Bazel vs CMake: What are the differences?

Bazel and CMake are popular build systems used in software development. Here are the key differences between Bazel and CMake:

1. Build Language and Configuration: Bazel uses a custom domain-specific language (DSL) called Starlark for defining build rules, while CMake relies on a script-based configuration language using CMakeLists.txt files. Bazel's declarative approach describes file relationships, dependencies, and targets, while CMake's script-based approach includes commands and directives to define the project's build process.

2. Build System Efficiency: Bazel uses a content-based build approach where each build target is associated with a unique key based on its inputs. Bazel can skip the build process for targets that have not changed, resulting in faster builds. It also supports distributed caching and remote execution for scalable and parallel builds across multiple machines or a distributed build farm. CMake, on the other hand, typically relies on more traditional build systems, and the efficiency of the build process may depend on the underlying build tool (e.g., make, Ninja) used by CMake.

3. Language Support and Ecosystem: Bazel is designed to be language-agnostic and supports various programming languages, including C++, Java, Python, and more. It provides built-in rules and toolchains for different languages, making it easier to set up and configure multi-language projects. Bazel also integrates well with other tools such as TensorFlow and Android, and has a growing community and ecosystem. CMake, while also supporting multiple programming languages, is more commonly associated with C and C++ projects. It has a mature ecosystem with wide adoption, especially in the C++ community, and supports integration with various build tools, libraries, and IDEs.

4. Project Size and Complexity: Bazel is ideal for large-scale projects with complex dependencies and multiple targets, providing fine-grained control and efficient handling of codebases. CMake is versatile, suited for smaller to medium-sized projects, and adaptable to different project sizes and structures with its modular approach and support for external dependencies.

In summary, Bazel and CMake are both powerful build systems. Bazel offers an efficient and language-agnostic build system with advanced caching and remote execution capabilities, while CMake provides a script-based configuration language and a mature ecosystem, particularly for C and C++ projects.