Bzlmod Migration Guide

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Due to the shortcomings of WORKSPACE, Bzlmod is going to replace the legacy WORKSPACE system. The WORKSPACE file will be disabled by default in Bazel 8 (late 2024) and will be removed in Bazel 9 (late 2025). This guide helps you migrate your project to Bzlmod and drop WORKSPACE for fetching external dependencies.

WORKSPACE vs Bzlmod

Bazel's WORKSPACE and Bzlmod offer similar features with different syntax. This section explains how to migrate from specific WORKSPACE functionalities to Bzlmod.

Define the root of a Bazel workspace

The WORKSPACE file marks the source root of a Bazel project, this responsibility is replaced by MODULE.bazel in Bazel version 6.3 and later. With Bazel version prior to 6.3, there should still be a WORKSPACE or WORKSPACE.bazel file at your workspace root, maybe with comments like:

  • WORKSPACE

    # This file marks the root of the Bazel workspace.
# See MODULE.bazel for external dependencies setup.

Enable Bzlmod in your bazelrc

.bazelrc lets you set flags that apply every time your run Bazel. To enable Bzlmod, use the --enable_bzlmod flag, and apply it to the common command so it applies to every command:

  • .bazelrc

    # Enable Bzlmod for every Bazel command
common --enable_bzlmod

Specify repository name for your workspace

  • WORKSPACE

    The workspace function is used to specify a repository name for your workspace. This allows a target //foo:bar in the workspace to be referenced as @<workspace name>//foo:bar. If not specified, the default repository name for your workspace is __main__.

    ## WORKSPACE
workspace(name = "com_foo_bar")

  • Bzlmod

    It's recommended to reference targets in the same workspace with the //foo:bar syntax without @<repo name>. But if you do need the old syntax , you can use the module name specified by the module function as the repository name. If the module name is different from the needed repository name, you can use repo_name attribute of the module function to override the repository name.

    ## MODULE.bazel
module(
    name = "bar",
    repo_name = "com_foo_bar",
)

Fetch external dependencies as Bazel modules

If your dependency is a Bazel project, you should be able to depend on it as a Bazel module when it also adopts Bzlmod.

  • WORKSPACE

    With WORKSPACE, it's common to use the http_archive or git_repository repository rules to download the sources of the Bazel project.

    ## WORKSPACE
load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")

http_archive(
    name = "bazel_skylib",
    urls = ["https://github.com/bazelbuild/bazel-skylib/releases/download/1.4.2/bazel-skylib-1.4.2.tar.gz"],
    sha256 = "66ffd9315665bfaafc96b52278f57c7e2dd09f5ede279ea6d39b2be471e7e3aa",
)
load("@bazel_skylib//:workspace.bzl", "bazel_skylib_workspace")
bazel_skylib_workspace()

http_archive(
    name = "rules_java",
    urls = ["https://github.com/bazelbuild/rules_java/releases/download/6.1.1/rules_java-6.1.1.tar.gz"],
    sha256 = "76402a50ae6859d50bd7aed8c1b8ef09dae5c1035bb3ca7d276f7f3ce659818a",
)
load("@rules_java//java:repositories.bzl", "rules_java_dependencies", "rules_java_toolchains")
rules_java_dependencies()
rules_java_toolchains()

    As you can see, it's a common pattern that users need to load transitive dependencies from a macro of the dependency. Assume both bazel_skylib and rules_java depends on platform, the exact version of the platform dependency is determined by the order of the macros.

  • Bzlmod

    With Bzlmod, as long as your dependency is available in Bazel Central Registry or your custom Bazel registry, you can simply depend on it with a bazel_dep directive.

    ## MODULE.bazel
bazel_dep(name = "bazel_skylib", version = "1.4.2")
bazel_dep(name = "rules_java", version = "6.1.1")

    Bzlmod resolves Bazel module dependencies transitively using the MVS algorithm. Therefore, the maximal required version of platform is selected automatically.

Override a dependency as a Bazel module

As the root module, you can override Bazel module dependencies in different ways.

Please read the overrides section for more information.

You can find some example usages in the examples repository.

Fetch external dependencies with module extensions

If your dependency is not a Bazel project or not yet available in any Bazel registry, you can introduce it using use_repo_rule or module extensions.

  • WORKSPACE

    Download a file using the http_file repository rule.

    ## WORKSPACE
load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_file")

http_file(
    name = "data_file",
    url = "http://example.com/file",
    sha256 = "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
)

  • Bzlmod

    With Bzlmod, you can use the use_repo_rule directive in your MODULE.bazel file to directly instantiate repos:

    ## MODULE.bazel
http_file = use_repo_rule("@bazel_tools//tools/build_defs/repo:http.bzl", "http_file")
http_file(
    name = "data_file",
    url = "http://example.com/file",
    sha256 = "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
)

    Under the hood, this is implemented using a module extension. If you need to perform more complex logic than simply invoking a repo rule, you could also implement a module extension yourself. You'll need to move the definition into a .bzl file, which also lets you share the definition between WORKSPACE and Bzlmod during the migration period.

    ## repositories.bzl
load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_file")
def my_data_dependency():
    http_file(
        name = "data_file",
        url = "http://example.com/file",
        sha256 = "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
    )

    Implement a module extension to load the dependencies macro. You can define it in the same .bzl file of the macro, but to keep compatibility with older Bazel versions, it's better to define it in a separate .bzl file.

    ## extensions.bzl
load("//:repositories.bzl", "my_data_dependency")
def _non_module_dependencies_impl(_ctx):
    my_data_dependency()

non_module_dependencies = module_extension(
    implementation = _non_module_dependencies_impl,
)

    To make the repository visible to the root project, you should declare the usages of the module extension and the repository in the MODULE.bazel file.

    ## MODULE.bazel
non_module_dependencies = use_extension("//:extensions.bzl", "non_module_dependencies")
use_repo(non_module_dependencies, "data_file")

Resolve conflict external dependencies with module extension

A project can provide a macro that introduces external repositories based on inputs from its callers. But what if there are multiple callers in the dependency graph and they cause a conflict?

Assume the project foo provides the following macro which takes version as an argument.

## repositories.bzl in foo {:#repositories.bzl-foo}
load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_file")
def data_deps(version = "1.0"):
    http_file(
        name = "data_file",
        url = "http://example.com/file-%s" % version,
        # Omitting the "sha256" attribute for simplicity
    )

  • WORKSPACE

    With WORKSPACE, you can load the macro from @foo and specify the version of the data dependency you need. Assume you have another dependency @bar, which also depends on @foo but requires a different version of the data dependency.

    ## WORKSPACE

# Introduce @foo and @bar.
...

load("@foo//:repositories.bzl", "data_deps")
data_deps(version = "2.0")

load("@bar//:repositories.bzl", "bar_deps")
bar_deps() # -> which calls data_deps(version = "3.0")

    In this case, the end user must carefully adjust the order of macros in the WORKSPACE to get the version they need. This is one of the biggest pain points with WORKSPACE since it doesn't really provide a sensible way to resolve dependencies.

  • Bzlmod

    With Bzlmod, the author of project foo can use module extension to resolve conflicts. For example, let's assume it makes sense to always select the maximal required version of the data dependency among all Bazel modules.

    ## extensions.bzl in foo
load("//:repositories.bzl", "data_deps")

data = tag_class(attrs={"version": attr.string()})

def _data_deps_extension_impl(module_ctx):
    # Select the maximal required version in the dependency graph.
    version = "1.0"
    for mod in module_ctx.modules:
        for data in mod.tags.data:
            version = max(version, data.version)
    data_deps(version)

data_deps_extension = module_extension(
    implementation = _data_deps_extension_impl,
    tag_classes = {"data": data},
)

    ## MODULE.bazel in bar
bazel_dep(name = "foo", version = "1.0")

foo_data_deps = use_extension("@foo//:extensions.bzl", "data_deps_extension")
foo_data_deps.data(version = "3.0")
use_repo(foo_data_deps, "data_file")

    ## MODULE.bazel in root module
bazel_dep(name = "foo", version = "1.0")
bazel_dep(name = "bar", version = "1.0")

foo_data_deps = use_extension("@foo//:extensions.bzl", "data_deps_extension")
foo_data_deps.data(version = "2.0")
use_repo(foo_data_deps, "data_file")

    In this case, the root module requires data version 2.0, while its dependency bar requires 3.0. The module extension in foo can correctly resolve this conflict and automatically select version 3.0 for the data dependency.

Integrate third party package manager

Following the last section, since module extension provides a way to collect information from the dependency graph, perform custom logic to resolve dependencies and call repository rules to introduce external repositories, this provides a great way for rules authors to enhance the rulesets that integrate package managers for specific languages.

Please read the module extensions page to learn more about how to use module extensions.

Here is a list of the rulesets that already adopted Bzlmod to fetch dependencies from different package managers:

A minimal example that integrates a pseudo package manager is available at the examples repository.

Detect toolchains on the host machine

When Bazel build rules need to detect what toolchains are available on your host machine, they use repository rules to inspect the host machine and generate toolchain info as external repositories.

  • WORKSPACE

    Given the following repository rule to detect a shell toolchain.

    ## local_config_sh.bzl
def _sh_config_rule_impl(repository_ctx):
    sh_path = get_sh_path_from_env("SH_BIN_PATH")

    if not sh_path:
        sh_path = detect_sh_from_path()

    if not sh_path:
        sh_path = "/shell/binary/not/found"

    repository_ctx.file("BUILD", """
load("@bazel_tools//tools/sh:sh_toolchain.bzl", "sh_toolchain")
sh_toolchain(
    name = "local_sh",
    path = "{sh_path}",
    visibility = ["//visibility:public"],
)
toolchain(
    name = "local_sh_toolchain",
    toolchain = ":local_sh",
    toolchain_type = "@bazel_tools//tools/sh:toolchain_type",
)
""".format(sh_path = sh_path))

sh_config_rule = repository_rule(
    environ = ["SH_BIN_PATH"],
    local = True,
    implementation = _sh_config_rule_impl,
)

    You can load the repository rule in WORKSPACE.

    ## WORKSPACE
load("//:local_config_sh.bzl", "sh_config_rule")
sh_config_rule(name = "local_config_sh")

  • Bzlmod

    With Bzlmod, you can introduce the same repository using a module extension, which is similar to introducing the @data_file repository in the last section.

    ## local_config_sh_extension.bzl
load("//:local_config_sh.bzl", "sh_config_rule")

sh_config_extension = module_extension(
    implementation = lambda ctx: sh_config_rule(name = "local_config_sh"),
)

    Then use the extension in the MODULE.bazel file.

    ## MODULE.bazel
sh_config_ext = use_extension("//:local_config_sh_extension.bzl", "sh_config_extension")
use_repo(sh_config_ext, "local_config_sh")

Register toolchains & execution platforms

Following the last section, after introducing a repository hosting toolchain information (e.g. local_config_sh), you probably want to register the toolchain.

  • WORKSPACE

    With WORKSPACE, you can register the toolchain in the following ways.

    1. You can register the toolchain the .bzl file and load the macro in the WORKSPACE file.

      ## local_config_sh.bzl
def sh_configure():
    sh_config_rule(name = "local_config_sh")
    native.register_toolchains("@local_config_sh//:local_sh_toolchain")

      ## WORKSPACE
load("//:local_config_sh.bzl", "sh_configure")
sh_configure()

    2. Or register the toolchain in the WORKSPACE file directly.

      ## WORKSPACE
load("//:local_config_sh.bzl", "sh_config_rule")
sh_config_rule(name = "local_config_sh")
register_toolchains("@local_config_sh//:local_sh_toolchain")

  • Bzlmod

    With Bzlmod, the register_toolchains and register_execution_platforms APIs are only available in the MODULE.bazel file. You cannot call native.register_toolchains in a module extension.

    ## MODULE.bazel
sh_config_ext = use_extension("//:local_config_sh_extension.bzl", "sh_config_extension")
use_repo(sh_config_ext, "local_config_sh")
register_toolchains("@local_config_sh//:local_sh_toolchain")

The toolchains and execution platforms registered in WORKSPACE, WORKSPACE.bzlmod and each Bazel module's MODULE.bazel file follow this order of precedence during toolchain selection (from highest to lowest):

  1. toolchains and execution platforms registered in the root module's MODULE.bazel file.
  2. toolchains and execution platforms registered in the WORKSPACE or WORKSPACE.bzlmod file.
  3. toolchains and execution platforms registered by modules that are (transitive) dependencies of the root module.
  4. when not using WORKSPACE.bzlmod: toolchains registered in the WORKSPACE suffix.

Introduce local repositories

You may need to introduce a dependency as a local repository when you need a local version of the dependency for debugging or you want to incorporate a directory in your workspace as external repository.

  • WORKSPACE

    With WORKSPACE, this is achieved by two native repository rules, local_repository and new_local_repository.

    ## WORKSPACE
local_repository(
    name = "rules_java",
    path = "/Users/bazel_user/workspace/rules_java",
)

  • Bzlmod

    With Bzlmod, you can use local_path_override to override a module with a local path.

    ## MODULE.bazel
bazel_dep(name = "rules_java")
local_path_override(
    module_name = "rules_java",
    path = "/Users/bazel_user/workspace/rules_java",
)

    It is also possible to introduce a local repository with module extension. However, you cannot call native.local_repository in module extension, there is ongoing effort on starlarkifying all native repository rules (check #18285 for progress). Then you can call the corresponding starlark local_repository in a module extension. It's also trivial to implement a custom version of local_repository repository rule if this is a blocking issue for you.

Bind targets

The bind rule in WORKSPACE is deprecated and not supported in Bzlmod. It was introduced to give a target an alias in the special //external package. All users depending on this should migrate away.

For example, if you have

## WORKSPACE
bind(
    name = "openssl",
    actual = "@my-ssl//src:openssl-lib",
)

This allows other targets to depend on //external:openssl. You can migrate away from this by:

  • Replace all usages of //external:openssl with @my-ssl//src:openssl-lib.

  • Or use the alias build rule

    • Define the following target in a package (e.g. //third_party)

      ## third_party/BUILD
alias(
    name = "openssl",
    actual = "@my-ssl//src:openssl-lib",
)

    • Replace all usages of //external:openssl with //third_party:openssl.

Fetch versus Sync

Fetch and sync commands are used to download external repos locally and keep them updated. Sometimes also to allow building offline using the --nofetch flag after fetching all repos needed for a build.

  • WORKSPACE

    Sync performs a force fetch for all repositories, or for a specific configured set of repos, while fetch is only used to fetch for a specific target.

  • Bzlmod

    The sync command is no longer applicable, but fetch offers various options. You can fetch a target, a repository, a set of configured repos or all repositories involved in your dependency resolution and module extensions. The fetch result is cached and to force a fetch you must include the --force option during the fetch process.

Migration

This section provides useful information and guidance for your Bzlmod migration process.

Know your dependencies in WORKSPACE

The first step of migration is to understand what dependencies you have. It could be hard to figure out what exact dependencies are introduced in the WORKSPACE file because transitive dependencies are often loaded with *_deps macros.

Inspect external dependency with workspace resolved file

Fortunately, the flag --experimental_repository_resolved_file can help. This flag essentially generates a "lock file" of all fetched external dependencies in your last Bazel command. You can find more details in this blog post.

It can be used in two ways:

  1. To fetch info of external dependencies needed for building certain targets.

    bazel clean --expunge
bazel build --nobuild --experimental_repository_resolved_file=resolved.bzl //foo:bar

  2. To fetch info of all external dependencies defined in the WORKSPACE file.

    bazel clean --expunge
bazel sync --experimental_repository_resolved_file=resolved.bzl

    With the bazel sync command, you can fetch all dependencies defined in the WORKSPACE file, which include:

    • bind usages
    • register_toolchains & register_execution_platforms usages

    However, if your project is cross platforms, bazel sync may break on certain platforms because some repository rules may only run correctly on supported platforms.

After running the command, you should have information of your external dependencies in the resolved.bzl file.

Inspect external dependency with bazel query

You may also know bazel query can be used for inspecting repository rules with

bazel query --output=build //external:<repo name>

While it is more convenient and much faster, bazel query can lie about external dependency version, so be careful using it! Querying and inspecting external dependencies with Bzlmod is going to achieved by a new subcommand.

Built-in default dependencies

If you check the file generated by --experimental_repository_resolved_file, you are going to find many dependencies that are not defined in your WORKSPACE. This is because Bazel in fact adds prefixes and suffixes to the user's WORKSPACE file content to inject some default dependencies, which are usually required by native rules (e.g. @bazel_tools, @platforms and @remote_java_tools). With Bzlmod, those dependencies are introduced with a built-in module bazel_tools , which is a default dependency for every other Bazel module.

Hybrid mode for gradual migration

Bzlmod and WORKSPACE can work side by side, which allows migrating dependencies from the WORKSPACE file to Bzlmod to be a gradual process.

WORKSPACE.bzlmod

During the migration, Bazel users may need to switch between builds with and without Bzlmod enabled. WORKSPACE.bzlmod support is implemented to make the process smoother.

WORKSPACE.bzlmod has the exact same syntax as WORKSPACE. When Bzlmod is enabled, if a WORKSPACE.bzlmod file also exists at the workspace root:

  • WORKSPACE.bzlmod takes effect and the content of WORKSPACE is ignored.
  • No prefixes or suffixes are added to the WORKSPACE.bzlmod file.

Using the WORKSPACE.bzlmod file can make the migration easier because:

  • When Bzlmod is disabled, you fall back to fetching dependencies from the original WORKSPACE file.
  • When Bzlmod is enabled, you can better track what dependencies are left to migrate with WORKSPACE.bzlmod.

Repository visibility

Bzlmod is able to control which other repositories are visible from a given repository, check repository names and strict deps for more details.

Here is a summary of repository visibilities from different types of repositories when also taking WORKSPACE into consideration.

From the main repo From Bazel module repos From module extension repos From WORKSPACE repos
The main repo Visible If the root module is a direct dependency If the root module is a direct dependency of the module hosting the module extension Visible
Bazel module repos Direct deps Direct deps Direct deps of the module hosting the module extension Direct deps of the root module
Module extension repos Direct deps Direct deps Direct deps of the module hosting the module extension + all repos generated by the same module extension Direct deps of the root module
WORKSPACE Repos All visible Not visible Not visible All visible

Migration process

A typical Bzlmod migration process can look like this:

  1. Understand what dependencies you have in WORKSPACE.
  2. Add an empty MODULE.bazel file at your project root.
  3. Add an empty WORKSPACE.bzlmod file to override the WORKSPACE file content.
  4. Build your targets with Bzlmod enabled and check which repository is missing.
  5. Check the definition of the missing repository in the resolved dependency file.
  6. Introduce the missing dependency as a Bazel module, through a module extension, or leave it in the WORKSPACE.bzlmod for later migration.
  7. Go back to 4 and repeat until all dependencies are available.

Migration tool

There is an interactive Bzlmod migration helper script that can get you started.

The script does the following things:

  • Generate and parse the WORKSPACE resolved file.
  • Print repository info from the resolved file in a human readable way.
  • Run bazel build command, detect recognized error messages, and recommend a way to migrate.
  • Check if a dependency is already available in the BCR.
  • Add a dependency to MODULE.bazel file.
  • Add a dependency through a module extension.
  • Add a dependency to WORKSPACE.bzlmod file.

To use it, make sure you have the latest Bazel release installed, and run the following command:

git clone https://github.com/bazelbuild/bazel-central-registry.git
cd <your workspace root>
<BCR repo root>/tools/migrate_to_bzlmod.py -t <your build targets>

Publish Bazel modules

If your Bazel project is a dependency for other projects, you can publish your project in the Bazel Central Registry.

To be able to check in your project in the BCR, you need a source archive URL of the project. Take note of a few things when creating the source archive:

  • Make sure the archive is pointing to a specific version.

    The BCR can only accept versioned source archives because Bzlmod needs to conduct version comparison during dependency resolution.

  • Make sure the archive URL is stable.

    Bazel verifies the content of the archive by a hash value, so you should make sure the checksum of the downloaded file never changes. If the URL is from GitHub, please create and upload a release archive in the release page. GitHub isn't going to guarantee the checksum of source archives generated on demand. In short, URLs in the form of https://github.com/<org>/<repo>/releases/download/... is considered stable while https://github.com/<org>/<repo>/archive/... is not. Check GitHub Archive Checksum Outage for more context.

  • Make sure the source tree follows the layout of the original repository.

    In case your repository is very large and you want to create a distribution archive with reduced size by stripping out unnecessary sources, please make sure the stripped source tree is a subset of the original source tree. This makes it easier for end users to override the module to a non-release version by archive_override and git_override.

  • Include a test module in a subdirectory that tests your most common APIs.

    A test module is a Bazel project with its own WORKSPACE and MODULE.bazel file located in a subdirectory of the source archive which depends on the actual module to be published. It should contain examples or some integration tests that cover your most common APIs. Check test module to learn how to set it up.

When you have your source archive URL ready, follow the BCR contribution guidelines to submit your module to the BCR with a GitHub Pull Request.

It is highly recommended to set up the Publish to BCR GitHub App for your repository to automate the process of submitting your module to the BCR.

Best practices

This section documents a few best practices you should follow for better managing your external dependencies.

Split targets into different packages to avoid fetching unnecessary dependencies.

Check #12835, where dev dependencies for tests are forced to be fetched unnecessarily for building targets that don't need them. This is actually not Bzlmod specific, but following this practices makes it easier to specify dev dependencies correctly.

Specify dev dependencies

You can set the dev_dependency attribute to true for bazel_dep and use_extension directives so that they don't propagate to dependent projects. As the root module, you can use the --ignore_dev_dependency flag to verify if your targets still build without dev dependencies and overrides.

Community migration progress

You can check the Bazel Central Registry to find out if your dependencies are already available. Otherwise feel free to join this GitHub discussion to upvote or post the dependencies that are blocking your migration.

Report issues

Please check the Bazel GitHub issue list for known Bzlmod issues. Feel free to file new issues or feature requests that can help unblock your migration!