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. Scaling version control software. 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 May 2017 Microsoft announced that virtually all of its Windows engineers use a Git 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.