Component manifests

A component manifest is a file that defines a component by encoding a component declaration. This document gives an overview of the concepts used by component declarations, and presents the syntax for writing component manifest source. Component declarations contain:

Manifests and declarations

This section explains the distinction between component manifests, component manifest sources, and component declarations.

Component manifest

A component manifest is a file that encodes a component declaration, usually distributed as part of a package. The binary format is a JSON file mapping one-to-one onto the component declaration, by convention ending in a .cm extension.

A fuchsia-pkg URL with a component manifest fragment identifies a component in a package.

Component manifest source

A component manifest source is a file that encodes part of a component manifest. Component manifest sources are written in CML (component manifest language), which is the developer-facing source format for component manifests. CML files are JSON5 files that end with a .cml extension. Descriptions and examples of the CML syntax are contained in this document: see Syntax.

Component manifest sources are compiled to component manifests by the cmc tool.

Component declaration

The ComponentDecl FIDL table is a component declaration. Component declarations are used by the component framework APIs to represent components and may be provided to components at runtime.

Concepts

Runtime

The program section of a component manifest declares how the component is run. For a component containing an ELF binary, this section consists of a path to a binary in the package and, optionally, a list of arguments. For a component that does not contain an executable, this section is omitted.

See also: ELF Runner

Child component instances

A child component instance is a component instance that is owned by another component (the "parent"). Child instances may be static or dynamic. Static children are declared by the children section of a component manifest. Dynamic children are created in a collection at runtime.

The offer declarations determine which capabilities child instances have access to (see Routing terminology).

Component collections

A collection is a container for component instances which may be created and destroyed at runtime using the fuchsia.sys2.Realm framework service. 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 (see Routing terminology).

Capability routing

Component manifests provide a syntax for routing capabilities between components. For a detailed walkthrough about what happens during capability routing, see Life of a service open

Capability types

The following capabilities can be routed:

  • service: A filesystem service node that can be used to open a channel to a service provider.
  • directory: A filesystem directory.
  • storage: A filesystem directory that is isolated to the component using it.

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 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 containing realm (i.e., 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 one significant difference between offer and expose. While a component may use a capability that was offered to it, a component is not allowed to use a capability that was exposed to it by its child. This restriction exists to prevent dependency cycles between parent and child.

Framework services

A framework service is a service provided by the component framework. Because the component framework itself is the provider of the service, any component may use it without an explicit offer. Fuchsia supports the following framework services:

  • fuchsia.sys2.Realm: Allows a component to manage and bind to its children. Scoped to the component's realm.

Capability paths

Service and directory capabilities are identified by paths. A path consists of a sequence of path components, starting with and separated by /, where each path component consists one or more non-/ characters.

A path may either be a source path or target path, whose meaning depends on context:

  • A source path is either a path in the component's outgoing directory (for offer or expose from self), or the path by which the capability was offered or exposed to this component.
  • A target path is either a path in the component's namespace (for use), or the path by which the capability is being offered or exposed to another component.

Storage capabilities

Storage capabilities are not directly provided from a component instance's outgoing directory, but are created from preexisting directory capabilities that are declared in storage in a component manifest. This declaration describes the source for a directory capability and can then be listed as a source for offering storage capabilities.

Storage capabilities cannot be exposed.

Storage types

Storage capabilities are identified by types. Valid storage types are data, cache, and meta, each having different semantics:

  • data: A mutable directory the component may store its state in. This directory is guaranteed to be unique and non-overlapping with directories provided to other components.
  • cache: Identical to the data storage type, but the framework may delete items from this directory to reclaim space.
  • meta: A directory where the framework can store metadata for the component instance. Features such as persistent collections must use this capability as they require component manager to store data on the component's behalf. The component cannot directly access this directory.

Examples

For an example of how these keywords interact, consider the following component instance tree:

Capability routing example

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

The routing sequence is:

  • echo hosts the /svc/echo service in its outgoing directory. Also, it exposes /svc/echo from self so the service is visible to its parent, system.
  • system exposes /svc/echo from its child echo to its parent, root.
  • root offers /svc/echo from its child system to its other child shell.
  • shell offers /svc/echo from realm (i.e., its parent) to its child echo_tool.
  • echo_tool uses /svc/echo. When echo_tool runs, it will find /svc/echo in its namespace.

A working example of capability routing can be found at //examples/components/routing.

Facet metadata

Facets are metadata that is ignored by the component framework itself, but may be interpreted by interested components. For example, a module component might contain module facets declaring intents the module subscribes to.

Syntax

This section explains the syntax for each section of the component manifest, in CML format. For the full schema, see cml_schema.json.

program

program varies depending on how the component is run. If the component contains no executable, program is to omitted. If the component contains an ELF binary, program will contain information on how to run the binary.

When program contains information on how to run a binary, it is an object with the following properties:

  • binary: Package-relative path to the executable binary
  • args (optional): List of arguments

Example:

"program": {
    "binary": "bin/hippo",
    "args": [ "Hello", "hippos!" ],
},

See also: ELF Runner

children

The children section declares child component instances as described in Child component instances.

children is an array of objects with the following properties:

  • name: The name of the child component instance, which is a string of one or more of the following characters: a-z, 0-9, _, ., -.
  • url: The component URL for the child component instance.
  • startup (optional): The component instance's startup mode.
    • lazy (default): Start the component instance only if another component instance binds to it.
    • eager: Start the component instance as soon as its parent starts.

Example:

"children": [
    {
        "name": "logger",
        "url": "fuchsia-pkg://fuchsia.com/logger#logger.cm",
    },
    {
        "name": "pkg_cache",
        "url": "fuchsia-pkg://fuchsia.com/pkg_cache#meta/pkg_cache.cm",
        "startup": "eager",
    },
],

collections

The collections section declares collections as described in Component collections.

collections is an array of objects with the following properties:

  • name: The name of the component collection, which is a string of one or more of the following characters: a-z, 0-9, _, ., -.
  • durability: The duration of child component instances in the collection.
    • transient: The instance exists until its containing realm is stopped or it is explicitly destroyed.
    • persistent: The instance exists until it is explicitly destroyed. This mode is not yet supported.

Example:

"collections": [
    {
        "name": "tests",
        "durability": "transient",
    },
],

use

The use section contains use declarations of child component instances as explained in Routing terminology.

use is an array of objects with the following properties:

  • A capability declaration, one of:
    • service: The source path of a service capability.
    • directory: The source path of a directory capability.
    • storage: The type of a storage capability. A manifest can only declare one use for each storage type.
  • as (optional): The explicit target path for the capability. If omitted, defaults to the source path for service and directory capabilities, and one of /data or /cache for storage capabilities. This property cannot be used for meta storage capabilities.

Example:

"use": [
    {
        "service": "/svc/fuchsia.log.Log",
    },
    {
        "directory": "/data/themes",
        "as": "/themes",
    },
    {
        "storage": "data",
        "as": "/my_data",
    },
],

expose

The expose section declares capabilities exposed by this component, as explained in Routing terminology.

expose is an array of objects with the following properties:

  • A capability declaration, one of:
  • from: The source of the capability, one of:
    • self: This component.
    • #<child-name>: A reference to a child component instance.
  • as (optional): The explicit target path for the capability. If omitted, defaults to the source path.

Example:

"expose: [
    {
        "directory": "/data/themes",
        "from": "self",
    },
    {
        "service": "/svc/pkg_cache",
        "from": "#pkg_cache",
        "as": "/svc/fuchsia.pkg.PackageCache",
    },
],

offer

The offer section declares capabilities offered by this component, as explained in Routing terminology.

offer is an array of objects with the following properties:

  • A capability declaration, one of:
    • service: The source path of a service capability.
    • directory: The source path of a directory capability.
    • storage: The type of a storage capability.
  • from: The source of the capability, one of:
    • realm: The component's containing realm (parent). This source can be used for all capability types.
    • self: This component. This source can only be used when offering service or directory capabilities.
    • #<child-name>: A reference to a child component instance. This source can only be used when offering service or directory capabilities.
    • #<storage-name> A reference to a storage declaration. This source can only be used when offering storage capabilities.
  • to: An array of target declarations, each of which is an object with the following properties:
    • dest: A reference to the target (child or collection) to which the capability is being offered, #<target-name>.
    • as (optional): The explicit target path for the capability. If omitted, defaults to the source path. This path cannot be used for storage capabilities.

Example:

"offer": [
    {
        "service": "/svc/fuchsia.log.Log",
        "from": "#logger",
        "to": [
            { "dest": "#fshost" },
            { "dest": "#pkg_cache" },
        ],
    },
    {
        "directory": "/data/blobfs",
        "from": "self",
        "to": [
            { "dest": "#pkg_cache", "as": "/blobfs" },
        ],
    },
    {
        "directory": "/data",
        "from": "realm",
        "to": [
            { "dest": "#fshost" },
        ],
    },
    {
        "storage": "meta",
        "from": "realm",
        "to": [
            { "dest": "#logger" },
        ],
    },
],

storage

A storage declaration creates three storage capabilities, for "data", "cache", and "meta" storage. These storage capabilities are backed by a preexisting directory capability, as explained in Storage capabilities.

storage is an array of objects with the following properties:

  • name: A name for this storage section which can be used by an offer.
  • from: The source of the directory capability backing the new storage capabilities, one of:
    • realm: The component's containing realm (parent).
    • self: This component.
    • #<child-name>: A reference to a child component instance.
  • path: The source path of a directory capability.

facets

The facets section is a JSON object containing facets, chunks of metadata which components may interpret for their own purposes. The component framework enforces no schema for this section, but third parties may expect their facets to adhere to a particular schema.

This section may be omitted.

References

A reference is a string of the form #<reference-name>, where <reference-name> is a string of one or more of the following characters: a-z, 0-9, _, ., -.

A reference may refer to: