RFC-0165: SDK categories

RFC-0165: SDK categories
  • Developer

SDK atoms have categories that describe their maturity and intended audience.

Gerrit change
Date submitted (year-month-day)2022-05-13
Date reviewed (year-month-day)2022-05-31


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 an existing mechanism for balancing these concerns. SDK categories predate the RFC process. This document simply documents the existing mechanism.

SDK categories are not the only mechanism Fuchsia uses to address this tension. For example, Fuchsia also limits which FIDL protocols a component can depend upon through capability routing. These mechanisms are complementary because without capability routing, component authors might be tempted to create local copies of internal FIDL definitions rather than wait for them to be added to the SDK. Similarly, without SDK categories, component authors could start depending upon internal FIDL protocols to communicate among their own components.


Who has a stake in whether this RFC is accepted? (This section is optional but encouraged.)


  • neelsa@google.com


  • dschuyler@google.com
  • jamesr@google.com
  • jeremymanson@google.com
  • neelsa@google.com
  • sebmarchand@google.com
  • sethladd@google.com


This RFC skipped the socalization phase because the mechanism is already fully implemented.


An SDK Atom is a collection of files that can be included in an SDK. Fuchsia represents SDK Atoms using the sdk_atom template in GN.

An SDK target is a build target that creates an SDK. For example, //sdk:core is the GN label for the build target that creates the Core SDK. It might be more accurate to refer to these targets as IDK targets because they create the JSON metadata for the IDK. A later stage of the SDK production pipeline uses this JSON metadata to produce a fully integrated Fuchsia SDK (e.g., with a build system like Bazel).


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 fuchsia.foo FIDL library has an SDK category of internal, that means only SDK consumers within the Fuchsia project can depend upon fuchsia.foo. If someone wants to change fuchsia.foo, 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 fuchsia.bar FIDL library with an SDK category of partner, which means fuchsia.bar can be used both within the Fuchsia project and by SDK consumers who have partnered1 with the Fuchsia project. When someone changes fuchsia.bar, they run a larger risk of breaking consumers because they might break the partners that depend upon fuchsia.bar.

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, fuchsia.foo can change dramatically from one day to the next because the internal category limits the exposure to the Fuchsia project itself. Someone changing fuchsia.foo 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. For example, we currently have an agreement with partners to maintain a six-week compatibility window, although this window is likely to change in a soon-to-be-published RFC. This agreement implies that fuchsia.bar cannot change dramatically from one day to the next. Instead, we need to change fuchsia.bar gradually and in a way that maintains the compatibility window.

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.

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.

This lifecycle is currently incomplete in that the lifecycle does not cover deprecation and removal of SDK Atoms. For example, we might a need to add a historical category for SDK Atoms that are not currently useful but still have historical value. Such an extension to the existing model are out of scope for this RFC.

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 GN rules for building SDKs. 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: 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.

The experimental category does not make much sense because we have better mechanisms (e.g., GN visibility) to control use of code within the Fuchsia platform source tree. Perhaps this category will be removed soon.

Each SDK target also has a category parameter that defines the set of consumers to whom that SDK ships. The build system enforces that everything included in an SDK target has an SDK category that is acceptable for that audience. For example, an SDK for partner can include SDK Atoms authorized for public (because public comes after partner in this list above) but cannot include SDK Atoms authorized only for internal use (because internal comes before partner in this list).

The excluded SDK category is used as a double-check to prevent certain targets from ever being included in an SDK. Effectively, excluded is documentation about that intent and is a hook for code reviewers to consider changes to that value carefully.

Backwards Compatibility

Typically, SDK categories change by exposing the SDK Atom to larger and larger audiences. Shrinking the set of consumers for an SDK Atom effectively deletes those APIs from their view, which can break those consumers if not coordinated correctly.

Security considerations

SDK categories are not a security mechanism. A malicious actor can read all of the FIDL definitions from the Fuchsia open source project and make use of them in whatever nefarious ways the attacker can imagine. This mechanism is limited to making the engineer process run more smoothly.

Privacy considerations

Everything about SDK categories and the SDK Atoms to which they apply is public.


SDK categories are enforced at build time through build configuration. However, we do not have many tests of this mechanism. There is a risk that changes to the SDK-related GN templates could break the mechanism, which would allow SDK Atoms to be included in inappropriate SDK targets. We have a few redundant mechanisms, including SDK manifests, to catch misconfigurations, but we are relying upon code reviewers to notice inconsistencies between SDK manifests and SDK categories in order to identify regressiosn in the SDK category mechanism.


The purpose of writing this RFC is to document the current state-of-the-world for SDK categories. There is other developer-facing documentation about what stability promises Fuchsia makes about the various SDKs that Fuchsia publishes.

Drawbacks, alternatives, and unknowns

The main drawback of this approach is the coarse granularity at which SDK categories can be applied. For example, you could imagine another approach in which individual FIDL protocol elements could be assigned SDK categories, similar to how individual protocol elements are assigned @available attributes.

The advantage of the existing SDK category mechanism is that it applies uniformly to all kinds of SDK Atoms. However, we might want a finer-grained mechanism for certain kinds of SDK Atoms (e.g., FIDL protocols) over time.

Prior art and references

Most platforms have a similar mechanism for gradually expanding the audience for APIs as they become more stable. For example, Apple uses a similar mechanism when developing APIs for macOS and iOS. In the development process for those operating systems, each framework has three sets of APIs: internal APIs that are available within the framework itself, private APIs that are available to other frameworks that are part of the operating system build, and public APIs that are available to the general public building applications for the operating system.

Some other platforms have a "header stripping" step in their SDK release process that removes text from headers prior to releasing SDKs to larger audiences. Consumers who use the "unstripped" headers can depend upon a larger set of APIs those consumers who use the stripped headers. This mechanism is similar to SDK categories but operates at a finer granularity, and often has fewer audience gradations.

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