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
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
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
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
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
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
An additional type of SDK category is required for the APIs used in the prebuilt
public SDK atoms when it's undesirable to expose these APIs to
SDK users. These
public_internal categories will enforce
the same API compatibility windows as the
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
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
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
category as well. Some SDK Atoms might remain in the
internal SDK category
indefinitely. Others might graduate to
partner but never graduate to
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
partnercategory 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.
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.
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
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).
partner_internal SDK category is used to give some APIs the same
compatibility constraints as
partner APIs without exposing them to the SDK
excluded SDK category is used as a double-check to prevent certain
targets from ever being included in an SDK. Effectively,
documentation about that intent and is a hook for code reviewers to consider
changes to that value carefully.
- First documented in RFC-0165: SDK categories.
Currently, the set of partners is not public. As the project scales, we will likely need to revisit our approach to partnerships. ↩