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Source Control Systems

7 posts · May 2013 – Oct 2025

What this trail covers

Meta's source control systems have evolved with the development and open-sourcing of Sapling, a Git-compatible client designed for extreme scalability and user-friendliness. This evolution includes the adoption of Rust for critical components like the Mononoke rewrite of Mercurial, demonstrating Rust's reliability and performance benefits for large-scale systems. The company has also established dedicated teams to support Rust development, improve interoperability with C++, and contribute to the broader open source community. Recent advancements include the introduction of directory branching, which addresses the challenges of managing multiple versions of code within a large monorepo by providing a scalable and developer-friendly alternative to full-repo branching. This new workflow allows for merging changes at the directory level while maintaining a linear commit graph at the repository level, resolving performance issues and enabling more flexible development workflows. Future work includes better integration of Git repositories into the Sapling monorepo through lightweight migration mechanisms.

All Meta trails
  1. Branching in a Sapling MonorepoThis post details the design and implementation of directory branching within Sapling, Meta's source control system for its monorepo. It introduces two key workflows: non-mergeable full-repo branching and mergeable directory branching. The core technical contribution is the 'directory branching' solution, implemented using `sl subtree copy`, `sl subtree graft`, and `sl subtree merge` commands. This approach allows for branching and merging at the directory level while appearing as linear commits in the monorepo's commit graph, thus avoiding performance issues associated with wide, non-linear commit graphs. The post also discusses integration with the Buck2 build system and user feedback on adoption for CI cost, experimental changes, and Git migration scenarios. Future work includes integrating external Git repositories via soft links.
  2. Indexing code at scale with GleanThis post introduces Glean, Meta's open-source code indexing system, detailing its design rationale and usage for developer tooling. Glean collects and derives facts about source code, storing them in RocksDB and querying them via the Angle language. It supports language-agnostic data storage and querying, enabling features like code navigation, search, and documentation generation. A key innovation is incremental indexing, achieved by stacking immutable databases to achieve O(changes) or O(fanout) indexing costs. The post also highlights how Glean powers code navigation through the Glass symbol server and contributes to IDE performance.
  3. DotSlash: Simplified executable deploymentThis post introduces DotSlash, a tool that simplifies the deployment of large executables by replacing them with lightweight scripts. DotSlash handles fetching, decompressing, and verifying platform-specific artifacts, reducing repository size and avoiding I/O-heavy clone operations. It is written in Rust for performance and cross-platform compatibility. The post details the DotSlash file format, its execution workflow, and how DotSlash files are generated and managed within Meta's automated build pipelines, including a complementary GitHub Action for external use. It also highlights how DotSlash can manage multiple executables within a single archive and its use in distributing Node.js and Python scripts.
  4. Meta developer tools: Working at scaleThis post introduces Sapling, a new scalable version control system implemented primarily in Rust, featuring a server, client, and virtual file system (EdenFS). It also details Buck2, a large-scale build system with remote caching and execution capabilities supporting multiple languages, and its integration with Sapling and Watchman. The post also highlights Infer, RacerD, and Jest as tools for static analysis and testing, with Sapienz for automated mobile app testing.
  5. Sapling: Source control that’s user-friendly and scalableThis post introduces Sapling, a new Git-compatible source control client developed at Meta, emphasizing usability and scalability for large repositories. It details Sapling's design principles, user experience features like 'smartlog' and easy mistake recovery, and its approach to handling commit stacks. The post also previews future server-side scaling features and introduces ReviewStack, a code review UI for stacked commits.
  7. Programming languages endorsed for server-side use at MetaThis post announces Rust as a new supported server-side language at Meta, alongside Hack, C++, and Python. It details the criteria for a 'supported' language (IDE support, debugging, build, deployment, core libraries, interoperability, long-term commitment) versus 'community supported'. The post explains the significant engineering investment required to support a language at Meta's scale, covering core libraries, security, privacy, operational risk, expertise, and developer experience. It highlights Rust's increasing footprint and its recommendation for performance-sensitive back-end services and CLI tools. The post also references a previous journey with Rust, indicating ongoing adoption and commitment.
  8. A brief history of Rust at FacebookThis post details the history and adoption of Rust at Facebook, focusing on its use in the Mononoke project for source control management, its broader adoption in developer tooling and backend services, and the establishment of dedicated Rust developer experience and programming language teams. It highlights the technical reasons for choosing Rust (safety, performance) and its integration with C++.
  9. Announcing Yarn 1.0This post announces the 1.0 release of Yarn, a JavaScript package manager, highlighting its adoption at Meta and across the industry. Key new features include Yarn Workspaces for monorepos, auto-merging of lockfiles to resolve merge conflicts, and selective version resolutions for managing transient dependencies. It also mentions various smaller fixes and improvements, emphasizing community involvement in its development. The post details how these features improve installation times, stability, and developer workflows, particularly for large-scale projects and monorepos.
  10. Dev Tools @Scale 2017 RecapThis post recaps the Dev Tools @Scale 2017 conference, highlighting presentations on scaling development tools. Key contributions include details on Facebook's Project OneWorld for large-scale device testing, Infer for static code analysis, Jupiter for CI job scheduling, and the Git Virtual File System (GVFS) for scaling Git to massive repositories. Other presentations covered Amazon's developer tools, GitHub's repository hosting challenges, Spotify's Android testing, ARM's cross-platform debugging tools, and Google's Bazel build system.
  11. Yarn: A new package manager for JavaScriptThis post introduces Yarn, a new JavaScript package manager developed collaboratively by Meta, Exponent, Google, and Tilde. It addresses issues of consistency, security, and performance encountered with the npm client at scale. Yarn utilizes lockfiles for deterministic dependency installation, a global cache for offline access and faster installs, and a three-step architecture (Resolution, Fetching, Linking) to parallelize operations. It also offers features like license restriction and a stable JS API.
  12. Automatically push commits to GitHub with FBShipItThis post introduces FBShipIt, an open-source tool developed to automate the process of pushing commits from internal Mercurial repositories to GitHub. It addresses challenges such as file movement, removal of confidential information, replacement of internal user names, and translation of Mercurial patches to Git format. The tool evolved from earlier internal scripts ('fboss', 'fboss2') to handle continuous, unsupervised pushes and has been expanded to support over 20 projects, including React Native and HHVM, managing thousands of pushes daily. The post details its Hack library structure, phases of operation (cloning, fetching, copying/filtering commits, pushing), and provides an example of project-specific filtering logic.
  13. Scaling Mercurial at FacebookThis post details the scaling of Mercurial at Facebook by introducing two key extensions: hgwatchman and remotefilelog. hgwatchman integrates with Watchman to monitor file system changes, making status commands over 5x faster than Git's. remotefilelog optimizes clone and pull operations by downloading only commit metadata and fetching file contents on demand via memcache, resulting in 10x faster clones/pulls and reducing server network load by over 10x. These extensions were open-sourced.
  14. Watchman: Faster builds with large source treesIntroduced Watchman, a file change detection service designed to significantly speed up build times for large source trees. Watchman provides a fast and reliable service to query for changed or deleted files since the last query, enabling incremental build processes and reducing interactive-user build times by 60%. It runs on Linux, OS X, FreeBSD, and Solaris and was open-sourced.