Capabilities

Capabilities can be created, routed, and used in a component manifest to control which parts of Fuchsia have ability to connect to and access which resources.

Routing terminology

Component manifests declare how capabilities are routed between components. The language of capability routing consists of the following three keywords:

• use: When a component uses a capability, the capability is installed in the component's namespace. A component may use any capability that has been offered or exposed to it.
• offer: A component may offer a capability to a target, which is either a child or collection. When a capability is offered to a child, the child instance may use the capability or offer it to one of its own targets. Likewise, when a capability is offered to a collection, any instance in the collection may use the capability or offer it.
• expose: When a component exposes a capability to its parent, the parent may offer the capability to one of its other children. A component may expose any capability that it provides, or that one of its children exposes.

When you use these keywords together, they express how a capability is routed from a component instance‘s outgoing directory to another component instance’s namespace:

• use describes the capabilities that populate a component instance's namespace.
• expose and offer describe how capabilities are passed between component instances. Aside from their directionality, there is another significant difference between offer and expose. A component may freely use a capability that was offered to it, however, in order to prevent dependency cycles between parents and children there are restrictions on using a capability that is exposed to a component by its child. If the parent offers no capabilities to the child which the parent itself provides, then it can use from its child. If the parent does offer capabilities that it provides, then the use from the child must be marked as weak.

Capability types

• Directory capabilities connect to directories provided by other components.
• Event capabilities receive lifecycle events about components at a certain scope.
• Protocol capabilities are service nodes that can be used to open a channel to a FIDL protocol.
• Resolver capabilities when registered in an environment, cause a component with a particular URL scheme to be resolved with that resolver.
• Runner capabilities determines which runner is responsible for instantiating the component and assisting with its lifecycle.
• Service capabilities connect to FIDL services (groups of protocols) provided by other components or the framework itself.
• Storage capabilities are special-cased directories with different semantics that are isolated to the components using them.

directory, event, protocol, service and storage capabilities are routed to components that use them. resolver and runner capabilities are routed to environments that include them.

For more information on what happens when connecting to a capability, see Life of a protocol open.