Loading an extension
Bazel extensions are files ending in
.bzl. Use the
load statement to
import a symbol from an extension.
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
.bzl file. If it is a relative label, it is resolved with
respect to the package (not directory) containing the current
Relative labels in
load statements should use a leading
load also supports aliases, i.e. you can assign different names to the
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).
load(":my_rules.bzl", "some_rule", nice_alias = "some_other_rule")
.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
exports_files to make a
.bzl file visible.
Macros and rules
A macro 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
A rule 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.
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
implementationfunction), 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
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 (
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.