Bazel modules

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A Bazel module is a Bazel project that can have multiple versions, each of which publishes metadata about other modules that it depends on. This is analogous to familiar concepts in other dependency management systems, such as a Maven artifact, an npm package, a Go module, or a Cargo crate.

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 direct dependencies, and other information. For a basic example:

module(name = "my-module", version = "1.0")

bazel_dep(name = "rules_cc", version = "0.0.1")
bazel_dep(name = "protobuf", version = "3.19.0")

To perform module resolution, Bazel starts by reading the root module's MODULE.bazel file, and then repeatedly requests any dependency's MODULE.bazel file from a Bazel registry until it discovers the entire dependency graph.

By default, Bazel then selects one version of each module to use. Bazel represents each module with a repo, and consults the registry again to learn how to define each of the repos.

Version format

Bazel has a diverse ecosystem and projects use various versioning schemes. The most popular by far is SemVer, but there are also prominent projects using different schemes such as Abseil, whose versions are date-based, for example 20210324.2).

For this reason, Bzlmod adopts a more relaxed version of the SemVer spec. The differences include:

  • SemVer prescribes that the "release" part of the version must consist of 3 segments: MAJOR.MINOR.PATCH. In Bazel, this requirement is loosened so that any number of segments is allowed.
  • In SemVer, each of the segments in the "release" part must be digits only. In Bazel, this is loosened to allow letters too, and the comparison semantics match the "identifiers" in the "prerelease" part.
  • Additionally, the semantics of major, minor, and patch version increases are not enforced. However, see compatibility level for details on how we denote backwards compatibility.

Any valid SemVer version is a valid Bazel module version. Additionally, two SemVer versions a and b compare a < b if and only if the same holds when they're compared as Bazel module versions.

Version selection

Consider the diamond dependency problem, a staple in the versioned dependency management space. Suppose you have the dependency graph:

       A 1.0
      /     \
   B 1.0    C 1.1
     |        |
   D 1.0    D 1.1

Which version of D should be used? To resolve this question, Bzlmod uses the Minimal Version Selection (MVS) algorithm introduced in the Go module system. MVS assumes that all new versions of a module are backwards compatible, and so picks the highest version specified by any dependent (D 1.1 in our example). It's called "minimal" because D 1.1 is the earliest version that could satisfy our requirements — even if D 1.2 or newer exists, we don't select them. Using MVS creates a version selection process that is high-fidelity and reproducible.

Yanked versions

The registry can declare certain versions as yanked if they should be avoided (such as for security vulnerabilities). Bazel throws an error when selecting a yanked version of a module. To fix this error, either upgrade to a newer, non-yanked version, or use the --allow_yanked_versions flag to explicitly allow the yanked version.

Compatibility level

In Go, MVS's assumption about backwards compatibility works because it treats backwards incompatible versions of a module as a separate module. In terms of SemVer, that means A 1.x and A 2.x are considered distinct modules, and can coexist in the resolved dependency graph. This is, in turn, made possible by encoding the major version in the package path in Go, so there aren't any compile-time or linking-time conflicts.

Bazel, however, cannot provide such guarantees, so it needs the "major version" number in order to detect backwards incompatible versions. This number is called the compatibility level, and is specified by each module version in its module() directive. With this information, Bazel can throw an error when it detects that versions of the same module with different compatibility levels exist in the resolved dependency graph.

Overrides

Specify overrides in the MODULE.bazel file to alter the behavior of Bazel module resolution. Only the root module's overrides take effect — if a module is used as a dependency, its overrides are ignored.

Each override is specified for a certain module name, affecting all of its versions in the dependency graph. Although only the root module's overrides take effect, they can be for transitive dependencies that the root module does not directly depend on.

Single-version override

The single_version_override serves multiple purposes:

  • With the version attribute, you can pin a dependency to a specific version, regardless of which versions of the dependency are requested in the dependency graph.
  • With the registry attribute, you can force this dependency to come from a specific registry, instead of following the normal registry selection process.
  • With the patch* attributes, you can specify a set of patches to apply to the downloaded module.

These attributes are all optional and can be mixed and matched with each other.

Multiple-version override

A multiple_version_override can be specified to allow multiple versions of the same module to coexist in the resolved dependency graph.

You can specify an explicit list of allowed versions for the module, which must all be present in the dependency graph before resolution — there must exist some transitive dependency depending on each allowed version. After resolution, only the allowed versions of the module remain, while Bazel upgrades other versions of the module to the nearest higher allowed version at the same compatibility level. If no higher allowed version at the same compatibility level exists, Bazel throws an error.

For example, if versions 1.1, 1.3, 1.5, 1.7, and 2.0 exist in the dependency graph before resolution and the major version is the compatibility level:

  • A multiple-version override allowing 1.3, 1.7, and 2.0 results in 1.1 being upgraded to 1.3, 1.5 being upgraded to 1.7, and other versions remaining the same.
  • A multiple-version override allowing 1.5 and 2.0 results in an error, as 1.7 has no higher version at the same compatibility level to upgrade to.
  • A multiple-version override allowing 1.9 and 2.0 results in an error, as 1.9 is not present in the dependency graph before resolution.

Additionally, users can also override the registry using the registry attribute, similarly to single-version overrides.

Non-registry overrides

Non-registry overrides completely remove a module from version resolution. Bazel does not request these MODULE.bazel files from a registry, but instead from the repo itself.

Bazel supports the following non-registry overrides:

Repository names and strict deps

The canonical name of a repo backing a module is module_name~version (for example, bazel_skylib~1.0.3). For modules with a non-registry override, replace the version part with the string override. Note that the canonical name format is not an API you should depend on and is subject to change at any time.

The apparent name of a repo backing a module to its direct dependents defaults to its module name, unless the repo_name attribute of the bazel_dep directive says otherwise. Note that this means a module can only find its direct dependencies. This helps prevent accidental breakages due to changes in transitive dependencies.

Module extensions can also introduce additional repos into the visible scope of a module.