API Stability

API and ABI

At a high level, an API – Application Programming Interface – is the boundary between two components when developing against them. It is closely related to an ABI – Application Binary Interface – 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 #defines, 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.

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.

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.

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.

Stability

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.

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.

Several types of stability commonly considered:

unstable

The API could change or be removed in future.

backwards compatible

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).

forwards compatible

Only changes which permit code compiled against the modified API to run against the unmodified API are allowed (for example, functions cannot be added).

totally stable

No changes are allowed to the API, only to the implementation.

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.

Versioning

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 Autotools Mythbuster or Versioning.

Package versioning (major.minor.micro) 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 Versioning for more information.

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 documentation on D-Bus API versioning for details.

For GIR APIs, their stability typically follows the C API stability, as they are generated from the C API.