SDK categories

Each SDK Atom has a category that defines which kinds of SDK consumers can see the Atom. As SDK Atoms mature, we can increase their visibility, which implies increasing their stability guarantees.


Fuchsia is built by combining many different components that interact using protocols with schemas defined in FIDL. Components that are part of the Fuchsia project interact with each other using the same mechanism that components written by third parties interact with the Fuchsia platform. For this reason, we benefit from having a uniform mechanism that can be used both to develop Fuchsia and to develop for Fuchsia.

The simplest approach would be to put all the FIDL definitions into the Fuchsia SDK, and then have all the developers use those same FIDL definitions in developing their components. However, this approach breaks down because of a common tension in designing APIs: API designers need the ability to iterate on their designs and API consumers need stability in order to build on top of the APIs.

This document describes SDK categories, which is Fuchsia's primary mechanism for balancing these concerns.


FIDL libraries are one example of an SDK Atom, but there are other kinds of SDK Atoms, including C++ client libraries, documentation, and tools. SDK categories apply to every kind of SDK Atom, but this document uses FIDL libraries as a running example.

SDK categories balance the needs for iteration and stability in APIs by recognizing that different API consumers have different stability needs. API consumers that are "closer" to API designers typically have less need for stability and often are the first customers that provide implementation feedback for API designers.

Each SDK Atom is annotated with an SDK category, which defines which SDK consumers can depend upon the SDK Atom. For example, if the FIDL library has an SDK category of internal, that means only SDK consumers within the Fuchsia project can depend upon If someone wants to change, they run the risk of breaking consumers inside the Fuchsia project but they do not run the risk of breaking consumers in other projects.

As another example, consider a FIDL library with an SDK category of partner, which means can be used both within the Fuchsia project and by SDK consumers who have partnered1 with the Fuchsia project. When someone changes, they run a larger risk of breaking consumers because they might break the partners that depend upon

Finally, consider a fuchsia.qux FIDL library with an SDK category of public, which means fuchsia.qux can be used by the general public. Changing fuchsia.qux is very risky because the set of software developed by the general public is potentially unbounded and unknowable.

Along with defining concentrically increasing sets of API consumers, SDK categories also define increasing stability windows. For example, can change dramatically from one day to the next because the internal category limits the exposure to the Fuchsia project itself. Someone changing can change all the clients and servers at the same time, which means the stability window needed for the API is either very small or zero. By way of contrast, the agreement that Fuchsia has with partner projects includes an expectation for compatibility windows.

Currently, Fuchsia do not have any SDK Atoms with an SDK category of public, which means Fuchsia has not made any commitments to supporting the general public using its APIs. However, at some point, the Fuchsia project will begin supporting the general public using its APIs. At that time, the Fuchsia project will need to define the compatibility window for those APIs, which will likely be longer than the compatibility window for partner APIs.

An additional type of SDK category is required for the APIs used in the prebuilt partner or public SDK atoms when it's undesirable to expose these APIs to SDK users. These partner_internal and public_internal categories will enforce the same API compatibility windows as the partner and public categories without requiring adding those APIs to the SDK API surface area. Only the partner_internal category will be introduced for now as there's no public SDK atoms.

A typical SDK Atom begins its lifecycle in the internal SDK category. At some point, the API Council might graduate the SDK Atom might to the partner SDK category, often when a partner needs access to an API contained in the Atom. Sometime in the future, when Fuchsia has a non-empty public SDK category, SDK Atoms will be able to graduate from the partner category to the public category as well. Some SDK Atoms might remain in the internal SDK category indefinitely. Others might graduate to partner but never graduate to public.

Please note that this mechanism is complementary to @available mechanism for platform versioning. The @available mechanism records when and how FIDL APIs change. The SDK category mechanism determines the policy for how quickly API designers can make changes.


SDK categories have been implemented in the `sdk_atom` GN Rule. Each SDK Atom has an category parameter with one of the following values:

  • excluded: the Atom may not be included in SDKs;
  • experimental: (this SDK category does not make much sense);
  • internal: supported for use within the Fuchsia platform source tree;
  • cts: supported for use in the Compatibility Tests for Fuchsia;
  • partner_internal: supported for use in non-source SDK atoms in the partner category but not exposed to the SDK users;
  • partner: supported for use by select partners;
  • public: supported for use by the general public.

These categories form an ordered list with a monotonically increasing audience. For example, an SDK Atom in the public category is necessarily available to select partners because public comes after partner in this list.


Adding an API to the partner or partner_internal category amounts to a commitment to our partners that we will not break their code or impose undue churn on them. Each team that owns an API in one of these categories has a responsibility to uphold these commitments.


APIs in the internal category have minimal commitments beyond those that apply to all code in the Fuchsia project.

If your API is only ever called by in-tree code, with both sides of the communication always having been built from the same revision of the Fuchsia source (as is the case for two platform components talking to each other), then it should be internal.

Note that even if all of your API's clients are in-tree, that's not sufficient to say it belongs in internal. For instance, the source for ffx is in the Fuchsia tree, but it doesn't meet the second requirement: ffx subtools built at one Fuchsia revision will frequently talk to a device built at another. As such, ffx subtools, and any other artifacts shipped in the SDK, must not depend on internal APIs.


Partners don't write their own code using partner_internal APIs, but they still depend on these APIs indirectly via tools, libraries, or packages written by the Fuchsia team. Since the Fuchsia team owns the code that uses these APIs, we can change these APIs without churning our partners. However, the tools, libraries, and packages that use partner_internal APIs will, in general, be built from a different revision than the platform components that they talk to. Thus, we must follow our ABI compatibility policies whenever we change partner_internal APIs.

Namely, the owners of an API in the partner_internal category agree to:

  • Use FIDL Versioning annotations on their APIs.
  • Only ever modify their API at the in-development API level, or at HEAD. Once an API level is declared stable, it should not be changed (see version_history.json).
  • Keep the platform components that implement those APIs compatible with all Fuchsia-supported API levels (see version_history.json).

See the API evolution guidelines for more details on API compatibility.


Partners use partner APIs directly. These APIs are the foundation on which our partners build their applications, and it is our responsibility to keep that foundation reliable and stable.

Owners of an API in the partner category agree to:

  • Make all the versioning commitments from the partner_internal section above.
  • Own our partners' developer experience when it comes to this API, including:

    • Providing good documentation.
    • Following consistent style.
    • Anything else you'd like to see in an SDK you were using.

    See the API Development Guide for relevant rules and suggestions.

  • Acknowledge that backwards-incompatible changes to new API levels impose a cost on our partners, even when we follow our API evolution guidelines. If and when a partner chooses to update their target API level, they will need to make modifications within their codebase to adapt to your change. As such, these changes should not be made lightly.

    If you do decide a backwards-incompatible change is worth making, you agree to pay most of the downstream costs of that change, in accordance with the churn policy.

    Changes are much easier to make to internal APIs than partner APIs, so any planned API refactoring should be done just before adding an API to the partner category, rather than after.

Change history

  1. Currently, the set of partners is not public. As the project scales, we will likely need to revisit our approach to partnerships.