Dependencies

Dependencies in Bazel are a crucial aspect of its build system, playing a central role in how Bazel determines the sequence and scope of the build process.

Definition and Importance

• What Are Dependencies? In Bazel, dependencies refer to the relationships between different targets (like libraries, binaries, tests) defined in BUILD files. A target's dependencies are other targets that must be built or are required before the target itself can be built.
• Role in Build Process: Dependencies dictate the order in which targets are built and ensure that all necessary components are available for each step of the build. They are fundamental in creating a dependency graph, which Bazel uses to manage the build process.

Types of Dependencies

• Source Dependencies: These are dependencies on source files, which are the basic inputs for most build targets.
• Target Dependencies: Dependencies on other build targets, often expressed in BUILD files. For example, a C++ binary might depend on several C++ library targets.
• Tool Dependencies: Dependencies on specific tools or compilers needed to build a target.
• External Dependencies: Dependencies on third-party libraries or resources that are not part of the primary codebase. These are often declared in the workspace file.

Dependency Graph

• Directed Acyclic Graph (DAG): Bazel constructs a DAG of all the targets and their dependencies. This graph is used to determine the order of build actions and to identify which parts of the codebase need to be rebuilt when changes occur.
• Efficient Builds: The dependency graph enables Bazel to perform incremental builds, rebuilding only the targets affected by a change, rather than the entire project.

Declaring Dependencies

• In BUILD Files: Dependencies are usually declared in BUILD files using attributes of build rules. For example, the deps attribute in a java_library rule specifies the Java libraries it depends on.
• Syntax: Dependencies are specified using target labels, which uniquely identify a target within the workspace.

Resolving Dependencies

• Automatic Resolution: Bazel automatically resolves the dependencies specified in BUILD files. It ensures that all dependencies are built in the correct order.
• Managing Complex Dependencies: Bazel is particularly adept at managing complex dependency trees, making it suitable for large-scale projects with intricate interdependencies.

Challenges and Best Practices

• Circular Dependencies: Bazel does not allow circular dependencies, as they can lead to build conflicts and are generally considered a bad practice.
• Granularity: The level of granularity in declaring dependencies (how detailed they are) can affect build performance. Overly granular dependencies might lead to unnecessarily complex builds, while too coarse dependencies can hinder incremental building efficiency.
• External Dependencies: Managing external dependencies, especially from various sources, requires careful organization, often facilitated by additional Bazel tools or rules like http_archive or git_repository.

External Dependency Management

• Workspace-Level Dependencies: Dependencies on external repositories are declared in the WORKSPACE file. Bazel fetches these dependencies from their respective sources and makes them available to the build.
• Version Management: Handling versioning of external dependencies is critical to ensure consistency and reproducibility of builds.