Capabilities

Components interact with one another through capabilities . A capability combines access to a resource and a set of rights, providing a access control and a means for interacting with the resource. Fuchsia capabilities typically access underlying kernel objects through handles provided in the component's namespace .

A component can interact with the system and other components only through the discoverable capabilities from its namespace and the few numbered handles it receives.

Capability routing

Components declare new capabilities that they offer to the system and capabilities provided by other components (or the framework) that they require in their component manifest. Component framework uses these declarations to populate the namespace.

For capabilities to be available at runtime, there must also be a valid capability route from the consuming component to a provider. Since capabilities are most often routed through parent components to their children, parent components play an important role in defining the sandboxes for their child components.

Some capability types are routed to environments rather than individual component instances. Environments configure the behavior of the framework for the realms where they are assigned. Capabilities routed to environments are accessed and used by the framework. Component instances do not have runtime access to the capabilities in their environment.

Routing terminology

Routing terminology divides into the following categories:

  1. Declarations of how capabilities are routed between the component, its parent, and its children:
    • offer: Declares that the capability listed is made available to a child component instance or a child collection.
    • expose: Declares that the capabilities listed are made available to the parent component or to the framework. It is valid to expose from self or from a child component.
  2. Declarations of capabilities consumed or provided by the component:
    • use: For executable components, declares capabilities that this component requires in its namespace at runtime. Capabilities are routed from the parent unless otherwise specified, and each capability must have a valid route from its source.
    • capabilities: Declares capabilities that this component provides. Capabilities that are offered or exposed from self must appear here. These capabilities often map to a node in the outgoing directory .

Capability types

The following capabilities can be routed:

type description routed to
protocol A filesystem node that is used to open a channel backed by a FIDL protocol. components
service A filesystem directory that is used to open a channel to one of several service instances. components
directory A filesystem directory. components
storage A writable filesystem directory that is isolated to the component using it. components
resolver A capability that, when registered in an environment, causes a component with a particular URL scheme to be resolved with that resolver. environments
runner A capability that, when registered in an environment, allows the framework to use that runner when starting components. environments

Examples

Consider the following example that describes capability routing through the component instance tree:


Capability routing example

In this example, the echo component instance provides an fuchsia.Echo protocol in its outgoing directory. This protocol is routed to the echo_tool component instance, which uses it. It is necessary for each component instance in the routing path to propagate fuchsia.Echo to the next component instance.

The routing sequence is:

  • echo hosts the fuchsia.Echo protocol in its outgoing directory. Also, it exposes fuchsia.Echo from self so the protocol is visible to its parent, services.
  • services exposes fuchsia.Echo from its child echo to its parent, shell.
  • shell offers fuchsia.Echo from its child services to its other child tools.
  • tools offers fuchsia.Echo from parent (i.e., its parent) to its child echo_tool.
  • echo_tool uses fuchsia.Echo. When echo_tool runs, it will find fuchsia.Echo in its namespace.

For a more detailed example of capability routing, see //examples/components/routing.

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