Introduction
At the core of BuildStream is a data model of Elements
which
are parsed from .bst
files in a project directory and configured from a few different
sources.
When BuildStream loads your project, various levels of composition occur, allowing configuration on various levels with different priority.
This page provides an introduction to the project directory structure, explains the basic directives supported inherently throughout the format, and outlines how composition occurs and what configurations are considered in which order.
The meaning of the various constructs expressed in the BuildStream format are covered in other sections of the documentation.
Directory structure
A BuildStream project is a directory consisting of:
A project configuration file
BuildStream element files
Optional user defined plugins
An optional project.refs file
A typical project structure may look like this:
myproject/project.conf
myproject/project.refs
myproject/elements/element1.bst
myproject/elements/element2.bst
myproject/elements/...
myproject/plugins/customelement.py
myproject/plugins/customelement.yaml
myproject/plugins/...
Except for the project configuration file, the user is allowed to structure their project directory in any way. For documentation on the format of the project configuration file, refer to the Project configuration documentation.
Simpler projects may choose to place all element definition files at the root of the project directory while more complex projects may decide to put stacks in one directory and other floating elements into other directories, perhaps placing deployment elements in another directory, this is all fine.
The important part to remember is that when you declare dependency relationships, a project relative path to the element one depends on must be provided.
Composition
Below are the various sources of configuration which go into an element or source in the order in which they are applied. Configurations which are applied later have a higher priority and override configurations which precede them.
1. Builtin defaults
The builtin defaults provide a set of builtin
default default values for project.conf
.
The project wide defaults defined in the builtin project configuration, such as the variables or environment sections, form the base configuration of all elements.
2. Project configuration
The project wide defaults specified in your
project.conf
are now applied on top of builtin defaults.
Defaults such as the variables or
environment which are specified in
your project.conf
override the builtin defaults for elements.
Note that plugin type specific configuration
in project.conf
is not applied until later.
3. Plugin defaults
Elements and Sources are all implemented as plugins.
Each Element plugin installs a .yaml
file along side their plugin to
define the default variables, environment and config. The config
is element specific and as such this is the first place where defaults
can be set on the config section.
The variables and environment specified in the declaring plugin’s
defaults here override the project configuration defaults for the given
element kind
.
Source plugins do not have a .yaml
file, and do not have variables or
environment.
4. Project configuration overrides
The project.conf
now gives you another opportunity to
override configuration on a per plugin basis.
Configurations specified in the elements or
sources sections of the project.conf
will override the given plugin’s defaults.
In this phase, it is possible to override any configurations of a given plugin, including configuration in element specific config sections.
See also Overriding plugin defaults
5. Plugin declarations
Finally, after having resolved any conditionals
in the parsing phase of loading element declarations; the configurations specified in a
.bst
file have the last word on any configuration in the data model.
Directives
(?) Conditionals
The (?)
directive allows expression of conditional statements which
test project option values.
The (?)
directive may appear as a key in any dictionary expressed
in YAML, and its value is a list of conditional expressions. Each conditional
expression must be a single key dictionary, where the key is the conditional
expression itself, and the value is a dictionary to be composited into the
parent dictionary containing the (?)
directive if the expression evaluates
to a truthy value.
Example:
variables:
prefix: "/usr"
enable-debug: False
(?):
- relocate == True:
prefix: "/opt"
- debug == True:
enable-debug: True
Expressions are evaluated in the specified order, and each time an expression evaluates to a truthy value, its value will be composited to the parent dictionary in advance of processing other elements, allowing for logically overriding previous decisions in the condition list.
Nesting of conditional statements is also supported.
Example:
variables:
enable-logging: False
enable-debug: False
(?):
- logging == True:
enable-logging: True
(?):
- debugging == True:
enable-debug: True
Conditionals are expressed in a pythonic syntax, the specifics for testing the individually supported option types are described in their respective documentation.
Compound conditionals are also allowed.
Example:
variables:
enable-debug: False
(?):
- (logging == True and debugging == True):
enable-debug: True
(!) Assertions
Assertions allow the project author to abort processing and present a custom error message to the user building their project.
This is only useful when used with conditionals, allowing the project author to assert some invalid configurations.
Example:
variables:
(?):
- (logging == False and debugging == True):
(!): |
Impossible to print any debugging information when
logging is disabled.
(<) List Prepend
Indicates that the list should be prepended to the target list, instead of the default behavior which is to replace the target list.
Example:
config:
configure-commands:
# Before configuring, lets make sure we're using
# the latest config.sub & config.guess
(<):
- cp %{datadir}/automake-*/config.{sub,guess} .
(>) List Append
Indicates that the list should be appended to the target list, instead of the default behavior which is to replace the target list.
Example:
public:
bst:
split-rules:
devel:
# This element also adds some extra stubs which
# need to be included in the devel domain
(>):
- "%{libdir}/*.stub"
(=) List Overwrite
Indicates that the list should be overwritten completely.
This exists mostly for completeness, and we recommend using literal lists most of the time instead of list overwrite directives when the intent is to overwrite a list.
This has the same behavior as a literal list, except that an error will be triggered in the case that there is no underlying list to overwrite; whereas a literal list will simply create a new list.
The added error protection can be useful when intentionally overwriting a list in an element’s public data, which is mostly free form and not validated.
Example:
config:
install-commands:
# This element's `make install` is broken, replace it.
(=):
- cp src/program %{bindir}
(@) Include
Indicates that content should be loaded from files.
This include directive expects a string, or a list of strings when
including multiple files. Each of these strings represent a project
relative filename to include. Files can be included from subprojects
by prefixing the string with the locally defined junction
element
and colon (‘:’).
The include directive can be used in any dictionary declared in the project.conf, in any .bst file, or recursively included in a another include file.
The including YAML fragment has priority over the files it includes, and overrides any values introduced by the includes. When including multiple files, files are included in the order they are declared in the include list, and each subsequent include file takes priority over the previous one.
Important
Cross junction include files are not processed when loading
junction elements
. Variables,
element overrides, source
overrides and mirrors used in the declaration of a junction
must be declared in the project.conf or in
included files which are local to the project declaring the
junction itself.
Junction elements
cannot use include directives.
Example:
elements:
(@): junction.bst:includes/element-overrides.bst
Note
The include directive is available since format version 12