BUILD files in Bazel are not just configuration files; they are the core descriptors that dictate how Bazel BUILDs, tests, and manages software. Their detailed and explicit nature allows for precise control over the BUILD process, supporting scalability, efficiency, and reproducibility.
BUILD files are a fundamental component of Bazel’s BUILD process. They serve as the blueprint for how Bazel constructs and manages software BUILDs.
Purpose and Functionality
What They Are: BUILD files are plain text files placed in directories throughout a Bazel workspace. Each file is associated with a single directory, and together, they define how Bazel BUILDs software outputs from the source files in that directory.
Role in Defining Targets: BUILD files define targets, which are named BUILD outputs like executables, libraries, or packages. These targets are the actionable units that Bazel BUILDs and tests.
Structure and Syntax
Language: BUILD files are written in a language called Starlark, which is a Python-like language designed for specifying BUILD and test actions.
Rules: A BUILD file contains one or more rules, where each rule describes a BUILD or test action.
Rules can be thought of as functions that declare how to produce outputs from inputs.
For example, a cc_binary rule specifies how to compile a C++ binary.
Types of Targets
Explicit Targets: These are directly named in a BUILD file. They include source files, rule instances, and sometimes generated files.
Implicit Targets: Generated by Bazel itself based on the rules and file types. For example, a C++ rule might implicitly create object file targets for each source file.
Components of a Rule
Attributes: Rules have attributes that specify inputs (like source files), dependencies (other targets), and parameters (like compiler flags).
Rule Classes: Predefined rule types in Bazel, like cc_binary for C++ binaries, java_library for Java libraries, etc. Each rule class comes with its own set of expected attributes and behaviors.
Dependency Management
Declaring Dependencies: BUILD files are also where dependencies between targets are declared. This helps Bazel determine the order of BUILDs and identify which parts of the project need to be rebuilt when changes occur.
Fine-Grained Dependencies: Because dependencies are defined at the target level in BUILD files, Bazel can perform fine-grained incremental BUILDs, only reBUILDing what is necessary.
BUILD Files and Large Projects
Scalability: BUILD files enable the scalable organization of large codebases. By breaking down a project into smaller modules (each with its own BUILD file), Bazel can manage complex dependencies and large-scale BUILDs efficiently.
Clarity and Maintenance: Having clear, well-structured BUILD files makes maintaining and understanding large codebases easier, as the BUILD specifications are modular and localized.
Impact on the BUILD Process
Incremental BUILDs: Bazel's ability to perform incremental BUILDs is heavily reliant on the information in BUILD files. It uses these files to create a dependency graph, which it then uses to determine what needs to be rebuilt.
Reproducibility: The explicit nature of BUILD files helps ensure BUILD reproducibility, as they declare exactly what is required for each part of the BUILD.