how bazel builds the project - step by step process

Building a project with Bazel involves several logical steps that ensure efficient, reproducible, and incremental builds. Here’s a step-by-step explanation of how Bazel builds a project:

Step-by-Step Logic of Bazel Build Process

1. Workspace Initialization

• WORKSPACE File: Bazel starts by reading the WORKSPACE file in the root directory of your project. This file defines the workspace and specifies external dependencies, such as other Bazel rules repositories (http_archive, git_repository).

2. Target Definition in BUILD Files

• BUILD Files: Bazel scans the project directory and subdirectories for BUILD files. These files define build targets (py_binary, java_library, etc.) and their dependencies.

3. Dependency Analysis

• Dependency Graph: Bazel analyzes the dependency graph defined in the BUILD files to understand the relationships between targets. It identifies direct and transitive dependencies for each target.

4. Action Graph Construction

• Action Graph: Bazel constructs an action graph based on the dependency graph. Each node in the action graph represents a build action (e.g., compile a file, link binaries, run tests), and edges represent dependencies between actions.

5. Incremental Build Optimization

• Caching: Bazel checks its local cache (output_base) to determine if any previously built artifacts (such as compiled binaries or test results) can be reused. This caching mechanism ensures that unchanged targets are not rebuilt unnecessarily, optimizing build times.

6. Parallel Execution

• Parallelization: Bazel determines the optimal execution order for actions based on their dependencies and available resources (--jobs). It parallelizes build actions across multiple cores or distributed systems, maximizing build performance.

7. Execution Phase

• Execution: Bazel executes build actions defined in the action graph. These actions include compiling source code, running tests, bundling assets, and producing executable binaries or libraries.

8. Artifact Output

• Output Directory: Bazel stores build artifacts (binaries, libraries, test results) in the bazel-bin and bazel-out directories within the workspace. These directories contain the final output of the build process.

9. Reproducibility and Environment Isolation

• Environment Settings: Bazel ensures reproducibility by controlling the build environment (--sandbox) and using hermetic execution. It isolates builds from system-level dependencies and ensures consistent build outputs across different environments.

10. Post-Build Actions

• Post-Processing: After completing the build actions, Bazel may perform post-processing tasks such as generating build reports, updating build status indicators (bazel build --status_command), or cleaning up temporary files.