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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.

Motivation

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.

Design

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.

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.

Categories

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.

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 GN 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 partner_internal SDK category is used to give some APIs the same compatibility constraints as partner APIs without exposing them to the SDK users.

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.

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. ↩