Packages

Packages in Bazel are a central aspect of its build philosophy. They provide a structured way to organize code, manage dependencies, and define build processes, all of which are crucial for efficient and scalable software development.

In Bazel, a package is a fundamental unit of code organization and a key concept in its build process.

Definition and Structure

• What is a Package? In Bazel, a package is defined by the presence of a BUILD or BUILD.bazel file in a directory. This directory, along with its subdirectories (excluding those which themselves contain a BUILD file), forms a package.
• Content: A package includes all files in its directory and its subdirectories, except those subdirectories which are packages themselves. This way, no file or directory can be a part of two different packages.

Role in the Bazel Build System

• Building Block: Packages are the basic building blocks in Bazel for organizing software and specifying build instructions. They represent a coherent unit of code and build configuration.
• Targets: Each package contains one or more targets, which are the actual elements Bazel builds or tests. These targets are defined in the package’s BUILD file.

Characteristics of Bazel Packages

• Hierarchical Nature: Packages in Bazel follow a hierarchical structure based on their directory layout. This hierarchy helps in managing and structuring large codebases.
• Isolation: Each package is isolated in terms of its build configuration. The BUILD file within a package defines how the targets in that package are built, independent of other packages.

Dependency Management

• Internal Dependencies: Within a package, targets can depend on each other, forming a dependency graph that Bazel uses to determine build order.
• External Dependencies: Packages can also depend on targets in other packages. These dependencies are explicitly stated, allowing Bazel to handle complex dependency trees across a large codebase.

Benefits of Using Packages

• Modularity: Packages allow for a modular approach to organizing code and build configurations. This modularity makes managing large projects more manageable.
• Incremental Builds: With clearly defined packages, Bazel can perform efficient incremental builds. It builds or tests only the packages affected by changes, reducing build times.
• Scalability: The package structure supports scalability in project size. As a project grows, new packages can be added without disrupting the existing structure.

Considerations in Package Design

• Granularity: The choice of how granular a package should be (how much code it should contain) can affect build performance and manageability. Finer granularity provides more precise control but can lead to a proliferation of BUILD files, while coarser granularity can reduce the number of BUILD files but may lead to less precise builds.
• Common Practices: In practice, a package often corresponds to a logical component of a software project, like a library, a service, or a group of related functionalities.