.. _api_lifecycle:

API Lifecycle
#############

Developers using Zephyr's APIs need to know how long they can trust that a
given API will not change in future releases. At the same time, developers
maintaining and extending Zephyr's APIs need to be able to introduce
new APIs that aren't yet fully proven, and to potentially retire old APIs when they're
no longer optimal or supported by the underlying platforms.


.. figure:: api_lifecycle.png
    :align: center
    :alt: API Life Cycle
    :figclass: align-center

    API Life Cycle

An up-to-date table of all APIs and their maturity level can be found in the
:ref:`api_overview` page.

Experimental
*************

Experimental APIs denote that a feature was introduced recently, and may change
or be removed in future versions. Try it out and provide feedback
to the community via the `Developer mailing list <https://lists.zephyrproject.org/g/devel>`_.

The following requirements apply to all new APIs:

- Documentation of the API (usage)
  explaining its design and assumptions, how it is to be used, current
  implementation limitations, and future potential, if appropriate.
- The API introduction should be accompanied by at least one implementation
  of said API (in the case of peripheral APIs, this corresponds to one driver)
- At least one sample using the new API (may only build on one single board)

Peripheral APIs (Hardware Related)
==================================

When introducing an API (public header file with documentation) for a new
peripheral or driver subsystem, review of the API is enforced and is driven by
the Architecture working group consisting of representatives from different vendors.

The API shall be promoted to ``unstable`` when it has at least two
implementations on different hardware platforms.

Unstable
********

The API is in the process of settling, but has not yet had sufficient real-world
testing to be considered stable. The API is considered generic in nature and can
be used on different hardware platforms.

.. note::

   Changes will not be announced.

Peripheral APIs (Hardware Related)
==================================

The API shall be promoted from ``experimental`` to ``unstable`` when it has at
least two implementations on different hardware platforms.

Hardware Agnostic APIs
=======================

For hardware agnostic APIs, multiple applications using it are required to
promote an API from ``experimental`` to ``unstable``.

Stable
*******

The API has proven satisfactory, but cleanup in the underlying code may cause
minor changes. Backwards-compatibility will be maintained if reasonable.

An API can be declared ``stable`` after fulfilling the following requirements:

- Test cases for the new API with 100% coverage
- Complete documentation in code. All public interfaces shall be documented
  and available in online documentation.
- The API has been in-use and was available in at least 2 development releases
- Stable APIs can get backward compatible updates, bug fixes and security fixes
  at any time.

In order to declare an API ``stable``, the following steps need to be followed:

#. A Pull Request must be opened that changes the corresponding entry in the
   :ref:`api_overview` table
#. An email must be sent to the ``devel`` mailing list announcing the API
   upgrade request
#. The Pull Request must be submitted for discussion in the next
   `Zephyr Architecture meeting`_ where, barring any objections, the Pull Request
   will be merged

.. _breaking_api_changes:

Introducing breaking API changes
================================

A stable API, as described above, strives to remain backwards-compatible through
its life-cycle. There are however cases where fulfilling this objective prevents
technical progress, or is simply unfeasible without unreasonable burden on the
maintenance of the API and its implementation(s).

A breaking API change is defined as one that forces users to modify their
existing code in order to maintain the current behavior of their application.
The need for recompilation of applications (without changing the application
itself) is not considered a breaking API change.

In order to restrict and control the introduction of a change that breaks the
promise of backwards compatibility, the following steps must be followed whenever
such a change is considered necessary in order to accept it in the project:

#. An :ref:`RFC issue <rfcs>` must be opened on GitHub with the following
   content:

   .. code-block:: none

      Title:     RFC: Breaking API Change: <subsystem>
      Contents:  - Problem Description:
                   - Background information on why the change is required
                 - Proposed Change (detailed):
                   - Brief description of the API change
                 - Detailed RFC:
                   - Function call changes
                   - Device Tree changes (source and bindings)
                   - Kconfig option changes
                 - Dependencies:
                   - Impact to users of the API, including the steps required
                     to adapt out-of-tree users of the API to the change

   Instead of a written description of the changes, the RFC issue may link to a
   Pull Request containing those changes in code form.
#. The RFC issue must be labeled with the GitHub ``Breaking API Change`` label
#. The RFC issue must be submitted for discussion in the next `Zephyr
   Architecture meeting`_
#. An email must be sent to the ``devel`` mailing list with a subject identical
   to the RFC issue title and that links to the RFC issue

The RFC will then receive feedback through issue comments and will also be
discussed in the Zephyr Architecture meeting, where the stakeholders and the
community at large will have a chance to discuss it in detail.

Finally, and if not done as part of the first step, a Pull Request must be
opened on GitHub. It is left to the person proposing the change to decide
whether to introduce both the RFC and the Pull Request at the same time or to
wait until the RFC has gathered consensus enough so that the implementation can
proceed with confidence that it will be accepted.
The Pull Request must include the following:

- A title that matches the RFC issue
- A link to the RFC issue
- The actual changes to the API

  - Changes to the API header file
  - Changes to the API implementation(s)
  - Changes to the relevant API documentation
  - Changes to Device Tree source and bindings

- The changes required to adapt in-tree users of the API to the change.
  Depending on the scope of this task this might require additional help from
  the corresponding maintainers
- An entry in the "API Changes" section of the release notes for the next
  upcoming release
- The labels ``API``, ``Breaking API Change`` and ``Release Notes``, as well as
  any others that are applicable

Once the steps above have been completed, the outcome of the proposal will
depend on the approval of the actual Pull Request by the maintainer of the
corresponding subsystem. As with any other Pull Request, the author can request
for it to be discussed and ultimately even voted on in the `Zephyr TSC meeting`_.

If the Pull Request is merged then an email must be sent to the ``devel`` and
``user`` mailing lists informing them of the change.

.. note::

   Breaking API changes will be listed and described in the migration guide.

Deprecated
***********

.. note::

   Unstable APIs can be removed without deprecation at any time.
   Deprecation and removal of APIs will be announced in the "API Changes"
   section of the release notes.

The following are the requirements for deprecating an existing API:

- Deprecation Time (stable APIs): 2 Releases
  The API needs to be marked as deprecated in at least two full releases.
  For example, if an API was first deprecated in release 1.14,
  it will be ready to be removed in 1.16 at the earliest.
  There may be special circumstances, determined by the Architecture working group,
  where an API is deprecated sooner.
- What is required when deprecating:

  - Mark as deprecated. This can be done by using the compiler itself
    (``__deprecated`` for  function declarations and ``__DEPRECATED_MACRO`` for
    macro definitions), or by introducing a Kconfig option (typically one that
    contains the ``DEPRECATED`` word in it) that, when enabled, reverts the APIs
    back to their previous form
  - Document the deprecation
  - Include the deprecation in the "API Changes" of the release notes for the
    next upcoming release
  - Code using the deprecated API needs to be modified to remove usage of said
    API
  - The change needs to be atomic and bisectable
  - Create a GitHub issue to track the removal of the deprecated API, and
    add it to the roadmap targeting the appropriate release
    (in the example above, 1.16).

During the deprecation waiting period, the API will be in the ``deprecated``
state. The Zephyr maintainers will track usage of deprecated APIs on
``docs.zephyrproject.org`` and support developers migrating their code. Zephyr
will continue to provide warnings:

- API documentation will inform users that the API is deprecated.
- Attempts to use a deprecated API at build time will log a warning to the
  console.


Retired
*******

In this phase, the API is removed.

The target removal date is 2 releases after deprecation is announced.
The Zephyr maintainers will decide when to actually remove the API: this
will depend on how many developers have successfully migrated from the
deprecated API, and on how urgently the API needs to be removed.

If it's OK to remove the API, it will be removed. The maintainers will remove
the corresponding documentation, and communicate the removal in the usual ways:
the release notes, mailing lists, Github issues and pull-requests.

If it's not OK to remove the API, the maintainers will continue to support
migration and update the roadmap with the aim to remove the API in the next
release.

.. _`Zephyr TSC meeting`: https://github.com/zephyrproject-rtos/zephyr/wiki/Zephyr-Committee-and-Working-Group-Meetings#technical-steering-committee-tsc
.. _`Zephyr Architecture meeting`: https://github.com/zephyrproject-rtos/zephyr/wiki/Architecture-Working-Group