Google's monolithic software repository, which is used by 95% of its software developers worldwide, meets the definition of an ultra-large-scale4 system, providing evidence the single-source repository model can be scaled successfully.
The Google codebase includes approximately one billion files and has a history of approximately 35 million commits spanning Google's entire 18-year existence. The repository contains 86TBa of data, including approximately two billion lines of code in nine million unique source files. The total number of files also includes source files copied into release branches, files that are deleted at the latest revision, configuration files, documentation, and supporting data files.
In 2014, approximately 15 million lines of code were changed in approximately 250,000 files in the Google repository on a weekly basis. The Linux kernel is a prominent example of a large open source software repository containing approximately 15 million lines of code in 40,000 files.
Google's codebase is shared by more than 25,000 Google software developers from dozens of offices in countries around the world. On a typical workday, they commit 16,000 changes to the codebase, and another 24,000 changes are committed by automated systems. Each day the repository serves billions of file read requests, with approximately 800,000 queries per second during peak traffic and an average of approximately 500,000 queries per second each workday. Most of this traffic originates from Google's distributed build-and-test systems.