Element Constructs

Element Basics

Here is a rather complete example using the autotools element kind and git source kind:

# 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

# 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.

To refer to a third party plugin, prefix the plugin with its package, for example:

kind: buildstream-plugins:dpkg_build

Depends

# 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

# 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:

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

# 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 Project Configuration

See Using Variables below for a more in depth discussion on variables in BuildStream.

Environment

# 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 Sandbox where build instructions are run for this element.

Environment variables can also be declared and overridden in the Project Configuration

Config

# 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.

Public

# 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.

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:

# Shorthand Dependencies
depends:
- elements/foo.bst
- elements/bar.bst

Dependency dictionary:

# Fully specified dependency
depends:
- filename: elements/foo.bst
  type: build
  junction: elements/baseproject.bst

The type attribute can be used to express the dependency type.

The junction attribute can be used to depend on elements in other projects. See junction.

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.

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.

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:

variables:
  hello: Hello World

You can refer to another variable while declaring a variable:

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:

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:

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}"