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Bazel Quick Start Guide

This is a hands-on guide to get you started with this Bazel tutorial.

Prerequisites

  • Bazel installed (see Installation)
  • Git for cloning the repository

Installation

Bazelisk automatically downloads and uses the correct Bazel version:

macOS: 

brew install bazelisk

Linux: 

npm install -g @bazel/bazelisk
# or
wget https://github.com/bazelbuild/bazelisk/releases/latest/download/bazelisk-linux-amd64 -O /usr/local/bin/bazel
chmod +x /usr/local/bin/bazel

Windows: 

choco install bazelisk

Option 2: Direct Bazel Installation

Follow the official guide: https://bazel.build/install

Verify Installation

bazel --version
# Should show: bazel 6.4.0 or later

Getting Started

1. Clone the Repository

git clone https://github.com/amulya-labs/bazel-tutorial.git
cd bazel-tutorial

2. Build Everything

The simplest command - builds all targets:

bazel build //...

What happens: - Bazel reads MODULE.bazel to resolve dependencies - Downloads Python and Go toolchains - Compiles all services - Caches results for future builds

3. Run the Go Service

bazel run //go_service:server

Visit http://localhost:8080 - you should see: 

Hello from Go! 🚀

Press Ctrl+C to stop.

4. Run the Python Service

bazel run //py_service:server

Visit http://localhost:8081 - you should see: 

{"message": "Hello from Python! 🐍"}

Press Ctrl+C to stop.

5. Run All Tests

bazel test //...

Output: 

//go_service:go_service_test        PASSED in 0.5s
//py_service:py_service_test        PASSED in 1.2s
//tests:test_go_service             PASSED in 0.3s
//tests:test_py_service             PASSED in 0.4s

Common Commands

Build Commands

# Build all targets
bazel build //...

# Build specific target
bazel build //go_service:server

# Build multiple packages
bazel build //go_service/... //py_service/...

Run Commands

# Run a binary target
bazel run //go_service:server

# Run with arguments
bazel run //go_service:server -- --port=9090

Test Commands

# Test all targets
bazel test //...

# Test specific package
bazel test //go_service:go_service_test

# Test with verbose output
bazel test //... --test_output=all

# Test and show only failures
bazel test //... --test_output=errors

Query Commands

# List all targets
bazel query //...

# List targets in a package
bazel query //py_service:all

# Show dependencies
bazel query 'deps(//go_service:server)'

# Find reverse dependencies
bazel query 'rdeps(//..., //py_service:server)'

Clean Commands

# Clean build outputs (keeps cache)
bazel clean

# Clean everything including cache
bazel clean --expunge

Understanding the Output

Build Output

INFO: Analyzed 2 targets (15 packages loaded, 89 targets configured).
INFO: Found 2 targets...
INFO: Elapsed time: 3.456s, Critical Path: 2.123s
  • Analyzed: How many targets Bazel processed
  • Packages loaded: How many BUILD files were read
  • Critical Path: Longest chain of dependent actions

Cached Builds

After the first build: 

INFO: Analyzed 2 targets (0 packages loaded, 0 targets configured).
INFO: Found 2 targets...
INFO: Elapsed time: 0.234s, Critical Path: 0.001s

Everything is cached! ⚡

Incremental Builds Demo

Try this to see Bazel's incremental build in action:

# 1. Build everything
bazel build //...

# 2. Change only Python code
echo "# New comment" >> py_service/main.py

# 3. Build again
bazel build //...
# Only Python targets rebuild - Go is cached!

# 4. Change Go code
echo "// New comment" >> go_service/main.go

# 5. Build again
bazel build //...
# Only Go targets rebuild - Python is cached!

Troubleshooting

"Cannot find MODULE.bazel"

Make sure you're in the repository root: 

cd bazel-tutorial
ls MODULE.bazel  # Should exist

"command not found: bazel"

Bazel isn't installed or not in PATH. See Installation.

"Failed to fetch external repository"

Network issue or typo in MODULE.bazel. Check: 

bazel sync

Build is slow

First build is always slow (downloads dependencies). Subsequent builds are fast due to caching.

Clean slate

Start fresh: 

bazel clean --expunge
bazel build //...

Next Steps

  1. Read the concepts: Check out Bazel Concepts
  2. Modify code: Change main.py and see incremental builds
  3. Add a dependency: Update requirements.txt and rebuild
  4. Create a test: Add a new test file and py_test target
  5. Explore queries: Use bazel query to understand the build graph

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