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
WORKSPACE file and enables non-hermetic operation at the loading phase
of Bazel. Each external repository rule creates its own workspace, with its
BUILD files and artifacts. They can be used to depend on third-party
libraries (such as Maven packaged libraries) but also to generate
specific to the host Bazel is running on.
Repository rule creation
.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> is the value of the
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
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
sha256 attributes as
local_repository = repository_rule(
To access an attribute within the implementation function, use
url = repository_ctx.attr.url
checksum = repository_ctx.attr.sha256
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
If an attribute name starts with
_ it is private and users cannot set it.
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
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
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
local_repository = repository_rule(
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
- The Starlark code comprising the implementation of the repository.
- The value of any environment variable declared with the
environattribute of the
repository_rule. The values of these environment variables can be hard-wired on the command line with the
--action_envflag (but this flag will invalidate every action of the build).
- The content of any file passed to the
execute()and similar methods of
repository_ctxwhich is referred to by a label (for example,
bazel syncis executed.
There are two parameters of
repository_rule that control when the repositories
- If the
configureflag is set, the repository is only re-fetched on
bazel syncwhen the
--configureparameter is passed to it (if the attribute is unset, this command will not cause a re-fetch)
localflag 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
WORKSPACEfile 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
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
definition has the
configure attribute set, use
bazel sync --configure.
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_ruleto defines the list of dependencies needed to use the Go rules.
rules_jvm_external creates an external repository called
@mavenby default that generates build targets for every Maven artifact in the transitive dependency tree.