Project: /_project.yaml
Book: /_book.yaml

# Extension Overview

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This page describes how to extend the BUILD language using macros
and rules.

Bazel extensions are files ending in `.bzl`. Use a
[load statement](/concepts/build-files#load) to import a symbol from an extension.

Before learning the more advanced concepts, first:

* Read about the [Starlark language](/rules/language), used in both the
  `BUILD` and `.bzl` files.

* Learn how you can [share variables](/build/share-variables)
  between two `BUILD` files.

## Macros and rules

A [macro](/extending/macros) is a function that instantiates rules. It is useful when a
`BUILD` file is getting too repetitive or too complex, as it allows you to reuse
some code. The function is evaluated as soon as the `BUILD` file is read. After
the evaluation of the `BUILD` file, Bazel has little information about macros:
if your macro generates a `genrule`, Bazel will behave as if you wrote the
`genrule`. As a result, `bazel query` will only list the generated `genrule`.

A [rule](/extending/rules) is more powerful than a macro. It can access Bazel
internals and have full control over what is going on. It may for example pass
information to other rules.

If you want to reuse simple logic, start with a macro. If a macro becomes
complex, it is often a good idea to make it a rule. Support for a new language
is typically done with a rule. Rules are for advanced users, and most
users will never have to write one; they will only load and call existing
rules.

## Evaluation model {:#evaluation-model}

A build consists of three phases.

* **Loading phase**. First, load and evaluate all extensions and all `BUILD`
  files that are needed for the build. The execution of the `BUILD` files simply
  instantiates rules (each time a rule is called, it gets added to a graph).
  This is where macros are evaluated.

* **Analysis phase**. The code of the rules is executed (their `implementation`
  function), and actions are instantiated. An action describes how to generate
  a set of outputs from a set of inputs, such as "run gcc on hello.c and get
  hello.o". You must list explicitly which files will be generated before
  executing the actual commands. In other words, the analysis phase takes
  the graph generated by the loading phase and generates an action graph.

* **Execution phase**. Actions are executed, when at least one of their outputs is
  required. If a file is missing or if a command fails to generate one output,
  the build fails. Tests are also run during this phase.

Bazel uses parallelism to read, parse and evaluate the `.bzl` files and `BUILD`
files. A file is read at most once per build and the result of the evaluation is
cached and reused. A file is evaluated only once all its dependencies (`load()`
statements) have been resolved. By design, loading a `.bzl` file has no visible
side-effect, it only defines values and functions.

Bazel tries to be clever: it uses dependency analysis to know which files must
be loaded, which rules must be analyzed, and which actions must be executed. For
example, if a rule generates actions that you don't need for the current build,
they will not be executed.

## Creating extensions

* [Create your first macro](/rules/macro-tutorial) in order to
  reuse some code. Then [learn more about macros](/extending/macros) and
  [using them to create "custom verbs"](/rules/verbs-tutorial).

* [Follow the rules tutorial](/rules/rules-tutorial) to get started with rules.
  Next, you can read more about the [rules concepts](/extending/rules).

The two links below will be very useful when writing your own extensions. Keep
them within reach:

* The [API reference](/rules/lib)

* [Examples](https://github.com/bazelbuild/examples/tree/master/rules)

## Going further

In addition to [macros](/extending/macros) and [rules](/extending/rules), you may want to write
[aspects](/extending/aspects) and [repository rules](/extending/repo).

* Use [Buildifier](https://github.com/bazelbuild/buildtools){: .external}
  consistently to format and lint your code.

* Follow the [`.bzl` style guide](/rules/bzl-style).

* [Test](/rules/testing) your code.

* [Generate documentation](https://skydoc.bazel.build/) to help your users.

* [Optimize the performance](/rules/performance) of your code.

* [Deploy](/rules/deploying) your extensions to other people.