Repository Rules

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This page covers how to create repository rules and provides examples for more details.

An external repository is a rule that can be used only in the WORKSPACE file and enables non-hermetic operation at the loading phase of Bazel. Each external repository rule creates its own workspace, with its own BUILD files and artifacts. They can be used to depend on third-party libraries (such as Maven packaged libraries) but also to generate BUILD files specific to the host Bazel is running on.

Repository rule creation

In a .bzl file, use the repository_rule function to create a new repository rule and store it in a global variable.

A custom repository rule can be used just like a native repository rule. It has a mandatory name attribute and every target present in its build files can be referred as @<name>//package:target where <name> is the value of the name attribute.

The rule is loaded when you explicitly build it, or if it is a dependency of the build. In this case, Bazel will execute its implementation function. This function describe how to create the repository, its content and BUILD files.

Attributes

Attribute are rule arguments passed as a dict to the attrs rule argument. The attributes and their types are defined are listed when you define a repository rule. An example definining url and sha256 attributes as strings:

local_repository = repository_rule(
    implementation=_impl,
    local=True,
    attrs={
        "url": attr.string(mandatory=True)
        "sha256": attr.string(mandatory=True)
    }
)

To access an attribute within the implementation function, use repository_ctx.attr.<attribute_name>:

def _impl(repository_ctx):
    url = repository_ctx.attr.url
    checksum = repository_ctx.attr.sha256

All repository_rules have implicitly defined attributes (just like build rules). The two implicit attributes are name (just like for build rules) and repo_mapping. The name of a repository rule is accessible with repository_ctx.name. The meaning of repo_mapping is the same as for the native repository rules local_repository and new_local_repository.

If an attribute name starts with _ it is private and users cannot set it.

Implementation function

Every repository rule requires an implementation function. It contains the actual logic of the rule and is executed strictly in the Loading Phase.

The function has exactly one input parameter, repository_ctx. The function returns either None to signify that the rule is reproducible given the specified parameters, or a dict with a set of parameters for that rule that would turn that rule into a reproducible one generating the same repository. For example, for a rule tracking a git repository that would mean returning a specific commit identifier instead of a floating branch that was originally specified.

The input parameter repository_ctx can be used to access attribute values, and non-hermetic functions (finding a binary, executing a binary, creating a file in the repository or downloading a file from the Internet). See the library for more context. Example:

def _impl(repository_ctx):
  repository_ctx.symlink(repository_ctx.attr.path, "")

local_repository = repository_rule(
    implementation=_impl,
    ...)

When is the implementation function executed?

The implementation function of a repository is executed when Bazel needs a target from that repository, for example when another target (in another repository) depends on it or if it is mentioned on the command line. The implementation function is then expected to create the repository in the file system. This is called "fetching" the repository.

In contrast to regular targets, repositories are not necessarily re-fetched when something changes that would cause the repository to be different. This is because there are things that Bazel either cannot detect changes to or it would cause too much overhead on every build (for example, things that are fetched from the network). Therefore, repositories are re-fetched only if one of the following things changes:

  • The parameters passed to the declaration of the repository in the WORKSPACE file.
  • The Starlark code comprising the implementation of the repository.
  • The value of any environment variable passed to repository_ctx's getenv() method or declared with the environ attribute of the repository_rule. The values of these environment variables can be hard-wired on the command line with the --repo_env flag.
  • The content of any file passed to the read(), execute() and similar methods of repository_ctx which is referred to by a label (for example, //mypkg:label.txt but not mypkg/label.txt)
  • When bazel sync is executed.

There are two parameters of repository_rule that control when the repositories are re-fetched:

  • If the configure flag is set, the repository is only re-fetched on bazel sync when the--configure parameter is passed to it (if the attribute is unset, this command will not cause a re-fetch)

  • If the local flag is set, in addition to the above cases, the repository is also re-fetched when the Bazel server restarts or when any file that affects the declaration of the repository changes (e.g. the WORKSPACE file or a file it loads), regardless of whether the changes resulted in a change to the declaration of the repository or its code.

    Non-local repositories are not re-fetched in these cases. This is because these repositories are assumed to talk to the network or be otherwise expensive.

Restarting the implementation function

The implementation function can be restarted while a repository is being fetched if a dependency it requests is missing. In that case, the execution of the implementation function will stop, the missing dependency is resolved and the function will be re-executed after the dependency has been resolved. To avoid unnecessary restarts (which are expensive, as network access might have to be repeated), label arguments are prefetched, provided all label arguments can be resolved to an existing file. Note that resolving a path from a string or a label that was constructed only during execution of the function might still cause a restart.

Forcing refetch of external repositories

Sometimes, an external repository can become outdated without any change to its definition or dependencies. For example, a repository fetching sources might follow a particular branch of a third-party repository, and new commits are available on that branch. In this case, you can ask bazel to refetch all external repositories unconditionally by calling bazel sync.

Moreover, some rules inspect the local machine and might become outdated if the local machine was upgraded. Here you can ask bazel to only refetch those external repositories where the repository_rule definition has the configure attribute set, use bazel sync --configure.

Examples

  • C++ auto-configured toolchain: it uses a repository rule to automatically create the C++ configuration files for Bazel by looking for the local C++ compiler, the environment and the flags the C++ compiler supports.

  • Go repositories uses several repository_rule to defines the list of dependencies needed to use the Go rules.

  • rules_jvm_external creates an external repository called @maven by default that generates build targets for every Maven artifact in the transitive dependency tree.