Bazel: What It Is, How It Works, and Why Developers Need It

In this article, you will understand what Bazel is, what it does, how it does it, and how you can use it.

What is Bazel?

Bazel is a build tool that supports multiple languages and platforms. It is known for handling large codebases and ensuring consistent, fast builds.

What does Bazel do?

1. Defining Build Actions

A fundamental aspect of Bazel is its ability to define precise build actions. Build actions are the individual tasks that must be executed to build software. This can include compiling source code, linking libraries, copying files, running tests, etc. Bazel defines these actions in a highly granular and controlled manner, allowing for precise build process management.

2. Analyzing Dependency Relations

One of Bazel’s key features is its advanced dependency analysis. It efficiently determines the project’s dependency graph, which means Bazel understands how different parts of your project depend on each other. For example, if a source file is changed, Bazel knows which binaries rely on this file and, therefore, need to be rebuilt. This dependency analysis is crucial for incremental builds, ensuring only the necessary parts of the project are rebuilt when changes are made.

3. Building in the Most Efficient Way

Bazel optimizes the build process. It uses advanced caching and parallelization strategies to ensure that builds are as fast as possible. The tool caches previous build outputs and reuses them when possible. If a part of the project has stayed the same, Bazel won’t rebuild it, saving time. Additionally, Bazel executes independent build actions in parallel, using the available CPU cores to speed up the build process.

How does Bazel do it?

Bazel operates through a well-defined structure and process:

Bazel building blocks

Workspace: The workspace is the root directory containing all source files and the configuration of external dependencies. It features a `WORKSPACE.bazel` file, which defines the Main repository and references External repositories needed for the build.

Repository: This is a directory with source code and build instructions. The workspace directory is the Main repository’s root (notated as “@”). Other External repositories are defined in the `WORKSPACE.bazel` file. A repository is a collection of packages.

Package: A subdirectory within a repository that contains build instructions. Each package directory includes a `BUILD.bazel` file, defining various build targets, and encompasses all files in its directory, including subdirectories.

Target: A target is a definition of an action. Most targets are one of two principal kinds: rules or labels.

Rule: A function that processes files as input and output, supporting chaining, where the output of one rule can serve as the input for another.

Label: An identifier for a target.

Why So Many Configurations?

Bazel encourages breaking down monolithic builds into smaller, manageable targets for efficiency.

Critical Concepts in Bazel’s Process

Action Graph: This is a Direct Acyclic Graph (DAG) that represents dependency relations between targets.

Effective Caching: During incremental builds, Bazel reconstructs the Action Graph to identify the minimum targets needing rebuilding.

Parallel Execution: Independent targets can be executed simultaneously.

Stages of the Bazel Build Process

1. Loading Phase: All `BUILD.bazel` files required for the build are loaded and evaluated, instantiating rules and adding them to the graph.

2. Analysis Phase: This phase involves executing the rules’ code and creating actions, transforming the graph generated in the Loading phase into an action graph.

3. Execution Phase: Actions are executed, and the build fails if any required output is missing or a command fails to generate an output.

Syntax of Bazel

Bazel’s build files are written in Starlark, a dialect of Python, making them accessible and familiar to many developers.

In conclusion, Bazel stands out as a powerful and versatile build tool, adept at managing complex software projects. Its ability to handle large codebases efficiently and its support for multiple languages and platforms make it an indispensable tool for modern developers. As you embark on your journey with Bazel, remember that mastering its functionalities will significantly streamline your build processes, ultimately leading to more robust and reliable software development.