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What is Bazel?

Bazel is a tool that automates software builds and tests. Supported build tasks include running compilers and linkers to produce executable programs and libraries, and assembling deployable packages for Android, iOS and other target environments. Bazel is similar to other tools like Make, Ant, Gradle, Buck, Pants and Maven.

What is special about Bazel?

Bazel was designed to fit the way software is developed at Google. It has the following features:

  • Multi-language support: Bazel supports many languages, and can be extended to support arbitrary programming languages.
  • High-level build language: Projects are described in the BUILD language, a concise text format that describes a project as sets of small interconnected libraries, binaries and tests. In contrast, with tools like Make, you have to describe individual files and compiler invocations.
  • Multi-platform support: The same tool and the same BUILD files can be used to build software for different architectures, and even different platforms. At Google, we use Bazel to build everything from server applications running on systems in our data centers to client apps running on mobile phones.
  • Reproducibility: In BUILD files, each library, test and binary must specify its direct dependencies completely. Bazel uses this dependency information to know what must be rebuilt when you make changes to a source file, and which tasks can run in parallel. This means that all builds are incremental and will always produce the same result.
  • Scalable: Bazel can handle large builds; at Google, it is common for a server binary to have 100k source files, and builds where no files were changed take about ~200ms.

Why doesn’t Google use...?

  • Make, Ninja: These tools give very exact control over what commands get invoked to build files, but it’s up to the user to write rules that are correct.
    • Users interact with Bazel on a higher level. For example, Bazel has built-in rules for “Java test”, “C++ binary”, and notions such as “target platform” and “host platform”. These rules have been battle tested to be foolproof.
  • Ant and Maven: Ant and Maven are primarily geared toward Java, while Bazel handles multiple languages. Bazel encourages subdividing codebases in smaller reusable units, and can rebuild only ones that need rebuilding. This speeds up development when working with larger codebases.
  • Gradle: Bazel configuration files are much more structured than Gradle’s, letting Bazel understand exactly what each action does. This allows for more parallelism and better reproducibility.
  • Pants, Buck: Both tools were created and developed by ex-Googlers at Twitter and Foursquare, and Facebook respectively. They have been modeled after Bazel, but their feature sets are different, so they aren’t viable alternatives for us.

Where did Bazel come from?

Bazel is a flavor of the tool that Google uses to build its server software internally. It has expanded to build other software as well, like mobile apps (iOS, Android) that connect to our servers.

Did you rewrite your internal tool as open-source? Is it a fork?

Bazel shares most of its code with the internal tool and its rules are used for millions of builds every day.

Why did Google build Bazel?

A long time ago, Google built its software using large, generated Makefiles. These led to slow and unreliable builds, which began to interfere with our developers’ productivity and the company’s agility. Bazel was a way to solve these problems.

Does Bazel require a build cluster?

Bazel runs build operations locally by default. However, Bazel can also connect to a build cluster for even faster builds and tests. See our documentation on remote execution and caching and remote caching for further details.

How does the Google development process work?

For our server code base, we use the following development workflow:

  • All our server code is in a single, gigantic version control system.
  • Everybody builds their software with Bazel.
  • Different teams own different parts of the source tree, and make their components available as BUILD targets.
  • Branching is primarily used for managing releases, so everybody develops their software at the head revision.

Bazel is a cornerstone of this philosophy: since Bazel requires all dependencies to be fully specified, we can predict which programs and tests are affected by a change, and vet them before submission.

More background on the development process at Google can be found on the eng tools blog.

Why did you open up Bazel?

Building software should be fun and easy. Slow and unpredictable builds take the fun out of programming.

Why would I want to use Bazel?

  • Bazel may give you faster build times because it can recompile only the files that need to be recompiled. Similarly, it can skip re-running tests that it knows haven’t changed.
  • Bazel produces deterministic results. This eliminates skew between incremental and clean builds, laptop and CI system, etc.
  • Bazel can build different client and server apps with the same tool from the same workspace. For example, you can change a client/server protocol in a single commit, and test that the updated mobile app works with the updated server, building both with the same tool, reaping all the aforementioned benefits of Bazel.

Can I see examples?

Yes; see a simple example or read the Bazel source code for a more complex example.

What is Bazel best at?

Bazel shines at building and testing projects with the following properties:

  • Projects with a large codebase
  • Projects written in (multiple) compiled languages
  • Projects that deploy on multiple platforms
  • Projects that have extensive tests

Where can I run Bazel?

Bazel runs on Linux, macOS (OS X), and Windows.

Porting to other UNIX platforms should be relatively easy, as long as a JDK is available for the platform.

What should I not use Bazel for?

  • Bazel tries to be smart about caching. This means that it is not good for running build operations whose outputs should not be cached. For example, the following steps should not be run from Bazel:
    • A compilation step that fetches data from the internet.
    • A test step that connects to the QA instance of your site.
    • A deployment step that changes your site’s cloud configuration.
  • If your build consists of a few long, sequential steps, Bazel may not be able to help much. You’ll get more speed by breaking long steps into smaller, discrete targets that Bazel can run in parallel.

How stable is Bazel’s feature set?

The core features (C++, Java, and shell rules) have extensive use inside Google, so they are thoroughly tested and have very little churn. Similarly, we test new versions of Bazel across hundreds of thousands of targets every day to find regressions, and we release new versions multiple times every month.

In short, except for features marked as experimental, Bazel should Just Work. Changes to non-experimental rules will be backward compatible. A more detailed list of feature support statuses can be found in our support document.

How stable is Bazel as a binary?

Inside Google, we make sure that Bazel crashes are very rare. This should also hold for our open source codebase.

How can I start using Bazel?

See Getting Started.

Doesn’t Docker solve the reproducibility problems?

With Docker you can easily create sandboxes with fixed OS releases, for example, Ubuntu 12.04, Fedora 21. This solves the problem of reproducibility for the system environment – that is, “which version of /usr/bin/c++ do I need?”

Docker does not address reproducibility with regard to changes in the source code. Running Make with an imperfectly written Makefile inside a Docker container can still yield unpredictable results.

Inside Google, we check tools into source control for reproducibility. In this way, we can vet changes to tools (“upgrade GCC to 4.6.1”) with the same mechanism as changes to base libraries (“fix bounds check in OpenSSL”).

Can I build binaries for deployment on Docker?

With Bazel, you can build standalone, statically linked binaries in C/C++, and self-contained jar files for Java. These run with few dependencies on normal UNIX systems, and as such should be simple to install inside a Docker container.

Bazel has conventions for structuring more complex programs, for example, a Java program that consumes a set of data files, or runs another program as subprocess. It is possible to package up such environments as standalone archives, so they can be deployed on different systems, including Docker images.

Can I build Docker images with Bazel?

Yes, you can use our Docker rules to build reproducible Docker images.

Will Bazel make my builds reproducible automatically?

For Java and C++ binaries, yes, assuming you do not change the toolchain. If you have build steps that involve custom recipes (for example, executing binaries through a shell script inside a rule), you will need to take some extra care:

  • Do not use dependencies that were not declared. Sandboxed execution (–spawn_strategy=sandboxed, only on Linux) can help find undeclared dependencies.
  • Avoid storing timestamps and user-IDs in generated files. ZIP files and other archives are especially prone to this.
  • Avoid connecting to the network. Sandboxed execution can help here too.
  • Avoid processes that use random numbers, in particular, dictionary traversal is randomized in many programming languages.

Do you have binary releases?

Yes, you can find the latest release binaries and review our release policy

I use Eclipse/IntelliJ/XCode. How does Bazel interoperate with IDEs?

For IntelliJ, check out the IntelliJ with Bazel plugin.

For XCode, check out Tulsi.

For Eclipse, check out E4B plugin.

For other IDEs, check out the blog post on how these plugins work.

I use Jenkins/CircleCI/TravisCI. How does Bazel interoperate with CI systems?

Bazel returns a non-zero exit code if the build or test invocation fails, and this should be enough for basic CI integration. Since Bazel does not need clean builds for correctness, the CI system should not be configured to clean before starting a build/test run.

Further details on exit codes are in the User Manual.

What future features can we expect in Bazel?

See our Roadmaps.

Can I use Bazel for my INSERT LANGUAGE HERE project?

Bazel is extensible. Anyone can add support for new languages. Many languages are supported: see the build encyclopedia for a list of recommendations and awesomebazel.com for a more comprehensive list.

If you would like to develop extensions or learn how they work, see the documentation for extending Bazel.

Can I contribute to the Bazel code base?

See our contribution guidelines.

Why isn’t all development done in the open?

We still have to refactor the interfaces between the public code in Bazel and our internal extensions frequently. This makes it hard to do much development in the open.

Are you done open sourcing Bazel?

Open sourcing Bazel is a work-in-progress. In particular, we’re still working on open sourcing:

  • Many of our unit and integration tests (which should make contributing patches easier).
  • Full IDE integration.

Beyond code, we’d like to eventually have all code reviews, bug tracking, and design decisions happen publicly, with the Bazel community involved. We are not there yet, so some changes will simply appear in the Bazel repository without clear explanation. Despite this lack of transparency, we want to support external developers and collaborate. Thus, we are opening up the code, even though some of the development is still happening internal to Google. Please let us know if anything seems unclear or unjustified as we transition to an open model.

Are there parts of Bazel that will never be open sourced?

Yes, some of the code base either integrates with Google-specific technology or we have been looking for an excuse to get rid of (or is some combination of the two). These parts of the code base are not available on GitHub and probably never will be.

How do I contact the team?

We are reachable at bazel-discuss@googlegroups.com.

Where do I report bugs?

Open an issue on GitHub.

What’s up with the word “Blaze” in the codebase?

This is an internal name for the tool. Please refer to Blaze as Bazel.

Why do other Google projects (Android, Chrome) use other build tools?

Until the first (Alpha) release, Bazel was not available externally, so open source projects such as Chromium and Android could not use it. In addition, the original lack of Windows support was a problem for building Windows applications, such as Chrome. Since the project has matured and become more stable, the Android Open Source Project is in the process of migrating to Bazel.

How do you pronounce “Bazel”?

The same way as “basil” (the herb) in US English: “BAY-zel”. It rhymes with “hazel”. IPA: /ˈbeɪzˌəl/