.. _formatintro: Introduction ============ At the core of BuildStream is a data model of :mod:`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. .. _format_structure: Directory Structure ------------------- A BuildStream project is a directory consisting of: * A project configuration file * BuildStream element files * User defined Plugins A typical project structure may look like this:: myproject/project.conf 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 :ref:`projectconf` 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. .. _format_composition: Composition ----------- Below are the various sources of configuration which go into an element 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 :ref:`projectconf` provides a set of default values for *variables* and the *environment* which are all documented with your copy of BuildStream. 2. Project Configuration ~~~~~~~~~~~~~~~~~~~~~~~~ The project wide defaults are now applied on top of builtin defaults. If you specify anything in the *variables* or *environment* sections in your ``project.conf`` then it will override the builtin defaults. 3. Element Defaults ~~~~~~~~~~~~~~~~~~~ Elements are all implemented as plugins. Each 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``. 4. Project Configuration ~~~~~~~~~~~~~~~~~~~~~~~~ The ``project.conf`` now gives you another opportunity to override *variables*, *environment* and *config* sections on a per element basis. Configurations specified in the *elements* section of the ``project.conf`` will override the given element's default. 5. Element Declarations ~~~~~~~~~~~~~~~~~~~~~~~ Finally, after having resolved any :ref:`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. .. _format_directives: Directives ---------- .. _format_directives_conditional: (?) Conditionals ~~~~~~~~~~~~~~~~ The ``(?)`` directive allows expression of conditional statements which test :ref:`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:** .. code:: yaml 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:** .. code:: yaml 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 :ref:`respective documentation `. Compound conditionals are also allowed. **Example:** .. code:: yaml variables: enable-debug: False (?): - (logging == True and debugging == True): enable-debug: True .. _format_directives_assertion: (!) 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:** .. code:: yaml variables: (?): - (logging == False and debugging == True): (!): | Impossible to print any debugging information when logging is disabled. .. _format_directives_list_prepend: (<) 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:** .. code:: yaml config: configure-commands: # Before configuring, lets make sure we're using # the latest config.sub & config.guess (<): - cp %{datadir}/automake-*/config.{sub,guess} . .. _format_directives_list_append: (>) 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:** .. code:: yaml public: bst: split-rules: devel: # This element also adds some extra stubs which # need to be included in the devel domain (>): - "%{libdir}/*.stub" .. _format_directives_list_overwrite: (=) 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:** .. code:: yaml config: install-commands: # This element's `make install` is broken, replace it. (=): - cp src/program %{bindir}