Fuchsia

Hub

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Definition

The hub provides access to detailed structural information about component instances at runtime. The hub provides information such as:

  • Instance name
  • Instance state
  • Instance ID
  • Children
  • Component URL
  • Exposed capabilities

Features

Immutability

The hub’s structure is mostly read-only. It is not possible to create, rename, delete, or otherwise modify directories and files that form the structure of the hub itself. However, the outgoing directory of an instance may include mutable directories, files, and services which can be accessed through the hub.

Scoping

The root of a hub directory is always scoped to a particular realm.

The hub of a realm gives information about:

  • The realm itself
  • All child realms
  • All component instances

The hub of a realm cannot give information about the parent realm. This structure makes it easy to constrain the parts of the hub particular clients can access.

If a realm requests access to its hub, it can also access the hub of all of its descendant realms, by traversing down the children/ subdirectory.

Requesting the hub

Like other capabilities, hub access is requested with a use declaration in the component manifest. This example shows the manifest file of a component that accesses its own hub.

{
    program: {
        runner: "elf",
        binary: "bin/program"
    },
    use: [
        {
            directory: "hub",
            from: "framework",
            rights: [ "r*" ],
        },
    ]
}

Since the hub is a directory capability, with the right routing it can be exposed upwards, offered downwards or even shared between sibling components. See the advanced routing section for more details.

Once the capability is added to the manifest, a component can access the hub from /hub.

Note: in shell environments, the v2 hub included in the namespace as /hub-v2, not /hub. So replace /hub with /hub-v2 for all the examples on this page when exploring the hub via the shell.

Structure of the hub

/hub

The directory structure of /hub looks like:

/hub
├── children
├── exec
├── component_type
├── id
└── url

A hub contains all the information pertaining to a component instance and its realm.

Directories:

Files:

  • /hub/component_type: Contains the string static or dynamic. A component is static when its definition exists in the parent’s component manifest at compile time. A component is dynamic when the parent defines the component at runtime.
  • /hub/id: Contains a non-negative integer corresponding to the version of a particular instance. A new instance with the same name is a new version and will be given a new ID.
  • /hub/url: Contains the URL string corresponding to this component. The URL is defined by the parent either in its component manifest file or at runtime.

/hub/children

The directory structure of /hub/children looks like:

/hub/children
├── foo
|   ├── children
|   ├── exec
|   ├── component_type
|   ├── id
|   └── url
├── bar
|   ├── children
|   ├── exec
|   ├── component_type
|   ├── id
|   └── url
└── baz:qux
    ├── children
    ├── component_type
    ├── id
    └── url

The /hub/children directory contains the hubs of all created child instances, whether they be static, dynamic, running or stopped.

Note: A child does not have access to the hub of its parent unless its parent offers the hub capability to it. For more information, see advanced routing.

The hub gives a name to each child’s hub directory based on this format:

  • For static instances, the format is <instance name>
  • For dynamic instances, the format is <collection name>:<instance name>

/hub/exec

The directory structure of /hub/exec looks like:

/hub/exec
├── exposed
├── in
├── out
├── runtime
├── used
└── resolved_url

The /hub/exec directory is only visible when the instance is started since it contains information about the current execution of the instance.

Directories:

Files:

  • /hub/exec/resolved_url: The component's resolved URL in text format.

/hub/exec/exposed

The instance's exposed services as listed in its manifest file. A component can connect directly to these services from the hub by opening the provided path.

/hub/exec/in

The instance's namespace supplied by the component manager. This contains a listing of services and directories accessible to the given component instance. A component can open the provided path to connect directly to these services from the Hub.

/hub/exec/out/

The instance's outgoing directory. A component can connect directly to these services from the hub by opening the provided path.

/hub/exec/runtime

The directory structure of /hub/exec/runtime looks like:

/hub/exec/runtime
└── elf
    ├── args
    |   ├── 0
    |   └── 1
    ├── process-id
    └── job-id

If a component has an ELF runtime, then it will have a /hub/exec/runtime/elf directory.

Directories:

  • args: command-line arguments, presented as a series of files from 0 onward.

Files:

  • process-id: the instance's process id in text format.
  • job-id: the instance's job id in text format.

/hub/exec/used

Lists services used at least once by the instance. A service is considered “used” when it is requested by the instance and has been routed successfully by the component manager.

Advanced Routing

Offering parent hub

In this example, the parent component instance passes its view of the hub to hub_client, which then maps it as /parent_hub in its namespace. hub_client can inspect information about its parent and siblings through /parent_hub.

In the parent component manifest:

{
    offer: [
        // Route the root hub to hub_client.
        {
          directory: "hub",
          from: "framework",
          to: "#hub_client",
        },
        // Route the ELF runner capability to hub_client.
        {
          runner: "elf",
          from: "parent",
          to: "#hub_client",
        },
    ],
    children: [
        {
            name: "hub_client",
            url: "fuchsia-pkg://fuchsia.com/hub_test#meta/hub_client.cm",
            startup: "eager",
        },
    ],

In hub_client.cml:

{
    program: {
        runner: "elf",
        binary: "bin/hub_client",
    },
    use: [
        {
          directory: "hub",
          from: "parent",
          rights: [ "r*" ],
          path: "/parent_hub"
        }
    ]
}

Exposing a sibling hub

In this example, hub_client_sibling exposes its view of the hub to its parent. The realm, in turn, offers that view of the hub as /sibling_hub to hub_client. hub_client maps that view of the hub to its namespace.

In hub_client_sibling.cml:

{
    program: {
        runner: "elf",
        binary: "bin/hub_client_sibling",
    },
    expose: [
        {
            directory: "hub",
            from: "framework",
        },
    ],
}

In the parent component manifest file:

{
    // Route hub_client_sibling's view of the hub to hub_client.
    offer: [
        {
            directory: "hub",
            from: "#hub_client_sibling",
            to: "#hub_client",
            as: "sibling_hub",
        }
    ],
    children: [
        {
            name: "hub_client_sibling",
            url: "fuchsia-pkg://fuchsia.com/hub_test#meta/hub_client_sibling.cm",
            startup: "eager",
        },
        {
            name: "hub_client",
            url: "fuchsia-pkg://fuchsia.com/hub_test#meta/hub_client.cm",
            startup: "eager",
        },
    ],
}

In hub_client.cml:

{
    program: {
        runner: "elf",
        binary: "bin/hub_client",
    },
    use: [
        {
            directory: "sibling_hub",
            from: "parent",
            rights: [ "r*" ],
            path: "/sibling_hub"
        }
    ]
}

Lifecycle of an instance

To illustrate how the hub changes in response to lifecycle events, consider an example involving 3 components named A, B and C. The manifest of each component is given below:

  • A.cml

    {
    program: {
        runner: "elf",
        binary: "bin/A"
    },
    use: [
        {
            directory: "hub",
            from: "framework",
            rights: [ "r*" ],
        },
    ]
    collections: [
        {
            name: "coll",
            durability: "transient",
        },
    ]
}

  • B.cml

    {
    program: {
        binary: "bin/B"
    },
    children: [
        {
            name: "baz",
            url: "fuchsia-pkg://fuchsia.com/example#meta/C.cm"
            startup: "eager",
        }
    ]
}

  • C.cml

    {
    program: {
        binary: "bin/C"
    }
}

Also consider 3 instances, one for each component:

  • A -> foo
  • B -> bar
  • C -> baz

foo can perform various actions that modify the hub:

View the hub

foo can see its own hub by listing the contents of /hub. In Rust, this looks like:

let entries = std::fs::read_dir("/hub")?;
for entry in entries {
    println!("{}", entry?.path());
}

Without making any changes to the component hierarchy, the hub will look like the following:

/hub
├── children
├── exec
|    ├── exposed
|    ├── in
|    ├── out
|    ├── runtime
|    └── resolved_url
├── component_type => "static"
├── id => "0"
└── url => "fuchsia-pkg://fuchsia.com/example#meta/A.cm"

Connect to the Realm framework protocol

A component must connect to the Realm framework protocol to manipulate its dynamic children. In Rust, this looks like:

let realm = connect_to_protocol::<fsys::RealmMarker>()?;

Create a dynamic child

foo can define an instance of B called bar under the collection coll. In Rust, this looks like:

// Define the child and the collection
let mut collection_ref = fsys::CollectionRef { name: String::from("coll") };
let child_decl = fsys::ChildDecl {
    name: Some(String::from("bar")),
    url: Some(String::from("fuchsia-pkg://fuchsia.com/example#meta/B.cm")),
    startup: Some(fsys::StartupMode::Lazy),
};

// Create the child
realm.create_child(&mut collection_ref, child_decl).await??;

After executing this code, the hub changes to the following:

/hub
├── children
|    └── coll:bar
|         ├── children
|         ├── component_type => "dynamic"
|         ├── id => "1"
|         └── url => "fuchsia-pkg://fuchsia.com/example#meta/B.cm"
├── exec
|    ├── exposed
|    ├── in
|    ├── out
|    ├── runtime
|    └── resolved_url
├── component_type => "static"
├── id => "0"
└── url => "fuchsia-pkg://fuchsia.com/example#meta/A.cm"

Note: The instance baz (which is a child of bar) does not appear in the hub yet.

Note: The instance bar has not been started, so it does not have an exec directory.

Start the dynamic child

foo can now bind to the instance bar and start it. In Rust, this looks like the following:

// Create a reference to the dynamic child
let mut child_ref = fsys::ChildRef {
    name: "bar".to_string(),
    collection: Some("coll".to_string()),
};

// Create a proxy for the exposed directory
let (dir_proxy, server_end) = create_proxy::<DirectoryMarker>()?;

// Bind to the child
realm.bind_child(&mut child_ref, server_end).await??;

After executing this code, the hub changes to the following:

/hub
├── children
|    └── coll:bar
|         ├── children
|         |    └── baz
|         |         ├── children
|         |         ├── exec
|         |         |    ├── exposed
|         |         |    ├── in
|         |         |    ├── out
|         |         |    ├── runtime
|         |         |    ├── used
|         |         |    └── resolved_url
|         |         ├── component_type => "static"
|         |         ├── id => "0"
|         |         └── url => "fuchsia-pkg://fuchsia.com/example#meta/C.cm"
|         ├── exec
|         |    ├── exposed
|         |    ├── in
|         |    ├── out
|         |    ├── runtime
|         |    ├── used
|         |    └── resolved_url
|         ├── component_type => "dynamic"
|         ├── id => "1"
|         └── url => "fuchsia-pkg://fuchsia.com/example#meta/B.cm"
├── exec
|    ├── exposed
|    ├── in
|    ├── out
|    ├── runtime
|    ├── used
|    └── resolved_url
├── component_type => "static"
├── id => "0"
└── url => "fuchsia-pkg://fuchsia.com/example#meta/A.cm"

Note: The instance baz is automatically created and started as a static child of bar.

Note: The instances bar and baz now have exec directories since they both have been started.

Destroy the dynamic child

foo can destroy the instance bar. In Rust, this looks like the following:

// Create a reference to the dynamic child
let mut child_ref = fsys::ChildRef {
    name: "bar".to_string(),
    collection: Some("coll".to_string()),
};

// Destroy the child
realm.destroy_child(&mut child_ref).await??;

The above code begins the deletion process for bar. This process has several stages, most of which occur asynchronously. As a result, the hub’s structure changes several times.

  1. baz is stopped. The hub changes to the following:

    /hub
├── children
|    └── coll:bar:1
|         ├── children
|         |    └── baz
|         |         ├── children
|         |         ├── component_type => "static"
|         |         ├── id => "0"
|         |         └── url => "fuchsia-pkg://fuchsia.com/example#meta/C.cm"
|         ├── exec
|         |    ├── exposed
|         |    ├── in
|         |    ├── out
|         |    ├── runtime
|         |    ├── used
|         |    └── resolved_url
|         ├── component_type => "dynamic"
|         ├── id => "1"
|         └── url => "fuchsia-pkg://fuchsia.com/example#meta/B.cm"
├── exec
|    ├── exposed
|    ├── in
|    ├── out
|    ├── runtime
|    ├── used
|    └── resolved_url
├── component_type => "static"
├── id => "0"
└── url => "fuchsia-pkg://fuchsia.com/example#meta/A.cm"

  2. bar is stopped. The hub changes to the following:

    /hub
├── children
|    └── coll:bar
|         ├── children
|         |    └── baz
|         |         ├── children
|         |         ├── component_type => "static"
|         |         ├── id => "0"
|         |         └── url => "fuchsia-pkg://fuchsia.com/example#meta/C.cm"
|         ├── component_type => "dynamic"
|         ├── id => "1"
|         └── url => "fuchsia-pkg://fuchsia.com/example#meta/B.cm"
├── exec
|    ├── exposed
|    ├── in
|    ├── out
|    ├── runtime
|    ├── used
|    └── resolved_url
├── component_type => "static"
├── id => "0"
└── url => "fuchsia-pkg://fuchsia.com/example#meta/A.cm"

  3. baz is deleted. The hub changes to the following:

    /hub
├── children
|    └── coll:bar:1
|         ├── children
|         ├── component_type => "dynamic"
|         ├── id => "1"
|         └── url => "fuchsia-pkg://fuchsia.com/example#meta/B.cm"
├── exec
|    ├── exposed
|    ├── in
|    ├── out
|    ├── runtime
|    ├── used
|    └── resolved_url
├── component_type => "static"
├── id => "0"
└── url => "fuchsia-pkg://fuchsia.com/example#meta/A.cm"

  4. bar is deleted. The hub changes to the following:

    /hub
├── children
├── exec
|    ├── exposed
|    ├── in
|    ├── out
|    ├── runtime
|    ├── used
|    └── resolved_url
├── component_type => "static"
├── id => "0"
└── url => "fuchsia-pkg://fuchsia.com/example#meta/A.cm"