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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.

Contents

General structure

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

Prefer declaring third party dependencies as remote repositories in the WORKSPACE file. If it's necessary to check third party dependencies into your repository, put them in a directory called third_party/ under your workspace directory. Note that all BUILD files in third_party/ must include license declarations.

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.

Building from source prevents a build from 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_/tools/bazel.rc (see bazelrc format).

For options that you do not want to check into source control, create the configuration file _your-workspace_/.bazelrc and add .bazelrc to your .gitignore. Note that this file has a different name than the file above (bazel.rc vs .bazelrc).

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.

BUILD files

See the BUILD file style guide.

.bzl files

.bzl files style guide

See the Style guide for .bzl files for Skylark rule guidelines.

Packaging rules

See Packaging rules for advice on how to structure and where to put new Skylark rules.

Rule choice

When using a language for which Bazel has built-in rules (e.g., C++), prefer using these rules to writing your own in Skylark. These rules are documented in the build encyclopedia.

WORKSPACE files

Repository rules

Prefer http_archive and new_http_archive to git_repository, new_git_repository, and maven_jar.

git_repository depends on jGit, which has several unpleasant bugs, and maven_jar uses Maven's internal API, which generally works but is less optimized for Bazel than http_archive's downloader logic. Track the following issues filed to remediate these problems:

Do not use bind(). See "Consider removing bind" for a long discussion of its issues and alternatives.

Custom BUILD files

When using a new_ repository rule, prefer to specify build_file_content, not build_file.

Skylark repository rules

A Skylark repository rule should generally be responsible for:

  • Detecting system settings and writing them to files.
  • Finding resources elsewhere on the system.
  • Downloading resources from URLs.
  • Generating or symlinking BUILD files into the external repository directory.

Avoid using repository_ctx.execute when possible. For example, when using a non-Bazel C++ library that has a build using Make, it is preferable to use repository_ctx.download() and then write a BUILD file that builds it, instead of running ctx.execute(["make"]).

Programming languages

This section describes best practices for specific programming languages.

C++ and Bazel

For best practices for C++ projects, see C++ and Bazel.

Java and Bazel

For best practices for Java projects, see Java and Bazel.

Protos and Bazel

Recommended code organization:

  • One proto_library rule per .proto file.
  • A file named foo.proto will be in a rule named foo_proto, which is located in the same package.
  • A [language]_proto_library that wraps a proto_library named foo_proto should be called foo_[language]_proto, and be located in the same package.