Best practices for Bazel

This document assumes that you are familiar with Bazel and provides advice on structuring your projects to take full advantage of Bazel’s features.

The overall goals are:

  • To use fine-grained dependencies to allow parallelism and incrementality.
  • To keep dependencies well-encapsulated.
  • To make code well-structured and testable.
  • To create a build configuration that is easy to understand and maintain.

These guidelines are not requirements: few projects will be able to adhere to all of them. As the man page for lint says, “A special reward will be presented to the first person to produce a real program that produces no errors with strict checking.” However, incorporating as many of these principles as possible should make a project more readable, less error-prone, and faster to build.

This document uses the requirement levels described in this RFC.

Running builds and tests

A project should always be able to run bazel build //... and bazel test //... successfully on its stable branch. Targets that are necessary but do not build under certain circumstances (e.g., require specific build flags, do not build on a certain platform, require license agreements) should be tagged as specifically as possible (e.g., “requires-osx”). This tagging allows targets to be filtered at a more fine-grained level than the “manual” tag and allows someone inspecting the BUILD file to understand what a target’s restrictions are.

Third party dependencies

You may declare third party dependencies:

  • Either declare them as remote repositories in the WORKSPACE file.
  • Or put them in a directory called third_party/ under your workspace directory.

Depending on binaries

Everything should be built from source whenever possible. Generally this means that, instead of depending on a library some-library.so, you’d create a BUILD file and build some-library.so from its sources, then depend on that target.

Always building from source ensures that a build is not using a library that was built with incompatible flags or a different architecture. There are also some features like coverage, static analysis, or dynamic analysis that will only work on the source.

Versioning

Prefer building all code from head whenever possible. When versions must be used, avoid including the version in the target name (e.g., //guava, not //guava-20.0). This naming makes the library easier to update (only one target needs to be updated). It is also more resilient to diamond dependency issues: if one library depends on guava-19.0 and one depends on guava-20.0, you could end up with a library that tries to depend on two different versions. If you created a misleading alias to point both targets to one guava library, then the BUILD files are misleading.

.bazelrc

For project-specific options, use the configuration file your <workspace>/.bazelrc (see bazelrc format).

If you want to support per-user options for your project that you do not want to check into source control, include the line

try-import user.bazelrc

(or any other file name) in your <workspace>/.bazelrc and add user.bazelrc to your .gitignore.

Packages

Every directory that contains buildable files should be a package. If a BUILD file refers to files in subdirectories (e.g., srcs = ["a/b/C.java"]) it is a sign that a BUILD file should be added to that subdirectory. The longer this structure exists, the more likely circular dependencies will be inadvertently created, a target’s scope will creep, and an increasing number of reverse dependencies will have to be updated.