Hub

Note: This document pertains to modern components with a manifest that ends in .cml. The Fuchsia platform team is currently migrating legacy components to the modern component framework.

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",
        },
    ]
}

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.

Structure of the hub

`/hub`

The directory structure of /hub looks like:

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

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

Directories:

/hub/children
/hub/deleting
/hub/exec

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
|   ├── deleting
|   ├── exec
|   ├── component_type
|   ├── id
|   └── url
├── bar
|   ├── children
|   ├── deleting
|   ├── exec
|   ├── component_type
|   ├── id
|   └── url
└── baz:qux
    ├── children
    ├── deleting
    ├── component_type
    ├── id
    └── url

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

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/deleting`

The directory structure of /hub/deleting looks like:

/hub/deleting
├── baz:qux:1
|   ├── children
|   ├── deleting
|   ├── component_type
|   ├── id
|   └── url
└── baz:qux:2
    ├── children
    ├── deleting
    ├── exec
    ├── component_type
    ├── id
    └── url

Since deletion is not an atomic process, the deleting directory contains information about children that are in the process of being deleted. When a child’s deletion is complete, it is removed from this directory and ceases to exist in its parent’s hub. The full deletion process is explained later in this document.

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

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

Unlike the /hub/children directory, the instance ID is a part of the format for the /hub/deleting directory because multiple versions of the same instance may be in the process of deletion and the instance ID is used to distinguish between them.

`/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:

/hub/exec/exposed
/hub/exec/in
/hub/exec/out
/hub/exec/runtime
/hub/exec/used

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",
          as: "/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",
        }
    ]
}

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.cmx

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

B.cmx

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

C.cmx

// C.cmx
{
    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
Connect to the Realm framework protocol
Create a dynamic child
Start a dynamic child
Destroy a dynamic child

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

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

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.

bar is marked for deletion. The hub changes to the following:

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

baz is stopped. The hub changes to the following:

/hub
├── children
├── deleting
|    └── coll:bar:1
|         ├── children
|         |    └── baz
|         |         ├── children
|         |         ├── deleting
|         |         ├── component_type => "static"
|         |         ├── id => "0"
|         |         └── url => "fuchsia-pkg://fuchsia.com/example#meta/C.cm"
|         ├── deleting
|         ├── 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"

bar is stopped. The hub changes to the following:

/hub
├── children
├── deleting
|    └── coll:bar
|         ├── children
|         |    └── baz
|         |         ├── children
|         |         ├── deleting
|         |         ├── component_type => "static"
|         |         ├── id => "0"
|         |         └── url => "fuchsia-pkg://fuchsia.com/example#meta/C.cm"
|         ├── deleting
|         ├── 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"

baz is marked for deletion. The hub changes to the following:

/hub
├── children
├── deleting
|    └── coll:bar:1
|         ├── children
|         ├── deleting
|         |    └── baz:0
|         |         ├── children
|         |         ├── deleting
|         |         ├── 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"

baz is deleted. The hub changes to the following:

/hub
├── children
├── deleting
|    └── coll:bar:1
|         ├── children
|         ├── deleting
|         ├── 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"

bar is deleted. The hub changes to the following:

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