<page xmlns="http://projectmallard.org/1.0/"
      xmlns:its="http://www.w3.org/2005/11/its"
      type="topic"
      id="api-stability">

  <info>
    <link type="guide" xref="index#maintainer-guidelines"/>

    <credit type="author copyright">
      <name>Philip Withnall</name>
      <email its:translate="no">philip.withnall@collabora.co.uk</email>
      <years>2015</years>
    </credit>

    <include href="cc-by-sa-3-0.xml" xmlns="http://www.w3.org/2001/XInclude"/>

    <desc>Backwards compatibility in APIs</desc>
  </info>

  <title>API stability</title>

  <synopsis>
    <title>Summary</title>

    <list>
      <item><p>
        Define API stability guarantees for your project.
        (<link xref="#stability"/>)
      </p></item>
      <item><p>
        Ensure version numbers are changed as appropriate when API changes.
        (<link xref="#versioning"/>)
      </p></item>
    </list>
  </synopsis>

  <section id="api-and-abi">
    <title>API and ABI</title>

    <p>
      At a high level, an API – <em>Application Programming Interface</em> – is
      the boundary between two components when developing against them. It is
      closely related to an ABI – <em>Application Binary Interface</em> – which
      is the boundary at runtime. It defines the possible ways in which other
      components can interact with a component. More concretely, this normally
      means the C headers of a library form its API, and compiled library
      symbols its ABI. The difference between an API and ABI is given by
      compilation of the code: there are certain things in a C header, such as
      <code>#define</code>s, which can cause a library’s API to change without
      changing its ABI. But these differences are mostly academic, and for all
      practical purposes, API and ABI can be treated interchangeably.
    </p>

    <p>
      Examples of API-incompatible changes to a C function would be to add a
      new parameter, change the function’s return type, or remove a parameter.
    </p>

    <p>
      However, many other parts of a project can form an API. If a daemon
      exposes itself on D-Bus, the interfaces exported there form an API.
      Similarly, if a C API is exposed in higher level languages by use of
      GIR, the GIR file forms another API — if it changes, any higher level
      code using it must also change.
    </p>

    <p>
      Other examples of more unusual APIs are configuration file locations and
      formats, and GSettings schemas. Any changes to these could require code
      using your library to change.
    </p>
  </section>

  <section id="stability">
    <title>Stability</title>

    <p>
      API stability refers to some level of guarantee from a project that its
      API will only change in defined ways in the future, or will not change at
      all. Generally, an API is considered ‘stable’ if it commits to
      backwards-compatibility (defined below); but APIs could also commit to
      being unstable or even forwards-compatible. The purpose of API stability
      guarantees is to allow people to use your project from their own code
      without worrying about constantly updating their code to keep up with
      API changes. Typical API stability guarantees mean that code which is
      compiled against one version of a library will run without problems
      against all future versions of that library with the same major version
      number — or similarly that code which runs against a daemon will
      continue to run against all future versions of that daemon with the same
      major version number.
    </p>

    <p>
      It is possible to apply different levels of API stability to components
      within a project. For example, the core functions in a library could be
      stable, and hence their API left unchanged in future; while the newer,
      less core functions could be left unstable and allowed to change wildly
      until the right design is found, at which point they could be marked as
      stable.
    </p>

    <p>
      Several types of stability commonly considered:
    </p>
    <terms>
      <item>
        <title>Unstable</title>
        <p>The API could change or be removed in future.</p>
      </item>
      <item>
        <title>Backwards compatible</title>
        <p>
          Only changes which permit code compiled against the unmodified API
          to continue running against the modified API are allowed (for
          example, functions cannot be removed).
        </p>
      </item>
      <item>
        <title>Forwards compatible</title>
        <p>
          Only changes which permit code compiled against the modified API to
          run against the unmodified API are allowed (for example, functions
          cannot be added).
        </p>
      </item>
      <item>
        <title>Totally stable</title>
        <p>No changes are allowed to the API, only to the implementation.</p>
      </item>
    </terms>

    <p>
      Typically, projects commit to backwards-compatibility when they say an
      API is ‘stable’. Very few projects commit to total stability because it
      would prevent almost all further development of the project.
    </p>
  </section>

  <section id="versioning">
    <title>Versioning</title>

    <p>
      API stability guarantees are strongly linked to project versioning; both
      package versioning and libtool versioning. Libtool versioning exists
      entirely for the purpose of tracking ABI stability, and is explained in
      detail on the
      <link href="https://autotools.io/libtool/version.html">Autotools
      Mythbuster</link> or <link xref="versioning"/>.
    </p>

    <p>
      Package versioning (<em>major.minor.micro</em>) is strongly linked to API
      stability: typically, the major version number is incremented when
      backwards-incompatible changes are made (for example, when functions are
      renamed, parameters are changed, or functions are removed). The minor
      version number is incremented when forwards-incompatible changes are
      made (for example, when new public API is added). The micro version
      number is incremented when code changes are made without modifying API.
      See <link xref="versioning"/> for more information.
    </p>

    <p>
      API versioning is just as important for D-Bus APIs and GSettings schemas
      (if they are likely to change) as for C APIs. See the
      <link href="http://dbus.freedesktop.org/doc/dbus-api-design.html#api-versioning">documentation
      on D-Bus API versioning</link> for details.
    </p>

    <p>
      For GIR APIs, their stability typically follows the C API stability, as
      they are generated from the C API. One complexity is that their stability
      additionally depends on the version of gobject-introspection used in
      generating the GIR, but recent versions have not changed much so this is
      not a major concern.
    </p>
  </section>

  <section id="external-links">
    <title>External Links</title>

    <p>
      The topic of API stability is covered in the following articles:
    </p>
    <list>
      <item><p>
        <link href="http://en.wikipedia.org/wiki/Application_programming_interface">Wikipedia
        page on APIs</link>
      </p></item>
      <item><p>
        <link href="http://en.wikipedia.org/wiki/Application_binary_interface">Wikipedia
        page on ABIs</link>
      </p></item>
      <item><p>
        <link href="http://dbus.freedesktop.org/doc/dbus-api-design.html#api-versioning">D-Bus
        API versioning documentation</link>
      </p></item>
    </list>
  </section>
</page>
