Directory capabilities

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Directory capabilities allows components to connect to directories provided by other components.

Providing directory capabilities

To provide a directory capability, a component must define the capability and route it from self. The component hosts the directory capability in its outgoing directory.

To define the capability, add a capabilities declaration for it:

{
    capabilities: [
        {
            directory: "data",
            rights: ["r*"],
            path: "/published-data",
        },
    ],
}

This defines a capability hosted by this component whose outgoing directory path is /published-data, and whose maximum usable rights are r*.

Routing directory capabilities

Components route directory capabilities by either:

• exposing them,
• or offering them.

When a component wants to make one of its directories available to other components, it specifies the path of that directory in its outgoing directory in one of the following ways:

Exposing

To expose the directory to a parent:

{
    expose: [
        {
            directory: "data",
            from: "#child-a",
        },
    ],
}

Optionally, you may narrow the rights on the directory:

{
    expose: [
        {
            directory: "data",
            from: "#child-a",
            rights: ["r*"],
        },
    ],
}

Offering

To offer a directory to a child:

{
    offer: [
        {
            directory: "data",
            from: "self",
            to: [ "#child-a", "#child-b" ],
        },
    ],
}

Optionally, you may narrow the rights on the directory:

{
    offer: [
        {
            directory: "data",
            from: "self",
            rights: ["rw*"],
            to: [ "#child-a", "#child-b" ],
        },
    ],
}

Consuming directory capabilities

When a component wants to make use of a directory from its parent, it does so by using the directory. This will make the directory accessible from the component's [namespace][glossary.namespace].

This example shows a directory named data that is included in the component‘s namespace. If the component instance accesses this directory during its execution, the component framework performs capability routing to find the component that provides it. Then, the framework connects the directory from the component’s namespace to this provider.

{
    use: [
        {
            directory: "data",
            rights: ["r*"],
            path: "/data",
        },
    ],
}

rights must be a subset of the rights attached to the directory.

See //examples/components/routing for a working example of routing a directory capability from one component to another.

Directory capability rights

As directories are offered and exposed throughout the system a user may want to restrict the actions that other components may perform with this directory. For example, a component might expose a directory as read-write to its parent realm, which could in turn expose that directory to its children as read-only.

Directory rights enable any directory declaration, as well as mentions of it in offer, expose, and use, to include a rights field that restricts the set of rights for that directory.

Example

This example shows component A requesting access to data with read-write rights:

// A.cml
{
    use: [
        {
            directory: "data",
            rights: ["rw*"],
            path: "/data",
        },
    ],
}

Furthermore, parent component B offers the directory data to component A but with only read-only rights. In this case the routing fails and data wouldn‘t be present in A’s namespace.

// B.cml
{
    capabilities: [
        {
            directory: "data",
            rights: ["r*"],
            path: "/published-data",
        },
    ],
    offer: [
        {
            directory: "data",
            from: "self",
            to: [ "#A" ],
        },
    ],
}

Inference Rules

Directory rights are required in the following situations:

• use - All directories use statements must specify their directory rights.
• capability - All directory capability declarations must specify rights.

If an expose or offer directory declaration does not specify optional rights, it will inherit the rights from the source of the expose or offer. Rights specified in a use, offer, or expose declaration must be a subset of the rights set on the capability's source.

Framework directory capabilities

Some directory capabilities are available to all components through the framework. When a component wants to use one of these directories, it does so by using the directory with a source of framework.

{
    use: [
        {
            directory: "hub",
            from: "framework",
            rights: ["r*"],
            path: "/hub",
        },
    ],
}