Dependency model
================
Elements in the data model are related by their *dependencies*. In BuildStream, there
are two types of relationship that an Element may have with a *dependency*,
:ref:`build and runtime dependencies <format_dependencies>`. More often than not,
an element will require its dependency both to *build* and also at *runtime*.

Consider a simple build scenario where you want to build an application, which
requires a service be present in order to function properly at *runtime*, a
compiler that need only be present at *build time*, and a runtime environment
or base system which must be required all the time:

.. image:: images/arch-dependency-model.svg

Note that in BuildStream we are only concerned with element level granularity
in our dependency model, and there is no way to depend on only a part of an element's
output *artifact*. Instead we can employ :mod:`compose <elements.compose>` and
:mod:`filter <elements.filter>` elements in conjunction with :ref:`split rules <public_split_rules>`
to achieve sub artifact granularity at build and deploy time.

When developing BuildStream, it is important to understand the distinction
between dependency types and element :class:`Scope <buildstream.types.Scope>`,
which acts as a selector of which elements to consider in the dependency
graph of a given element when performing recursive activities.


Scope
~~~~~

* **Scope.ALL**

  In the :func:`Scope.ALL <buildstream.types.Scope.ALL>` scope, all elements
  are considered.

  This is used in some cases to forcefully fetch, pull or build all dependencies
  of a given element, even when not all of them are needed.

  This scope simply includes all of the dependencies, including the element itself.

* **Scope.RUN**

  In the :func:`Scope.RUN <buildstream.types.Scope.RUN>` scope, only elements
  which are required to run are considered, including the element itself. Note
  that these are transitive - the service also requires the base runtime.

  This is used when for example, launching a ``bst shell`` environment
  for the purpose of running, or in any case we need to consider which
  elements are required to run.

  .. image:: images/arch-dependency-model-runtime.svg
     :align: center

* **Scope.BUILD**

  In the :func:`Scope.BUILD <buildstream.types.Scope.BUILD>` scope, only
  elements which are required to build are considered, *excluding* the
  element we intend to build.

  .. image:: images/arch-dependency-model-build.svg
     :align: center

  Note that build type dependencies are not transitive, which is why the
  *Bootstrap* element is not selected when pulling in the *Compiler* to
  build the *Application*.

  Further, note that we still follow the *Compiler* dependency on the
  *Base Runtime*, this is because when we depend on an element for the
  purpose of *building*, we expect that element to *run* and as such
  we include all of the *runtime dependencies* of *build dependencies*
  when selecting the *Scope.BUILD* elements.