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:


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.


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.


(?) 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.


  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.


  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.


  enable-debug: False
  - (logging == True and debugging == True):
      enable-debug: True


Conditional statements are guaranteed to always be resolved in the context of the project where the conditional statement is declared.

When including a file from a subproject, any conditionals expressed in that file will already be resolved in the context of the subproject which the file was included from.

(!) 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.


  - (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.


    # 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.


        # 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.


    # This element's `make install` is broken, replace it.
    - cp src/program %{bindir}

(@) Include

Indicates that content should be loaded from files.

The 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 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.


  (@): junction.bst:includes/environment.bst


Files included across a junction cannot be used to inform the declaration of a junction element, as this can present a circular dependency.

Any variables, element overrides, source overrides or 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.