Bazel supports external dependencies, source files (both text and binary) used in your build that are not from your workspace. For example, they could be a ruleset hosted in a GitHub repo, a Maven artifact, or a directory on your local machine outside your current workspace.
As of Bazel 6.0, there are two ways to manage external dependencies with Bazel:
the traditional, repository-focused
WORKSPACE system, and
the newer module-focused
MODULE.bazel system (codenamed Bzlmod,
and enabled with the flag
--enable_bzlmod). The two systems can be used
together, but Bzlmod is replacing the
WORKSPACE system in future Bazel
This article explains the concepts surrounding external dependency management in Bazel, before going into a bit more detail about the two systems in order.
A directory with a
WORKSPACE.bazel file, containing source
files to be used in a Bazel build. Often shortened to just repo.
The repository in which the current Bazel command is being run.
The environment shared by all Bazel commands run in the same main repository.
Note that historically the concepts of "repository" and "workspace" have been conflated; the term "workspace" has often been used to refer to the main repository, and sometimes even used as a synonym of "repository".
Canonical repository name
The canonical name a repository is addressable by. Within the context of a
workspace, each repository has a single canonical name. A target inside a repo
whose canonical name is
canonical_name can be addressed by the label
@@canonical_name//pac/kage:target (note the double
The main repository always has the empty string as the canonical name.
Apparent repository name
The name a repository is addressable by in the context of a certain other repo.
This can be thought of as a repo's "nickname": The repo with the canonical name
michael might have the apparent name
mike in the context of the repo
alice, but might have the apparent name
mickey in the context of the repo
bob. In this case, a target inside
michael can be addressed by the label
@mike//pac/kage:target in the context of
alice (note the single
Conversely, this can be understood as a repository mapping: each repo maintains a mapping from "apparent repo name" to a "canonical repo name".
A schema for repository definitions that tells Bazel how to materialize a
repository. For example, it could be "download a zip archive from a certain URL
and extract it", or "fetch a certain Maven artifact and make it available as a
java_import target", or simply "symlink a local directory". Every repo is
defined by calling a repo rule with an appropriate number of arguments.
See Repository rules for more information on how to write your own repository rules.
The most common repo rules by far are
http_archive, which downloads an archive
from a URL and extracts it, and
local_repository, which symlinks a
local directory that is already a Bazel repository.
Fetching a repository
The action of making a repo available on local disk by running its associated repo rule. The repos defined in a workspace are not available on local disk before they are fetched.
Normally, Bazel will only fetch a repo when it needs something from the repo, and the repo hasn't already been fetched. If the repo has already been fetched before, Bazel will only re-fetch it if its definition has changed.
After being fetched, the repo can be found in the subdirectory
external in the
output base, under its canonical name.
You can run the following command to see the contents of the repo with the
ls $(bazel info output_base)/external/ canonical_name
Managing external dependencies with Bzlmod
Bzlmod, the new external dependency subsystem, does not directly work with repo definitions. Instead, it builds a dependency graph from modules, runs extensions on top of the graph, and defines repos accordingly.
A Bazel module is a Bazel project that can have multiple
versions, each of which publishes metadata about other modules that it depends
on. A module must have a
MODULE.bazel file at its repo root, next to the
WORKSPACE file. This file is the module's manifest, declaring its name,
version, list of dependencies, among other information. The following is a basic
module(name = "my-module", version = "1.0") bazel_dep(name = "rules_cc", version = "0.0.1") bazel_dep(name = "protobuf", version = "3.19.0")
A module must only list its direct dependencies, which Bzlmod looks up in a
Bazel registry — by default, the Bazel Central
Registry. The registry provides the
MODULE.bazel files, which allows Bazel to discover the entire
transitive dependency graph before performing version resolution.
After version resolution, in which one version is selected for each module,
Bazel consults the registry again to learn how to define a repo for each module
(in most cases, using
Modules can also specify customized pieces of data called tags, which are consumed by module extensions after module resolution to define additional repos. These extensions have capabilities similar to repo rules, enabling them to perform actions like file I/O and sending network requests. Among other things, they allow Bazel to interact with other package management systems while also respecting the dependency graph built out of Bazel modules.
External links on Bzlmod
- Bzlmod usage examples in bazelbuild/examples
- Bazel External Dependencies Overhaul (original Bzlmod design doc)
- BazelCon 2021 talk on Bzlmod
Defining repos with
Historically, you can manage external dependencies by defining repos in
WORKSPACE.bazel) file. This file has a similar syntax to
BUILD files, employing repo rules instead of build rules.
The following snippet is an example to use the
http_archive repo rule in the
load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive") http_archive( name = "foo", urls = ["https://example.com/foo.zip"], sha256 = "c9526390a7cd420fdcec2988b4f3626fe9c5b51e2959f685e8f4d170d1a9bd96", )
The snippet defines a repo whose canonical name is
foo. In the
system, by default, the canonical name of a repo is also its apparent name to
all other repos.
Shortcomings of the
In the years since the
WORKSPACE system was introduced, users have reported
many pain points, including:
- Bazel does not evaluate the
WORKSPACEfiles of any dependencies, so all transitive dependencies must be defined in the
WORKSPACEfile of the main repo, in addition to direct dependencies.
- To work around this, projects have adopted the "deps.bzl" pattern, in which
they define a macro which in turn defines multiple repos, and ask users to
call this macro in their
- This has its own problems: macros cannot
.bzlfiles, so these projects have to define their transitive dependencies in this "deps" macro, or work around this issue by having the user call multiple layered "deps" macros.
- Bazel evaluates the
WORKSPACEfile sequentially. Additionally, dependencies are specified using
http_archivewith URLs, without any version information. This means that there is no reliable way to perform version resolution in the case of diamond dependencies (
Cboth depend on different versions of
- This has its own problems: macros cannot