Configurations
Starlark configuration is Bazel’s API for customizing how your project builds.
This makes it possible to:
- define custom flags for your project, obsoleting the need for
--define
-
write transitions to configure deps in different configurations than their parents (e.g.
--compilation_mode=opt
or--cpu=arm
) - bake better defaults into rules (e.g. automatically build
//my:android_app
with a specified SDK)
and more, all completely from .bzl files (no Bazel release required). See the
bazelbuild/examples
repo for
examples.
Current status
Ongoing bug/feature work can be found in the Bazel configurability roadmap.
This feature may have memory and performance impacts and we are still working on ways to measure and mitigate those impacts.
User-defined build settings
A build setting is a single piece of
configuration
information. Think of a configuration as a key/value map. Setting --cpu=ppc
and --copt="-DFoo"
produces a configuration that looks like
{cpu: ppc, copt: "-DFoo"}
. Each entry is a build setting.
Traditional flags like cpu
and copt
are native settings i.e.
their keys are defined and their values are set inside native bazel java code.
Bazel users can only read and write them via the command line
and other APIs maintained natively. Changing native flags, and the APIs
that expose them, requires a bazel release. User-defined build
settings are defined in .bzl
files (and thus, don’t need a bazel release to
register changes). They also can be set via the command line
(if they’re designated as flags
, see more below), but can also be
set via user-defined transitions.
Defining build settings
The build_setting
rule()
parameter
Build settings are rules like any other rule and are differentiated using the
Starlark rule()
function’s build_setting
attribute.
# example/buildsettings/build_settings.bzl
string_flag = rule(
implementation = _impl,
build_setting = config.string(flag = True)
)
The build_setting
attribute takes a function that designates the type of the
build setting. The type is limited to a set of basic Starlark types like
bool
and string
. See the config
module
documentation for details. More complicated typing can be done in the rule’s
implementation function. More on this below.
The config
function also takes an optional boolean parameter, flag
, which is
set to false by default. if flag
is set to true, the build setting can be set
on the command line by users as well as internally by rule writers via default
values and
transitions.
Not all settings should be settable by users. For example if you as a rule
writer have some debug mode that you’d like to turn on inside test rules,
you don’t want to give users the ability to indiscriminately turn on that
feature inside other non-test rules.
Using ctx.build_setting_value
Like all rules, build setting rules have implementation functions.
The basic Starlark-type value of the build settings can be accessed via the
ctx.build_setting_value
method. This method is only available to
ctx
objects of build setting rules. These implementation methods can directly
forward the build settings value or do additional work on it, like type checking
or more complex struct creation. Here’s how you would implement an enum
-typed
build setting:
# example/buildsettings/build_settings.bzl
TemperatureProvider = provider(fields = ['type'])
temperatures = ["HOT", "LUKEWARM", "ICED"]
def _impl(ctx):
raw_temperature = ctx.build_setting_value
if raw_temperature not in temperatures:
fail(str(ctx.label) + " build setting allowed to take values {"
+ ", ".join(temperatures) + "} but was set to unallowed value "
+ raw_temperature)
return TemperatureProvider(type = raw_temperature)
temperature = rule(
implementation = _impl,
build_setting = config.string(flag = True)
)
Note: if a rule depends on a build setting, it will receive whatever providers the build setting implementation function returns, like any other dependency. But all other references to the value of the build setting (e.g. in transitions) will see its basic Starlark-typed value, not this post implementation function value.
Instantiating Build Settings
Rules defined with the build_setting
parameter have an implicit mandatory
build_setting_default
attribute. This attribute takes on the same type as
declared by the build_setting
param.
# example/buildsettings/build_settings.bzl
FlavorProvider = provider(fields = ['type'])
def _impl(ctx):
return FlavorProvider(type = ctx.build_setting_value)
flavor = rule(
implementation = _impl,
build_setting = config.string(flag = True)
)
# example/buildsettings/BUILD
load("//example/buildsettings:build_settings.bzl", "flavor")
flavor(
name = "favorite_flavor",
build_setting_default = "APPLE"
)
Predefined settings
The Skylib library includes a set of predefined settings you can instantiate without having to write custom Starlark.
For example, to define a setting that accepts a limited set of string values:
# example/BUILD
load("@bazel_skylib//rules:common_settings.bzl", "string_flag")
string_flag(
name = "myflag",
values = ["a", "b", "c"],
build_setting_default = "a",
)
For a complete list, see Common build setting rules.
Using build settings
Depending on build settings
If a target would like to read a piece of configuration information, it can directly depend on the build setting via a regular attribute dependency.
# example/rules.bzl
load("//example/buildsettings:build_settings.bzl", "FlavorProvider")
def _rule_impl(ctx):
if ctx.attr.flavor[FlavorProvider].type == "ORANGE":
...
drink_rule = rule(
implementation = _rule_impl,
attrs = {
"flavor": attr.label()
}
)
# example/BUILD
load("//example:rules.bzl", "drink_rule")
load("//example/buildsettings:build_settings.bzl", "flavor")
flavor(
name = "favorite_flavor",
build_setting_default = "APPLE"
)
drink_rule(
name = "my_drink",
flavor = ":favorite_flavor",
)
Languages may wish to create a canonical set of build settings which all rules
for that language depend on. Though the native concept of fragments
no longer
exists as a hardcoded object in Starlark configuration world, one way to
translate this concept would be to use sets of common implicit attributes. For
example:
# kotlin/rules.bzl
_KOTLIN_CONFIG = {
"_compiler": attr.label(default = "//kotlin/config:compiler-flag"),
"_mode": attr.label(default = "//kotlin/config:mode-flag"),
...
}
...
kotlin_library = rule(
implementation = _rule_impl,
attrs = dicts.add({
"library-attr": attr.string()
}, _KOTLIN_CONFIG)
)
kotlin_binary = rule(
implementation = _binary_impl,
attrs = dicts.add({
"binary-attr": attr.label()
}, _KOTLIN_CONFIG)
Settings Build Settings on the command line
Build settings are set on the command line like any other flag. Boolean build settings understand no-prefixes and both equals and space syntaxes are supported. The name of build settings is their full target path:
$ bazel build //my/target --//example:favorite_flavor="PAMPLEMOUSSE"
There are plans to implement shorthand mapping of flag labels so users don’t need to use their entire target path each time i.e.:
$ bazel build //my/target --cpu=k8 --noboolean_flag
instead of
$ bazel build //my/target --//third_party/bazel/src/main:cpu=k8 --no//my/project:boolean_flag
Label-typed build settings
Unlike other build settings, label-typed settings cannot be defined using the
build_setting
rule parameter. Instead, bazel has two built-in rules:
label_flag
and label_setting
. These rules forward the providers of the
actual target to which the build setting is set. label_flag
and
label_setting
can be read/written by transitions and label_flag
can be set
by the user like other build_setting
rules can. Their only difference is they
can’t customely defined.
Label-typed settings will eventually replace the functionality of late-bound
defaults. Late-bound default attributes are Label-typed attributes whose
final values can be affected by configuration. In Starlark, this will replace
the configuration_field
API.
# example/rules.bzl
MyProvider = provider(fields = ["my_field"])
def _dep_impl(ctx):
return MyProvider(my_field = "yeehaw")
dep_rule = rule(
implementation = _dep_impl
)
def _parent_impl(ctx):
if ctx.attr.my_field_provider[MyProvider].my_field == "cowabunga":
...
parent_rule = rule(
implementation = _parent_impl,
attrs = { "my_field_provider": attr.label() }
)
# example/BUILD
load("//example:rules.bzl", "dep_rule", "parent_rule")
dep_rule(name = "dep")
parent_rule(name = "parent", my_field_provider = ":my_field_provider")
label_flag(
name = "my_field_provider",
build_setting_default = ":dep"
)
TODO(bazel-team): Expand supported build setting types.
Build settings and select()
Users can configure attributes on build settings by using
select()
. Build setting targets can be passed to the flag_values
attribute of
config_setting
. The value to match to the configuration is passed as a
String
then parsed to the type of the build setting for matching.
config_setting(
name = "my_config",
flag_values = {
"//example:favorite_flavor": "MANGO"
}
)
User-defined transitions
A configuration transition is how we change configuration of configured targets in the build graph.
IMPORTANT: In order to use Starlark transitions, you need to attach a special attribute to the rule to which the transition is attached:
"_whitelist_function_transition": attr.label( default = "@bazel_tools//tools/whitelists/function_transition_whitelist" )
By adding transitions you can pretty easily explode the size of your build graph. This sets an allowlist on the packages in which you can create targets of this rule. The default value in the codeblock above allowlists everything. But if you’d like to restrict who is using your rule, you can set that attribute to point to your own custom allowlist. Contact bazel-discuss@googlegroups.com if you’d like advice or assistance understanding how transitions can affect on your build performance.
Defining
Transitions define configuration changes between rules. For example, a request like “compile my dependency for a different CPU than its parent” is handled by a transition.
Formally, a transition is a function from an input configuration to one or more
output configurations. Most transitions are 1:1 e.g. “override the input
configuration with --cpu=ppc
”. 1:2+ transitions can also exist but come
with special restrictions.
In Starlark, transitions are defined much like rules, with a defining
transition()
function
and an implementation function.
# example/transitions/transitions.bzl
def _impl(settings, attr):
_ignore = (settings, attr)
return {"//example:favorite_flavor" : "MINT"}
hot_chocolate_transition = transition(
implementation = _impl,
inputs = [],
outputs = ["//example:favorite_flavor"]
)
The transition()
function takes in an implementation function, a set of
build settings to read(inputs
), and a set of build settings to write
(outputs
). The implementation function has two parameters, settings
and
attr
. settings
is a dictionary {String
:Object
} of all settings declared
in the inputs
parameter to transition()
.
attr
is a dictionary of attributes and values of the rule to which the
transition is attached. When attached as an
outgoing edge transition, the values of these
attributes are all configured i.e. post-select() resolution. When attached as
an incoming edge transition, attr
does not
include any attributes that use a selector to resolve their value. If an
incoming edge transition on --foo
reads attribute bar
and then also
selects on --foo
to set attribute bar
, then there’s a chance for the
incoming edge transition to read the wrong value of bar
in the transition.
Note: since transitions are attached to rule definitions and select()
s are
attached to rule instantiations (i.e. targets), errors related to select()
s on
read attributes will pop up when users create targets rather than when rules are
written. It may be worth taking extra care to communicate to rule users which
attributes they should be wary of selecting on or taking other precautions.
The implementation function must return a dictionary (or list of
dictionaries, in the case of
transitions with multiple output configurations)
of new build settings values to apply. The returned dictionary keyset(s) must
contain exactly the set of build settings passed to the outputs
parameter of the transition function. This is true even if a build setting is
not actually changed over the course of the transition - its original value must
be explicitly passed through in the returned dictionary.
Defining 1:2+ transitions
Outgoing edge transition can map a single input configuration to two or more output configurations. These are defined in Starlark by returning a list of dictionaries in the transition implementation function.
# example/transitions/transitions.bzl
def _impl(settings, attr):
_ignore = (settings, attr)
return [
{"//example:favorite_flavor" : "LATTE"},
{"//example:favorite_flavor" : "MOCHA"},
]
coffee_transition = transition(
implementation = _impl,
inputs = [],
outputs = ["//example:favorite_flavor"]
)
Attaching transitions
Transitions can be attached in two places: incoming edges and outgoing edges. Effectively this means rules can transition their own configuration (incoming edge transition) and transition their dependencies’ configurations (outgoing edge transition).
NOTE: There is currently no way to attach Starlark transitions to native rules. If you need to do this, contact bazel-discuss@googlegroups.com and we can help you try to figure out a workaround.
Incoming edge transitions
Incoming edge transitions are activated by attaching a transition
object
(created by transition()
) to rule()
’s cfg
parameter:
# example/rules.bzl
load("example/transitions:transitions.bzl", "hot_chocolate_transition")
drink_rule = rule(
implementation = _impl,
cfg = hot_chocolate_transition,
...
Incoming edge transitions must be 1:1 transitions.
Outgoing edge transitions
Outgoing edge transitions are activated by attaching a transition
object
(created by transition()
) to an attribute’s cfg
parameter:
# example/rules.bzl
load("example/transitions:transitions.bzl", "coffee_transition")
drink_rule = rule(
implementation = _impl,
attrs = { "dep": attr.label(cfg = coffee_transition)}
...
Outgoing edge transitions can be 1:1 or 1:2+.
Transitions on native options
WARNING: Long term, we plan to reimplement all native options as build settings. When that happens, this syntax will be deprecated. Currently other issues are blocking that migration but be aware you may have to migrate your transitions at some point in the future.
Starlark transitions can also declare reads and writes on native options via a special prefix to the option name.
# example/transitions/transitions.bzl
def _impl(settings, attr):
_ignore = (settings, attr)
return {"//command_line_option:cpu": "k8"}
cpu_transition = transition(
implementation = _impl,
inputs = [],
outputs = ["//command_line_option:cpu"]
NOTE: Transitioning on –define using “//command_line_option:define” is not supported - create a custom build setting to cover this functionality.
Accessing attributes with transitions
When attaching a transition to an outgoing edge
(regardless of whether the transition is a 1:1 or 1:2+ transition) access to
values of that attribute in the rule implementation changes. Access through
ctx.attr
is forced to be a list if it isn’t already. The order of elements in
this list is unspecified.
# example/transitions/rules.bzl
def _transition_impl(settings, attr):
return {"//example:favorite_flavor" : "LATTE"},
coffee_transition = transition(
implementation = _transition_impl,
inputs = [],
outputs = ["//example:favorite_flavor"]
)
def _rule_impl(ctx):
# Note: List access even though "dep" is not declared as list
transitioned_dep = ctx.attr.dep[0]
# Note: Access doesn't change, other_deps was already a list
for other dep in ctx.attr.other_deps:
# ...
coffee_rule = rule(
implementation = _rule_impl,
attrs = {
"dep": attr.label(cfg = coffee_transition)
"other_deps": attr.label_list(cfg = coffee_transition)
})
Access to the value of a single branch of a 1:2+ has not been implemented yet.
Integration with platforms and toolchains
Many native flags today, like --cpu
and --crosstool_top
are related to
toolchain resolution. In the future, explicit transitions on these types of
flags will likely be replaced by transitioning on the
target platform
Also see
- Starlark Build Configuration
- Bazel Configurability Roadmap
- Full set of end to end examples
Memory and performance considerations
Adding transitions, and therefore new configurations, to your build comes at a cost: larger build graphs, less comprehensible build graphs, and slower builds. It’s worth considering these costs when considering using transitions in your build rules. Below is an example of how a transition might create exponential growth of your build graph.
Badly behaved builds: a case study
Say you have the following target structure:
Building //pkg:app
requires \(2n+2\) targets:
//pkg:app
//pkg:dep
//pkg:i_0
and//pkg:i_1
for \(i\) in \([1..n]\)
Imagine you implement) a flag
--//foo:owner=<STRING>
and //pkg:i_b
applies
depConfig = myConfig + depConfig.owner="$(myConfig.owner)$(b)"
In other words, //pkg:i_b
appends b
to the old value of --owner
for all
its deps.
This produces the following configured targets:
//pkg:app //foo:owner=""
//pkg:1_0 //foo:owner=""
//pkg:1_1 //foo:owner=""
//pkg:2_0 (via //pkg:1_0) //foo:owner="0"
//pkg:2_0 (via //pkg:1_1) //foo:owner="1"
//pkg:2_1 (via //pkg:1_0) //foo:owner="0"
//pkg:2_1 (via //pkg:1_1) //foo:owner="1"
//pkg:3_0 (via //pkg:1_0 → //pkg:2_0) //foo:owner="00"
//pkg:3_0 (via //pkg:1_0 → //pkg:2_1) //foo:owner="01"
//pkg:3_0 (via //pkg:1_1 → //pkg:2_0) //foo:owner="10"
//pkg:3_0 (via //pkg:1_1 → //pkg:2_1) //foo:owner="11"
...
//pkg:dep
produces \(2^n\) configured targets: config.owner=
“\(b_0b_1…b_n\)” for all \(b_i\) in \({0,1}\).
This makes the build graph exponentially larger than the target graph, with corresponding memory and performance consequences.
TODO: Add strategies for measurement and mitigation of these issues.