Component manifests

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

• Information about how to run the component.
• The realm's child component instances.
• Routing rules that describe how capabilities are used, exposed, and offered between components.
• Freeform data (“facets”) which is ignored by the component framework but can be interpreted by third parties.

Formats

A component manifest can be represented in multiple formats, depending on the context.

CML format

CML (component manifest language) 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.

FIDL format

The ComponentDecl FIDL table is the canonical representation of a component manifest. It is the representation used by the component framework APIs and the form in which component manifests are provided to components at runtime.

Binary format

The binary format stores component manifests at rest (for example, as part of a package). The binary format is a JSON file mapping one-to-one onto the FIDL representation, by convention ending in a .cm extension.

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

The children section of a component manifest may declare zero or more child component instances. The offer declarations determine which capabilities the child instances have access to (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.

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: When a component offers a capability to one of its child component instances, the child may use that capability or offer it to one of its own children. A component may offer any capability that it provides itself, has been offered to it by a parent, or has been exposed to it by a child.
• 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.

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.

Examples

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

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",
    },
],

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.
• as (optional): The explicit target path for the capability. If omitted, defaults to the source path.

Example:

"use": [
    {
        "service": "/svc/fuchsia.log.Log",
    },
    {
        "directory": "/data/themes",
        "as": "/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:
  • service: The source path of a service capability.
  • directory: The source path of a directory capability.
• 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.
• from: The source of the capability, one of:
  • realm: The component's containing realm (parent).
  • self: This component.
  • #<child-name>: A reference to a child component instance.
• to: An array of target declarations, each of which is an object with the following properties:
  • dest: A reference to the child to which the capability is being offered, #<child-name>.
  • as (optional): The explicit target path for the capability. If omitted, defaults to the source path.

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" },
        ],
    },
],

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, _, ., -.

Currently, the only type of supported reference is a child reference, which refers to the child component instance whose name is <reference-name>.