Why Google stores billions of lines of code in a single repository

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Presented in Communications of the ACM 2016.

Authors: Rachel Potvin, Josh Levenberg (Google)

Understanding the paper

TL;DR

  • Google chose to stick with the central repository due to its advantages.

  • The monolithic model of source code management is not for everyone, e.g., organizations where large parts of the codebase are private or hidden between groups.

Key systems

  • Piper: The distributed source-code repository

    • Implemented on top of standard Google infrastructure (originally Bigtable, now Spanner)

    • Reply on the Paxos algorithm to guarantee consistency across replicas

  • CitC (Clients in the Cloud): The workspace client

    • With a cloud-based storage backend and a Linux-only FUSE13 file system

  • Critique: The code-review tool

  • Tricorder: Static analysis system

    • Code quality, test coverage, and test results

  • Rosie: large-scale cleanups and code changes

    1. Create a large patch; find-and-replace

    2. Split the large patch into smaller patches; test them independently; send for code review; commit them automatically once they pass tests and a code review

Statistics

  • Google’s monolithic software repository is used by 95% of its software developers worldwide.

  • The Google codebase includes

    • approximately 1 billion files

    • a history of 35 million commits

    • 86TB of data (excluding release branches)

  • Over 99% of files stored in Piper are visible to all full-time Google engineers.

  • Over 80% of Piper users today use CitC.

Advantages of a monolithic codebase

  • Unified versioning → a single source of truth

  • Code sharing and reuse

  • Simplified dependency management

    • Avoid diamond dependency problem

  • Atomic changes

  • Large-scale refactoring

  • Collaboration across teams

  • Flexible team boundaries and code ownership

  • Code visibility and clear tree structure → implicit team namespacing

Costs and trade-offs

  • Tooling investments for both development and execution

    • Code-indexing system

    • Automated test infrastructure

    • Build infrastructure

    • Code search and browsing tools

  • Codebase complexity

    • Unnecessary dependencies → binary size bloating

  • Efforts invested in code health

Alternatives

  • Git (distributed version control systems)

    • A team at Google is focused on supporting Git, which is used by Google’s Android and Chrome teams outside the main Google repository.

    • Important for these teams due to external partner and open source collaborations.

    • The Git community strongly suggests and prefers developers have more and smaller repositories.

      • Git-clone will copy all content to one’s local machine.

  • Mercurial

    • An experimental effort

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