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Component resolvers

A component resolver is a protocol-backed capability responsible for resolving a URL to a component manifest.

Component resolver capabilities are registered with an environment for a particular URL scheme (http, fuchsia-pkg, etc) and are used by the component manager on behalf of a component to resolve its children.

Registering a component resolver

Component resolvers are registered with environments to resolve a particular URL scheme. If the environment extends from a parent environment, and the same scheme is registered in both parent and child environments, the child registration takes precedence.

environments: [
    {
        name: "my-environ",
        extends: "realm",
        resolvers: [
            {
                resolver: "my-resolver",
                scheme: "my-scheme",
                from: "parent",
            }
        ],
    }

An environment must be assigned to a child in order for the registered resolver to take effect. Then, the registered resolver will be used to resolve any component URLs in the environment whose URL scheme matches scheme.

children: [
    {
        name: "my-child",
        url: "my-scheme://myhost.com/my-path",
        environment: "#my-environ"
    },
]

Implementing a component resolver

A component resolver can be implemented by doing the following:

When the component manager is asked to resolve a component URL, it finds the component resolver registered to the URL’s scheme in the relevant environment and asks it to resolve the URL using the fuchsia.sys2.ComponentResolver FIDL protocol.

If resolution succeeds, the component resolver returns a ComponentDecl, the FIDL representation of a component manifest. If the component being resolved has an associated package, the component resolver should also return a fuchsia.io.Directory handle for the package directory.

Before registering a resolver with an environment, it must be created and routed to the environment.

capabilities: [
    {
        resolver: "my-resolver",
        path: "/svc/fuchsia.sys2.ComponentResolver",
    },
],
expose: [
    {
        resolver: "my-resolver",
        from: "self",
    }
]

resolver capabilities are different from protocol capabilities in that they cannot be used directly by a component. They can only be registered with an environment. See registering a component resolver.

Built-in boot resolver

The component manager provides a built-in component resolver called boot-resolver, which is registered to the fuchsia-boot scheme in component manager's built-in environment.

This resolver can be routed, and the built-in environment can be extended. See environments.