Component organization

All components in the system are composed into a single rooted component instance tree. This tree structure governs several important aspects of component behavior.

![Tree diagram illustrating how component instances are organized. These parent and child relationships govern several aspects of component behavior.] (/docs/get-started/images/intro/component-organization.png){: width=“712”}

Parent components in the tree are responsible for creating instances of other components as their children and providing them with the necessary capabilities. At the same time, child components can expose capabilities back to the parent. Child components can be created one of two ways:

• Statically: The parent declares the existence of the child in its own component declaration.
• Dynamically: The parent adds the child to a component collection at runtime using the fuchsia.component.Realm protocol.

Any parent component and all its children form a group within the tree called a realm. Realms enable a parent to control which capabilities flow into and out of its sub-tree of components, creating a capability boundary. Components decide whether to export capabilities outside their realm using the expose keyword:

expose: [
    {
        protocol: "fuchsia.example.Foo",
        from: "self",
    },
],

Once a capability is exposed to the realm, the parent can share it with other components within the same realm. This is done using the offer keyword:

offer: [
    {
        protocol: "fuchsia.example.Foo",
        from: "self",
    },
],

Component manager is responsible for resolving requests to access a capability (such as a directory or protocol) with the component providing that capability. This is known as capability routing. Component Manager can only resolve capabilities that are exposed and offered within the same realm.

![Diagram showing how components share capabilities through “capability routing,” which describes how resources are made available within a particular realm.] (/docs/get-started/images/intro/capability-routing.png){: width=“614”}