• Buck2, our new open source, large-scale build system, is now available on GitHub.
• Buck2 is an extensible and performant build system written in Rust and designed to make your build experience faster and more efficient. 
• In our internal tests at Meta, we observed that Buck2 completed builds 2x as fast as Buck1.

Buck2, Meta’s open source large-scale build system, is now publicly available via the Buck2 website and the Buck2 GitHub repository. While it shares some commonalities with other build systems (like Buck1 and Bazel), Buck2 is a from-scratch rewrite. Buck2 features a complete separation of the core and language-specific rules, with increased parallelism, integration with remote execution and virtual file systems, and a redesigned console output. All of these changes are aimed at helping engineers and developers spend less time waiting, and more time iterating on their code.

Thousands of developers at Meta are already using Buck2 and performing millions of builds per day, with builds completing twice as fast as with Buck1. Our own internal analysis has shown that engineers were able to produce meaningfully more code when their builds were executed by Buck2, and we hope the wider industry will also see benefits.

Why rebuild Buck?

Build systems stand between a programmer and running their code, so anything we can do to make the experience quicker or more productive directly impacts how effective a developer can be. The goal of Buck2 was to keep what we liked about Buck1 (the core concepts and workflows), draw inspiration from innovations after Buck1 (including Bazel, Adapton, and Shake), and focus on speed and enabling new experiences.

Buck2’s design is based on the following principles:

• The core build system has no knowledge of any language-specific rules. Having the rules separated from the core means that the rules are easier to change and understand. The core of Buck2 is written in Rust, and its language rules (like how to build C++) are written in Starlark. This separation is in contrast to Buck1 (where all rules are written in the core) and Bazel (where C++/Java are written in the core).
• The build system is powered by a single incremental dependency graph, avoiding any phases (in contrast to Buck1 or Bazel). This decision eliminates many types of bugs and increases parallelism.
• The rules API is designed to contain advanced features for performance, along with dynamic (or monadic) dependency features for expressibility. At the same time, these features are carefully restricted to ensure other properties (for example, fast queries or hermeticity) are not harmed.
• The open source release is almost identical to our internal version. The only pieces swapped out are the toolchains (which point at the internal copies of our compilers) and remote execution (which points at our internal servers) — both have open source alternatives supplied. We are also releasing all the rules exactly as they are used internally. Furthermore, we have separated some of the logical components into separate crates (e.g. Starlark, Superconsole, Allocative, Gazebo) so that they can be used outside Buck2.
• Buck2 is written to integrate with remote execution, with the ability to run actions on remote machines. We use the same API as Bazel, and have been testing remote execution with Buildbarn and EngFlow. While not required (and not really expected for people starting out with the open source version), we are able to efficiently compute recursive digests and send them to remote exec