Bazel

Bazel is an open-source build and test tool developed by Google to support large codebases with fast, reliable builds. It emphasizes hermeticity and reproducibility by isolating build inputs and using a dependency graph to determine build order. Bazel is designed to work across multiple languages and platforms, aiming to provide consistent results in local development and continuous integration environments.

Key concepts in Bazel include BUILD files that describe build targets, a WORKSPACE file that defines external dependencies, and labels used to reference targets (for example //path:target). Build rules define how to transform inputs into outputs, and users can extend Bazel with new rules written in the Starlark language (a Python-like language). Bazel can handle multi-language projects and supports custom toolchains to build for various platforms.

Bazel offers several features intended to speed up development and improve reliability. It provides extensive caching (local and remote) to avoid redundant work, supports incremental builds, and can perform remote execution or caching to scale builds across machines. It includes testing support, provides build and test workflows, and exposes a query tool for inspecting dependencies and configurations. The rule ecosystem allows developers to add language support and domain-specific workflows.

Bazel runs on major desktop platforms (Linux, macOS, Windows) and can build software for Android, iOS, and other targets via toolchains. It is used within Google and in a number of open-source projects, including TensorFlow and Angular, and is distributed under the Apache License 2.0. Bazel aims to provide fast, reproducible builds for large, multi-language codebases.