Targets

Targets in Bazel are a central concept that defines what the build system should produce. They are the actionable units within a Bazel build process.

Definition and Types

• What is a Target? In Bazel, a target is a named entity that represents a piece of output that Bazel can build, such as a binary, a library, or a test. Targets are defined in BUILD files within packages.
• Types of Targets: There are primarily two kinds of targets in Bazel:
  • File Targets: These include source files (like .java or .cc files) and generated files (files produced by a build rule).
  • Rule Targets: These are created by build rules in BUILD files and can represent various kinds of outputs, like executables, libraries, or test suites.

Rule Targets

• Build Rules: Rule targets are created by build rules, which specify how to produce outputs from inputs. Each rule target is an instance of a rule class, like cc_binary for C++ binaries or java_library for Java libraries.
• Attributes: Rules have attributes that define their inputs (like source files), dependencies, and parameters (like compiler flags).
• Dependencies: Rule targets can depend on other targets, forming a dependency graph that Bazel uses to determine the build order.

File Targets

• Source Files: Directly referenced files in the repository are considered file targets. They are the starting point of any build dependency graph.
• Generated Files: Files that are produced by other targets during the build process. They can be intermediate files like object files in C++ or final artifacts like a packaged JAR.

Target Labels

• Naming and Referencing: Targets are uniquely identified by labels, which are used to reference them in BUILD files and Bazel commands. A label has a specific syntax, typically //path/to/package:target_name, which uniquely identifies a target in the workspace.
• Purpose: Labels allow Bazel to precisely determine which parts of the codebase to build, test, or run, based on the dependencies and requirements specified in the BUILD files.

Importance in the Build Process

• Dependency Graph: Bazel uses targets to create a dependency graph. This graph helps Bazel understand the order in which targets should be built and which targets need to be rebuilt when changes occur.
• Incremental Builds: The target-based structure allows Bazel to perform incremental builds efficiently, rebuilding only the targets affected by changes, rather than the entire codebase.

Targets and Scalability

• Modularity: Targets contribute to the modularity of the build process. They allow a large project to be broken down into smaller, manageable units.
• Parallelization: Bazel can build independent targets in parallel, leveraging multi-core processors to speed up the build process.

Best Practices

• Granularity: The granularity of targets (how large or small they are) can significantly impact build performance. Finely granular targets can lead to more efficient incremental builds but might increase complexity.
• Organization: Organizing targets in a logical and coherent manner within a BUILD file is key to maintaining a scalable and manageable build system.