Realms

A realm is a subtree of the component instance tree. Every component instance is the root instance of a realm, known as the "component instance's realm", which is closely associated with the component instance.

TODO: link to component instance tree

Component instances may contain children. Each child component instance in turn defines its own sub-realm. The union of these sub-realms, along with the parent component instance, is equivalent to a subtree. Therefore, it is common to conceive of a realm as a component instance along with its set of children.

Realms play a special role in the component framework. A realm is an encapsulation boundary for component instances. This means:

  • Realms act as a capability boundary. It's up to the realm to decide whether a capability originating in the realm can be routed to component instances outside of the realm. This is accomplished through an expose declaration in a component manifest.
  • The internal structure of a sub-realm is opaque to the parent component instance. For example, the sub-realm could be structured either as one or multiple component instances, and from the perspective of the parent component instance this looks the same as long as the sub-realm exposes the same set of capabilities.

TODO: Sample diagram of a realm

Definitions

This section contains definitions for basic terminology about realms.

  • A realm is a subtree of the component instance tree.
  • A child component instance is a component instance that is owned by another instance, the parent.
  • A sub-realm is the realm corresponding to a child component instance.
  • A containing realm is the realm corresponding to the parent of a component instance.

Child component instances

Component instances may contain children. Child component instances are considered part of the parent instance's definition and are wholly owned by the parent. This has the following implications:

  • A component instance decides what children it contains, and when its children are created and destroyed.
  • A component instance cannot exist without its parent.
  • A component instance may not execute unless its parent is executing.
  • A component instance determines the capabilities available to its children by making offer declarations to them.
  • A component instance has some degree of control over the behavior of its children. For example, a component instance may bind to capabilities exposed from the child's realm through the Realm framework service, or set hooks to intercept child lifecycle events. This control is not absolute, however. For example, a component instance cannot use a capability from a sub-realm that was not explicitly exposed to it.

There are two varieties of child component instances, static and dynamic.

TODO: Diagram of a realm showing static and dynamic children

Static children

A static child is a component instance that was statically declared in the component's manifest by a children declaration. This declaration is necessary and sufficient to establish the child component instance's existence.

Typically, a child should be statically declared unless it has a reason to be dynamic (see Dynamic children). When a child is statically declared, its definition and capabilities can be audited and capabilities can be statically routed from it.

A static child is defined, foremost, by two pieces of information:

  • The child instance's name. The name is local to the parent component instance, and is used to form monikers. It is valid to declare multiple children with the same URL and different names.
  • The child instance's component URL.

For information on providing additional configuration information to child declarations, see children.

TODO: link to component URL

TODO: link to monikers

Dynamic children

A dynamic child is a component instance that was created at runtime in a component collection. A dynamic child is always scoped to a particular collection. Dynamic children can be used to support use cases where the existence or cardinality of component instances cannot be determined in advance. For example, a testing realm might declare a collection in which test component instances can be created.

Most of the metadata to create a dynamic child is identical to that used to declare a static instance, except that it's provided at runtime. The name of a dynamic child is implicitly scoped to its collection; thus it is possible to have two dynamic children in two different collections with the same name.

Capabilities cannot be statically routed from dynamic instances. This is an inherent restriction: there's no way to statically declare a route from a capability exposed by a dynamic instance. However, certain capabilities can be routed from the collection as a whole. TODO: service directories as an example

Component collections

A collection is a container for dynamic children which may be created and destroyed at runtime using the Realm framework service.

Collections support two modes of durability:

  • Transient: The instances in a transient collection are automatically destroyed when the instance containing the collection is stopped.
  • Persistent: The instances in a persistent collection exist until they are explicitly destroyed or the entire collection is removed. meta storage must be offered to the component for this option to be available.

TODO: Link to lifecycle

Collections are declared in the collections section of a component manifest. When an offer declaration targets a collection, the offered capability is made available to every instance in the collection. Some capabilities can be exposed or offered from the collection as a whole, as an aggregation over the corresponding capabilities exposed by the instances in the collection.

TODO: service directories as an example

The Realm framework service

There is a framework service available to every component, fuchsia.sys2.Realm. The Realm service provides APIs for a component instance to manage the children in its realm, such as binding to children and creating dynamic children. See the linked FIDL definitions for full documentation.