Frequently asked questions

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This page answers some frequently asked questions about external dependencies in Bazel.

MODULE.bazel

Why does MODULE.bazel not support loads?

During dependency resolution, the MODULE.bazel file of all referenced external dependencies are fetched from registries. At this stage, the source archives of the dependencies are not fetched yet; so if the MODULE.bazel file loads another file, there is no way for Bazel to actually fetch that file without fetching the entire source archive. Note the MODULE.bazel file itself is special, as it's directly hosted on the registry.

There are a few use cases that people asking for loads in MODULE.bazel are generally interested in, and they can be solved without loads:

• Ensuring that the version listed in MODULE.bazel is consistent with build metadata stored elsewhere, for example in a .bzl file: This can be achieved by using the native.module_version method in a .bzl file loaded from a BUILD file.
• Splitting up a very large MODULE.bazel file into manageable sections, particularly for monorepos: The root module can use the include directive to split its MODULE.bazel file into multiple segments. For the same reason we don't allow loads in MODULE.bazel files, include cannot be used in non-root modules.
• Users of the old WORKSPACE system might remember declaring a repo, and then immediately loading from that repo to perform complex logic. This capability has been replaced by module extensions.

Can I specify a SemVer range for a bazel_dep?

No. Some other package managers like npm and Cargo support version ranges (implicitly or explicitly), and this often requires a constraint solver (making the output harder to predict for users) and makes version resolution nonreproducible without a lockfile.

Bazel instead uses Minimal Version Selection like Go, which in contrast makes the output easy to predict and guarantees reproducibility. This is a tradeoff that matches Bazel's design goals.

Furthermore, Bazel module versions are a superset of SemVer, so what makes sense in a strict SemVer environment doesn't always carry over to Bazel module versions.

Can I automatically get the latest version for a bazel_dep?

Some users occasionally ask for the ability to specify bazel_dep(name = "foo", version = "latest") to automatically get the latest version of a dep. This is similar to the question about SemVer ranges, and the answer is also no.

The recommended solution here is to have automation take care of this. For example, Renovate supports Bazel modules.

Sometimes, users asking this question are really looking for a way to quickly iterate during local development. This can be achieved by using a local_path_override.

Why all these use_repos?

Module extension usages in MODULE.bazel files sometimes come with a big use_repo directive. For example, a typical usage of the go_deps extension from gazelle might look like:

go_deps = use_extension("@gazelle//:extensions.bzl", "go_deps")
go_deps.from_file(go_mod = "//:go.mod")
use_repo(
    go_deps,
    "com_github_gogo_protobuf",
    "com_github_golang_mock",
    "com_github_golang_protobuf",
    "org_golang_x_net",
    ...  # potentially dozens of lines...
)

The long use_repo directive may seem redundant, since the information is arguably already in the referenced go.mod file.

The reason Bazel needs this use_repo directive is that it runs module extensions lazily. That is, a module extension is only run if its result is observed. Since a module extension's "output" is repo definitions, this means that we only run a module extension if a repo it defines is requested (for instance, if the target @org_golang_x_net//:foo is built, in the example above). However, we don't know which repos a module extension would define until after we run it. This is where the use_repo directive comes in; the user can tell Bazel which repos they expect the extension to generate, and Bazel would then only run the extension when these specific repos are used.

To help the maintain this use_repo directive, a module extension can return an extension_metadata object from its implementation function. The user can run the bazel mod tidy command to update the use_repo directives for these module extensions.

Bzlmod migration

Which is evaluated first, MODULE.bazel or WORKSPACE?

When both --enable_bzlmod and --enable_workspace are set, it's natural to wonder which system is consulted first. The short answer is that MODULE.bazel (Bzlmod) is evaluated first.

The long answer is that "which evaluates first" is not the right question to ask; rather, the right question to ask is: in the context of the repo with canonical name @@foo, what does the apparent repo name @bar resolve to? Alternatively, what is the repo mapping of @@base?

Labels with apparent repo names (a single leading @) can refer to different things based on the context they're resolved from. When you see a label @bar//:baz and wonder what it actually points to, you need to first find out what the context repo is: for example, if the label is in a BUILD file located in the repo @@foo, then the context repo is @@foo.

Then, depending on what the context repo is, the "repository visibility" table in the migration guide can be used to find out which repo an apparent name actually resolves to.

• If the context repo is the main repo (@@):
  1. If bar is an apparent repo name introduced by the root module's MODULE.bazel file (through any of bazel_dep, use_repo, module, use_repo_rule), then @bar resolves to what that MODULE.bazel file claims.
  2. Otherwise, if bar is a repo defined in WORKSPACE (which means that its canonical name is @@bar), then @bar resolves to @@bar.
  3. Otherwise, @bar resolves to something like @@[unknown repo 'bar' requested from @@], and this will ultimately result in an error.
• If the context repo is a Bzlmod-world repo (that is, it corresponds to a non-root Bazel module, or is generated by a module extension), then it will only ever see other Bzlmod-world repos, and no WORKSPACE-world repos.
  • Notably, this includes any repos introduced in a non_module_deps-like module extension in the root module, or use_repo_rule instantiations in the root module.
• If the context repo is defined in WORKSPACE:
  1. First, check if the context repo definition has the magical repo_mapping attribute. If so, go through the mapping first (so for a repo defined with repo_mapping = {"@bar": "@baz"}, we would be looking at @baz below).
  2. If bar is an apparent repo name introduced by the root module's MODULE.bazel file, then @bar resolves to what that MODULE.bazel file claims. (This is the same as item 1 in the main repo case.)
  3. Otherwise, @bar resolves to @@bar. This most likely will point to a repo bar defined in WORKSPACE; if such a repo is not defined, Bazel will throw an error.

For a more succinct version:

• Bzlmod-world repos (excluding the main repo) will only see Bzlmod-world repos.
• WORKSPACE-world repos (including the main repo) will first see what the root module in the Bzlmod world defines, then fall back to seeing WORKSPACE-world repos.

Of note, labels in the Bazel command line (including Starlark flags, label-typed flag values, and build/test target patterns) are treated as having the main repo as the context repo.

Other

How do I prepare and run an offline build?

Use the bazel fetch command to prefetch repos. You can use the --repo flag (like bazel fetch --repo @foo) to fetch only the repo @foo (resolved in the context of the main repo, see question above), or use a target pattern (like bazel fetch @foo//:bar) to fetch all transitive dependencies of @foo//:bar (this is equivalent to bazel build --nobuild @foo//:bar).

The make sure no fetches happen during a build, use --nofetch. More precisely, this makes any attempt to run a non-local repository rule fail.

If you want to fetch repos and modify them to test locally, consider using the bazel vendor command.

How do I use HTTP proxies?

Bazel respects the http_proxy and HTTPS_PROXY environment variables commonly accepted by other programs, such as curl.

How do I make Bazel prefer IPv6 in dual-stack IPv4/IPv6 setups?

On IPv6-only machines, Bazel can download dependencies with no changes. However, on dual-stack IPv4/IPv6 machines Bazel follows the same convention as Java, preferring IPv4 if enabled. In some situations, for example when the IPv4 network cannot resolve/reach external addresses, this can cause Network unreachable exceptions and build failures. In these cases, you can override Bazel's behavior to prefer IPv6 by using the java.net.preferIPv6Addresses=true system property. Specifically:

• Use --host_jvm_args=-Djava.net.preferIPv6Addresses=true startup option, for example by adding the following line in your .bazelrc file:

  startup --host_jvm_args=-Djava.net.preferIPv6Addresses=true

• When running Java build targets that need to connect to the internet (such as for integration tests), use the --jvmopt=-Djava.net.preferIPv6Addresses=true tool flag. For example, include in your .bazelrc file:

  build --jvmopt=-Djava.net.preferIPv6Addresses

• If you are using rules_jvm_external for dependency version resolution, also add -Djava.net.preferIPv6Addresses=true to the COURSIER_OPTS environment variable to provide JVM options for Coursier.

Can repo rules be run remotely with remote execution?

No; or at least, not yet. Users employing remote execution services to speed up their builds may notice that repo rules are still run locally. For example, an http_archive would be first downloaded onto the local machine (using any local download cache if applicable), extracted, and then each source file would be uploaded to the remote execution service as an input file. It's natural to ask why the remote execution service doesn't just download and extract that archive, saving a useless roundtrip.

Part of the reason is that repo rules (and module extensions) are akin to "scripts" that are run by Bazel itself. A remote executor doesn't necessarily even have a Bazel installed.

Another reason is that Bazel often needs the BUILD files in the downloaded and extracted archives to perform loading and analysis, which are performed locally.

There are preliminary ideas to solve this problem by re-imagining repo rules as build rules, which would naturally allow them to be run remotely, but conversely raise new architectural concerns (for example, the query commands would potentially need to run actions, complicating their design).

For more previous discussion on this topic, see A way to support repositories that need Bazel for being fetched.