Rules

Bazel rules and rule classes are foundational elements in Bazel's build process, dictating how specific types of targets are built, tested, or otherwise processed.

Rules in Bazel

• Definition: A rule in a Bazel BUILD file is an instance of a rule class. It specifies how to generate one or more outputs from a set of inputs. Rules define actions that Bazel takes to build software, such as compiling source code or linking a library.
• Components: A rule consists of attributes that describe its inputs (like source files), outputs, and other necessary parameters. These attributes are used to determine what the rule does and how it interacts with other parts of the build process.

Rule Classes

• Definition: Rule classes are templates for creating rules. They define a specific type of build action, like compiling a binary or creating a library.
• Predefined Rule Classes: Bazel comes with a set of predefined rule classes for common tasks and programming languages. For example, cc_binary for compiling a C++ binary, java_library for building a Java library, or py_test for running a Python test suite.
• Custom Rule Classes: Users can define custom rule classes using Bazel's build language, Starlark. This allows for the creation of specialized build actions tailored to specific needs or non-standard build processes.

How Rules Work

• Attribute Specification: When you declare a rule in a BUILD file, you specify its attributes, which can include names of source files, dependencies on other targets, and configuration parameters.
• Dependency Handling: Rules can specify dependencies on other targets. Bazel uses these dependencies to create a dependency graph, determining the order in which targets should be built.
• Action Execution: Each rule translates into a set of actions that Bazel executes to produce the target outputs. These actions involve tools like compilers, linkers, or custom scripts.

The Role of Rules in the Build Process

• Build Instructions: Rules provide the instructions that Bazel needs to build software. They are the primary mechanism through which Bazel understands what needs to be done with the source code.
• Scalability and Modularity: By organizing build processes into discrete rules, Bazel can handle complex builds in a scalable and modular way. Each rule focuses on a specific task, making the build process more manageable and easier to understand.

Advantages of Using Rules and Rule Classes

• Flexibility: Rules and rule classes offer flexibility in defining custom build and test processes. This flexibility is particularly valuable in complex builds that involve multiple languages or require custom build steps.
• Reusability: Custom rule classes can be reused across different projects. Once a rule class is defined, it can be applied to any number of targets within a project or shared across multiple projects.
• Incremental Builds: Rules are essential for Bazel's ability to perform incremental builds. Bazel tracks the inputs and outputs of each rule, rebuilding targets only when their inputs have changed.

Challenges and Best Practices

• Complexity: Writing custom rules can be complex and requires a good understanding of the build process and the Starlark language.
• Best Practices: It's important to balance the granularity of rules (not too broad nor too fine) and to maintain clarity in their definition. This ensures that the build process remains efficient and manageable.