Bazel vs CMake

Choosing the right build tool

Choosing the right build system directly affects your team's productivity and project success. Two major options are Bazel and CMake, each taking different approaches to building software. CMake has served as the standard for C++ development with over 2 million monthly downloads, while Bazel emerged from Google's internal infrastructure to challenge traditional build systems with its focus on scalability and reproducibility. This comparison will help you understand when to use each tool and why your choice matters for your software development.

Introduction to Bazel

Bazel represents a new approach to build systems, born from Google's need to manage software development at massive scale. As an open-source version of Google's internal Blaze build tool, Bazel brings enterprise-grade build capabilities to the wider development community.

Overview

What is Bazel

Bazel's story started in 2006 when Google began developing "Blaze," an internal build tool designed to handle the company's massive codebase. After nearly a decade of internal refinement, Google open-sourced this technology as Bazel in March 2015, with the name being an anagram of "Blaze." The tool entered beta in September 2015 and reached its 1.0 milestone in October 2019, signaling production readiness. The recent release of Bazel 8.0 LTS in December 2024 marks another major step forward, introducing Bzlmod as the default dependency management system and modularizing core rulesets.

Hermetic Builds

Hermetic Builds serve as Bazel's cornerstone feature, creating isolated build environments through sandboxing. This means builds don't depend on tools or libraries installed outside the build environment, guaranteeing that identical inputs produce identical outputs across different machines and environments.

Lots of connected dots

Remote Caching and Execution

Remote Caching and Execution lets teams share build artifacts through a distributed caching system. This feature allows developers to avoid rebuilding components that teammates have already built, while remote execution distributes build tasks across worker pools for massive parallelization.

A diagram that shows the Bazel communicating with the bazel-remote service and its multiple backend options.

Multi-Language Support

Multi-Language Support makes Bazel versatile, with built-in rules for Java, C++, Go, Python, Rust, JavaScript, Objective-C, Kotlin, Scala, Android, iOS, and more. The system uses Starlark, a Python-like language, for writing custom rules and extending functionality

Tower and Logos

What are the trade offs?

Let’s take a look at the advantages and disadvantages of choosing Bazel.

Advantages

Here are the advantages of using Bazel.

  1. Reproducible builds: Bazel delivers reproducible builds that produce identical outputs for identical inputs across different environments, solving the "works on my machine" problem. Databricks reports that their builds always produce the same result regardless of build environment state.
  2. Scalability: Bazel handles scalability well, managing massive codebases like Google's projects with over 100,000 source files. Uber's Go monorepo with 900+ active developers shows Bazel's ability to manage one of the largest Go repositories in production.
  3. Speed improvements can be significant: Canva achieved a large decrease in average CI build times and Redfin reduced CI builds from 40-90 minutes to a 5-6 minute average.
  4. Cross-platform support: means the same BUILD files work across Windows, macOS, and Linux, allowing teams to develop on different systems while maintaining consistency.
  5. Advanced caching: with both local and distributed options eliminates the need for "clean builds" due to cache corruption. Asana reports, "We no longer need to clean because of incorrect caches," highlighting the reliability improvement.

Disadvantages

Here are the disadvantages of using Bazel.

  1. Steep Learning Curve: The steep learning curve presents a significant barrier, as developers must learn the Starlark language and Bazel concepts from scratch. Migration requires expertise that few developers possess, making adoption challenging.
  2. Verbose configuration: can become overwhelming, with BUILD files requiring explicit declaration of all dependencies. This explicitness, while providing correctness, can be tedious for simple projects.
  3. Frontend development challenges: are particularly acute. Node.js and JavaScript projects face difficulties due to NPM package management conflicts, and developers lose hot reloading capabilities, significantly slowing the development feedback loop.
  4. Migration complexity: compounds these issues, as no automated tools exist for converting from other build systems. Manual conversion of large existing projects can be extremely time-consuming.
  5. Workspace management: requires careful coordination across teams, as all projects must agree on common dependency versions through the WORKSPACE file (though Bzlmod in Bazel 8.0 addresses this).

Introduction to CMake

CMake has established itself as the standard for building C++ applications, providing a mature, cross-platform solution that generates native build files for various development environments.

Overview

What is CMake

CMake's story began in 1999 when Kitware Inc. started development, with the first release arriving in 2000. The project received initial funding from the United States National Library of Medicine as part of the Visible Human Project, specifically to support building the Insight Segmentation and Registration Toolkit (ITK) across multiple platforms. The development team made an early decision to create a custom scripting language rather than using Tcl, avoiding external dependencies. CMake 3.0's release in June 2014 marked the beginning of "Modern CMake" with its target-based approach, fundamentally changing how developers structure their build configurations. The current CMake 4.0.2, released in May 2025, continues this evolution while maintaining the tool's core philosophy.

Cross-Platform Support

Cross-Platform Support defines CMake's primary value proposition, generating native build files for Make, Ninja, Visual Studio, Xcode, Qt Creator, Android Studio, and more. This capability extends across Windows, Linux, macOS, Unix-like systems, and embedded platforms, working seamlessly with GCC, Clang, MSVC, Intel compilers, and others.

Connect dots with check marks

Find_Package System

Find_Package System provides sophisticated dependency management for locating and configuring external libraries. Modern CMake supports both traditional FindXXX.cmake modules and newer XXXConfig.cmake files, with target-based integration using imported targets for better dependency management.

Build model

Modern CMake Practices

Modern CMake Practices introduced post-3.0 emphasize a target-based approach focusing on targets and properties rather than global variables. Usage requirements with PRIVATE, PUBLIC, and INTERFACE scopes provide proper dependency propagation, while generator expressions enable advanced conditional logic for different build configurations.

# CMake simple example

## [main]

# Almost all CMake files should start with this
# You should always specify a range with the newest
# and oldest tested versions of CMake. This will ensure
# you pick up the best policies.
cmake_minimum_required(VERSION 3.15...4.0)

# This is your project statement. You should always list languages;
# Listing the version is nice here since it sets lots of useful variables
project(
  ModernCMakeExample
  VERSION 1.0
  LANGUAGES CXX)

# If you set any CMAKE_ variables, that can go here.
# (But usually don't do this, except maybe for C++ standard)

# Find packages go here.

# You should usually split this into folders, but this is a simple example

# This is a "default" library, and will match the *** variable setting.
# Other common choices are STATIC, SHARED, and MODULE
# Including header files here helps IDEs but is not required.
# Output libname matches target name, with the usual extensions on your system
add_library(MyLibExample simple_lib.cpp simple_lib.hpp)

# Link each target with other targets or add options, etc.

# Adding something we can run - Output name matches target name
add_executable(MyExample simple_example.cpp)

# Make sure you link your targets with this command. It can also link libraries and
# even flags, so linking a target that does not exist will not give a configure-time error.
target_link_libraries(MyExample PRIVATE MyLibExample)

## [main]

# This part is so the Modern CMake book can verify this example builds. For your code,
# you'll probably want tests too
enable_testing()
add_test(NAME MyExample COMMAND MyExample)

What are the trade offs?

Let’s take a look at the advantages and disadvantages of choosing CMake.

Advantages

Here are the advantages of using CMake.

  1. Widespread adoption: makes CMake the standard, with over 2 million monthly downloads and use by major projects including Android NDK, Netflix, MySQL, Boost, KDE, Qt, LLVM, Blender, FreeCAD, and WebKit. This adoption translates to extensive community knowledge and support.
  2. Platform independence: allows a single CMakeLists.txt file to work across all supported platforms. Out-of-source builds keep source directories clean, while the system handles compiler-specific flags and options automatically.
  3. Comprehensive Ecosystem: The comprehensive ecosystem includes integration with package managers like vcpkg, Conan, and Hunter, built-in CTest for automated testing, and CPack for creating platform-specific installers.
  4. Advanced features: include C++20 module support in CMake 3.28+, multiple language support (C, C++, Fortran, CUDA, HIP, Swift, C#, and assembly), and built-in parallel build capabilities.
  5. Open source: licensing under the BSD 3-clause license permits commercial use, while continuous development provides regular feature additions and improvements.

Disadvantages

Here are the disadvantages of using CMake.

  1. Complex syntax and learning curve: frustrate many developers, as the CMake scripting language differs significantly from traditional programming languages. The lack of classes or advanced constructs limits expressiveness, and functions cannot return values in conventional ways.
  2. Backwards compatibility issues: create tension between old and new practices, with the policy system often confusing developers trying to maintain compatibility. Documentation gaps for complex scenarios compound these difficulties.
  3. Caching: problems arise from command-line variables being cached and requiring explicit clearing. Hidden dependencies can cause unexpected behavior in CI/CD environments, while CMakeCache.txt corruption necessitates manual intervention.
  4. Generator limitations: restrict projects to single toolchains, preventing mixing of different tools. Separate builds are required for x86/x64 or iOS/macOS targets, and some IDE-specific features don't translate across generators.
  5. Build complexity: stems from the two-step configure-then-build process. Limited support for dynamic dependency generation and hard-coded platform-specific behaviors reduce flexibility in certain scenarios.

How do Bazel and CMake compare?

The choice between Bazel and CMake often comes down to specific project requirements and team capabilities. Understanding their differences across key dimensions helps make an informed decision.

Performance and Scalability

Build performance and scalability characteristics form critical decision factors when choosing between these build systems. The performance profile of each system varies significantly based on project complexity and team size.

Bazel

Bazel demonstrates exceptional performance at enterprise scale, particularly for large codebases and complex dependency graphs. For small projects under 1,000 lines of code, Bazel incurs higher configuration overhead, but this investment pays dividends as projects grow. The Drake robotics project study revealed that while initial builds take approximately 20 minutes for both systems, Bazel achieves up to 50% faster incremental builds and completes no-op builds in under 2 seconds for tens of thousands of C++ files.

CMake

CMake with Ninja typically outperforms Bazel for smaller projects due to lower configuration overhead and simpler setup requirements. The system works efficiently for moderate-sized codebases where teams need quick iteration cycles without extensive build infrastructure investment.

Developer Experience

The developer experience encompasses learning curves, documentation quality, and overall accessibility for development teams with varying expertise levels.

Bazel

Bazel presents a steep learning curve that consistently appears in developer feedback. The declarative Starlark language requires a mindset shift from imperative programming, and documentation gaps particularly affect third-party library integration. Multiple companies report that "nobody knows Bazel" as a significant adoption barrier, necessitating dedicated training programs for development teams.

CMake

CMake offers a moderate learning curve with a familiar scripting approach and extensive online resources, including strong Stack Overflow support. Most C++ developers already have some CMake experience, making it the path of least resistance for many teams.

Language Support and Ecosystem Integration

The breadth and depth of language support directly impacts a build system's applicability across different development scenarios and organizational needs.

Bazel

Bazel takes a fundamentally different approach as a multi-language build system by design. Consistent interfaces across Java, C++, Python, Go, Rust, Scala, Kotlin, JavaScript, and Swift support true polyglot development. This unified approach enables teams to manage complex applications spanning multiple technologies with consistent build patterns.

CMake

CMake primarily serves the C/C++ ecosystem with deep integration and mature tooling. While it supports other languages, its design philosophy centers on C++ development patterns. The vast ecosystem means most C++ libraries ship with CMake build scripts, and package managers like vcpkg and Conan provide seamless integration.

Build Reproducibility and Caching Mechanisms

Reproducible builds and effective caching strategies become increasingly important as development teams scale and deployment reliability requirements increase.

Bazel

Bazel's hermetic builds guarantee reproducibility through comprehensive sandboxing and dependency tracking. The built-in local and remote caching system shares artifacts across teams, dramatically reducing build times for large organizations. Google's internal usage demonstrates the effectiveness of this approach at scale.

CMake

CMake lacks built-in caching mechanisms, relying instead on underlying build system capabilities and third-party solutions like ccache. This limitation becomes more pronounced in distributed development environments where teams cannot easily share build artifacts.

IDE and Tooling Support

Development environment integration significantly impacts daily developer productivity and adoption friction within development teams.

Bazel

Bazel's IDE support remains less mature despite recent improvements. Official plugins for IntelliJ and CLion remain in beta status, while VS Code support relies on third-party extensions. This gap in tooling support affects developer productivity, particularly during the learning phase.

CMake

CMake enjoys excellent IDE integration as the industry standard. Visual Studio, CLion, Qt Creator, and VS Code provide native support, with the CMake Tools extension for VS Code being particularly popular. This mature ecosystem means developers can use their preferred tools without compromise.

Which tool should you pick?

Your choice between Bazel and CMake should match your team's needs, existing toolchain, and operational priorities.

Bazel is ideal for:

  1. Teams managing massive codebases with 100,000+ source files need Google-scale build infrastructure to handle enterprise-level complexity.
  2. Applications spanning Java, C++, Python, Go, and other languages require consistent build patterns across all technologies in polyglot development environments.
  3. Organizations with distributed development teams can eliminate redundant builds across multiple developers and CI systems through remote caching and execution.
  4. Companies requiring hermetic builds that guarantee identical outputs across all environments can eliminate "works on my machine" issues in enterprise settings.
  5. Projects seeking dramatic build time reductions from 40-90 minutes to 5-6 minutes can achieve significant business value through high-performance CI/CD pipelines.

CMake is ideal for:

  1. Applications primarily written in C++ benefit from mature tooling, extensive library ecosystem, and seamless third-party integration in traditional development workflows.
  2. Software targeting Windows, macOS, and Linux requires generating native build files for each platform in cross-platform desktop applications.
  3. Development groups under 50 people need quick setup without investing time in learning complex build system concepts for small to medium-sized teams.
  4. Existing projects require gradual migration to modern build practices while maintaining compatibility with current workflows during legacy codebase modernization.
  5. Projects requiring widespread adoption benefit from CMake's industry-standard status that ensures maximum compatibility and contributor accessibility for open-source C++ libraries.

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