---
layout: documentation
title: Extension Overview
---

# Extension Overview

<!-- [TOC] -->

## Loading an extension

Bazel extensions are files ending in `.bzl`. Use the `load` statement to
import a symbol from an extension.

```python
load("//build_tools/rules:maprule.bzl", "maprule")
```

This code will load the file `build_tools/rules/maprule.bzl` and add the
`maprule` symbol to the environment. This can be used to load new rules,
functions or constants (e.g. a string, a list, etc.). Multiple symbols can be
imported by using additional arguments to the call to `load`. Arguments must
be string literals (no variable) and `load` statements must appear at
top-level, i.e. they cannot be in a function body.

The first argument of `load` is a [label](https://docs.bazel.build/versions/master/build-ref.html#labels)
identifying a `.bzl` file. If it is a relative label, it is resolved with
respect to the package (not directory) containing the current `.bzl` file.
Relative labels in `load` statements should use a leading `:`.

`load` also supports aliases, i.e. you can assign different names to the
imported symbols.

```python
load("//build_tools/rules:maprule.bzl", maprule_alias = "maprule")
```

You can define multiple aliases within one `load` statement. Moreover, the
argument list can contain both aliases and regular symbol names. The following
example is perfectly legal (please note when to use quotation marks).

```python
load(":my_rules.bzl", "some_rule", nice_alias = "some_other_rule")
```

In a `.bzl` file, symbols starting with `_` are not exported and cannot be loaded
from another file. Visibility doesn't affect loading (yet): you don't need to
use `exports_files` to make a `.bzl` file visible.

## Macros and rules

A [macro](macros.md) 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](rules.md) 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: we expect that most
people will never have to write one, they will only load and call existing
rules.

## Evaluation model

A build consists of three phases.

* **Loading phase**. First, we 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, e.g. "run gcc on hello.c and get
  hello.o". It is important to note that we have to 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 we don't need for the current build,
they will not be executed.