.. _format: The BuildStream Format ====================== 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. This page should tell you everything you need to know about the base YAML format which BuildStream uses. This will not cover the configurations needed for various plugins, plugin configurations are documented in the plugins themselves. The Project Directory --------------------- 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. Element 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 `Architecture Conditionals`_ or `Variant 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. Element Basics -------------- Here is a rather complete example using the autotools element kind and git source kind: .. code:: yaml # Specify the kind of element this is kind: autotools # Specify some dependencies depends: - elements/element1.bst - elements/element2.bst # Specify the source which should be built sources: - kind: git url: upstream:modulename.git track: master ref: d0b38561afb8122a3fc6bafc5a733ec502fcaed6 # Override some variables variables: sysconfdir: %{prefix}/etc # Tweak the sandbox shell environment environment: LD_LIBRARY_PATH: /some/custom/path # Specify the configuration of the element config: # Override autotools element default configure-commands configure-commands: - "%{configure} --enable-fancy-feature" # Specify public domain data, visible to other elements. public: bst: integration-commands: - /usr/bin/update-fancy-feature-cache For most use cases you would not need to specify this much detail, we've provided details here in order to have a more complete initial example. Let's break down the above and give a brief explanation of what these attributes mean. Kind ~~~~ .. code:: yaml # Specify the kind of element this is kind: autotools The ``kind`` attribute specifies which plugin will be operating on the element's input to produce its output. Plugins define element types and each of them can be referred to by name with the ``kind`` attribute. Depends ~~~~~~~ .. code:: yaml # Specify some dependencies depends: - elements/element1.bst - elements/element2.bst Relationships between elements are specified with the ``depends`` attribute. Element definitions may depend on other elements by specifying the project relative path to the elements on which they depend here. See `Dependencies`_ for more information on the dependency model. Sources ~~~~~~~ .. code:: yaml # Specify the source which should be built sources: - kind: git url: upstream:modulename.git track: master ref: d0b38561afb8122a3fc6bafc5a733ec502fcaed6 Here we specify some input for the element, any number of sources may be specified. By default the sources will be staged in the root of the element's build directory in the build sandbox, but sources may specify a ``directory`` attribute to control where the sources will be staged. The ``directory`` attribute may specify a build sandbox relative subdirectory. For example, one might encounter a component which requires a separate data package in order to build itself, in this case the sources might be listed as: .. code:: yaml sources: # Specify the source which should be built - kind: git url: upstream:modulename.git track: master ref: d0b38561afb8122a3fc6bafc5a733ec502fcaed6 # Specify the data package we need for build frobnication, # we need it to be unpacked in a src/frobdir - kind: tarball directory: src/frobdir url: data:frobs.tgz sha256sum: 9d4b1147f8cf244b0002ba74bfb0b8dfb3... Like Elements, Source types are plugins which are indicated by the ``kind`` attribute. Asides from the common ``kind`` and ``directory`` attributes which may be applied to all Sources, refer to the Source specific documentation for meaningful attributes for the particular Source. Variables ~~~~~~~~~ .. code:: yaml # Override some variables variables: sysconfdir: "%{prefix}/etc" Variables can be declared or overridden from an element. Variables can also be declared and overridden in the :ref:`projectconf` See `Using Variables`_ below for a more in depth discussion on variables in BuildStream. Environment ~~~~~~~~~~~ .. code:: yaml # Tweak the sandbox shell environment environment: LD_LIBRARY_PATH: /some/custom/path Environment variables can be set to literal values here, these environment variables will be effective in the :mod:`Sandbox ` where build instructions are run for this element. Environment variables can also be declared and overridden in the :ref:`projectconf` Config ~~~~~~ .. code:: yaml # Specify the configuration of the element config: # Override autotools element default configure-commands configure-commands: - "%{configure} --enable-fancy-feature" Here we configure the element itself. The autotools element provides sane defaults for building sources which use autotools. Element default configurations can be overridden in the ``project.conf`` file and additionally overridden in the declaration of an element. For meaningful documentation on what can be specified in the ``config`` section for a given element ``kind``, refer to the element specific documentation. .. _format_public: Public ~~~~~~ .. code:: yaml # Specify public domain data, visible to other elements. public: bst: integration-commands: - /usr/bin/update-fancy-feature-cache Metadata declared in the ``public`` section of an element is visible to any other element which depends on the declaring element in a given pipeline. BuildStream itself consumes public data from the ``bst`` domain. The ``integration-commands`` demonstrated above for example, describe commands which should be run in an environment where the given element is installed but before anything should be run. An element is allowed to read domain data from any element it depends on, and users may specify additional domains to be understood and processed by their own element plugins. .. _format_dependencies: Dependencies ------------ The dependency model in BuildStream is simplified by treating software distribution and software building as separate problem spaces. This is to say that one element can only ever depend on another element but never on a subset of the product which another element produces. In this section we'll quickly go over the few features BuildStream offers in its dependency model. Expressing Dependencies ~~~~~~~~~~~~~~~~~~~~~~~ Dependencies in BuildStream are parameterizable objects, however as demonstrated in the above example, they can also be expressed as strings as a convenience shorthand whenever the default dependency attributes are suitable. Shorthand: .. code:: yaml # Shorthand Dependencies depends: - elements/foo.bst - elements/bar.bst Dependency dictionary: .. code:: yaml # Fully specified dependency depends: - filename: elements/foo.bst variant: bar type: build The ``variant`` attribute is explained below in `Variant Conditionals`_, and the ``type`` attribute can be used to express the dependency type. Dependency Types ~~~~~~~~~~~~~~~~ The dependency ``type`` attribute defines what the dependency is required for and is essential to how BuildStream plots a build plan. There are two types which one can specify for a dependency, ``build`` and ``runtime``. A ``build`` dependency type states that the given element's product must be staged in order to build the depending element. Depending on an element which has ``build`` dependencies will not implicitly depend on that element's ``build`` dependencies. A ``runtime`` dependency type states that the given element's product must be present for the depending element to function. An element's ``runtime`` dependencies need not be staged in order to build the element. If ``type`` is not specified, then it is assumed that the dependency is required both at build time and runtime. .. note:: It is assumed that a dependency which is required for building an element must run while building the depending element. This means that ``build`` depending on a given element implies that that element's ``runtime`` dependencies will also be staged for the purpose of building. .. _format_variables: Using Variables --------------- Variables in BuildStream are a way to make your build instructions and element configurations more dynamic. Referring to Variables ~~~~~~~~~~~~~~~~~~~~~~ Variables are expressed as ``%{...}``, where ``...`` must contain only alphanumeric characters and the separators ``_`` and ``-``. Further, the first letter of ``...`` must be an alphabetic character. .. code:: yaml This is release version %{version} Declaring and Overriding Variables ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ To declare or override a variable, one need only specify a value in the relevant *variables* section: .. code:: yaml variables: hello: Hello World You can refer to another variable while declaring a variable: .. code:: yaml variables: release-text: This is release version %{version} The order in which you declare variables is arbitrary, so long as there is no cyclic dependency and that all referenced variables are declared, the following is fine: .. code:: yaml variables: release-text: This is release version %{version} version: 5.5 .. note:: It should be noted that variable resolution only happens after all `Element Composition`_ has already taken place. This is to say that overriding ``%{version}`` at a higher priority will effect the final result of ``%{release-text}``. **Example:** .. code:: yaml kind: autotools # Declare variable, expect %{version} was already declared variables: release-text: This is release version %{version} config: # Customize the installation install-commands: - | %{make-install} RELEASE_TEXT="%{release-text}" Architecture Conditionals ------------------------- To BuildStream, an architecture is simply an arbitrary name that is associated with the target architecture and compiler tuning. Conditional YAML segments can be applied for a given target architecture, like so: .. code:: yaml kind: autotools config: something: 5 arches: x86_64: config: something: 6 x86_32: config: something: 7 The ``arches`` attribute, if provided, overrides the element for a given architecture name. It is not considered an error if the element does not provide an architecture clause for the specific architecture BuildStream was launched to build for. There is also a ``host-arches`` attribute, which operates in the same manner but follows the *host* architecture rather than the *target* architecture. In the above example we demonstrate that a given ``config`` attribute can be overridden by an architecture conditional, this can however be done for any segment of the element such as ``depends``, ``sources`` and ``public`` as well. It is however illegal to override the element ``kind`` in any conditional. Further, it should be noted that when applying elements to a list in the element YAML, the conditional segments are *appended* to the parent list and do not replace the list entirely. Consider for example: .. code:: yaml kind: autotools depends: - elements/foo.bst arches: x86_64: depends: - elements/bar.bst When targetting the ``x86_64`` architecture name, the above element YAML will expand to the following YAML: .. code:: yaml kind: autotools depends: - elements/foo.bst - elements/bar.bst Variant Conditionals -------------------- Variants are a way for a single element to provide multiple features. In contrast with the architecture conditionals described above, which are resolved once for the entirety of a pipeline; variant conditionals are selected by way of dependency. Declaring Variants ~~~~~~~~~~~~~~~~~~ If an element declares any variants, it must declare at least two variants. One of the variant declarations may be left empty so that they do not override or effect the base element declaration, but at least two variant names must be declared. The first declared variant is the default. It may have whatever name you decide to give it, but the default variant is what will be selected if all dependencies on the given element are *ambivalent* of the variant. Here is an example of how an element declares multiple variants: .. code:: yaml # Unconditionally depend on foo.bst kind: autotools depends: - elements/foo.bst variants: # The default variant needs to disable flying ponies, or else # our configure script bails out if the ponies are not found - variant: default config: configure-commands: - "%{configure} --without-flying-ponies" # For the flying-ponies variant, we want to pull in the extra # ponies so they will be available for flying - variant: flying-ponies depends: - elements/ponies.bst Depending on Variants ~~~~~~~~~~~~~~~~~~~~~ To depend on a specific variant of a given element, one must simply use the ``variant`` attribute in a dependency that is expressed as a dictionary: .. code:: yaml # Depend on the flying-ponies variant of the foo element depends: - filename: elements/foo.bst variant: flying-ponies When depending on an element which advertizes variants without specifying any particular variant, the dependency is said to be *ambivalent*. Variant Resolution ~~~~~~~~~~~~~~~~~~ Variants of an element may augment the given element's dependencies, as such there may be many possible ways in which a pipeline can be constructed. As a rule, every variant of a given element should be buildable without presenting any conflict when building the element as your pipeline *target*. When resolving variants in a complex pipeline however, it is possible that sibling elements depend on specific variants of common dependencies. BuildStream will resolve which variants to build deterministically by traversing an element's variants in the order of declaration, always choosing the first buildable variant for any *ambivalent* dependency. If there is no suitable build plan found for the selected variant of the pipeline *target*, then it is considered a variant disagreement error and the build will be aborted during the parse phase.