Fuchsia

Component-based software

<<../../_common/intro/_components_intro.md>>

<<../../_common/intro/_components_manager.md>>

<<../../_common/intro/_components_capabilities.md>>

<<../../_common/intro/_components_organization.md>>

Exercise: Components

In this exercise, you'll explore the component instance tree and look in detail at capability routing in action using some core system components.

<<../_common/_start_femu.md>>

Explore system components

Open another terminal window and use the component list command to dump the system's component tree:

ffx component list

You should see output similar to the (truncated) list below:

/
/bootstrap
/bootstrap/archivist
/bootstrap/base_resolver
/bootstrap/console
/bootstrap/console-launcher
/bootstrap/decompressor
/bootstrap/device_name_provider
/bootstrap/driver_manager
/bootstrap/fshost
/bootstrap/miscsvc
/bootstrap/netsvc
/bootstrap/power_manager
/bootstrap/ptysvc
/bootstrap/pwrbtn-monitor
/bootstrap/shutdown_shim
/bootstrap/svchost
/bootstrap/sysinfo
/bootstrap/virtual_console
/core
/core/activity
/core/appmgr
...
/core/debug_serial
/core/detect
/core/font_provider
/core/log-stats
/core/remote-control
/core/remote-diagnostics-bridge
/core/sampler
/core/system-update-committer
/core/temperature-logger
/core/test_manager
/core/full-resolver
/startup

This list represents the component instance tree, with organizational components like bootstrap, core, and startup forming sub-trees underneath the root.

The component show command provides more details about each component.

Use this command to see the details of http-client — a component that provides an HTTP request service:

ffx component show http-client.cm

The command outputs the following report:

               Moniker: /core/network/http-client
                   URL: #meta/http-client.cm
                  Type: CML static component
       Component State: Resolved
 Incoming Capabilities: config
                        fuchsia.logger.LogSink
                        fuchsia.net.name.Lookup
                        fuchsia.posix.socket.Provider
                        pkg
  Exposed Capabilities: fuchsia.net.http.Loader
           Merkle root: d9e73f5b061f2f227e596e2e0079ff3a095fc69e192cf85e0d7621826c76356c
       Execution State: Running
          Start reason: '/core/feedback' requested capability 'fuchsia.net.http.Loader'
         Running since: ...
                Job ID: 41268
            Process ID: 41311
 Outgoing Capabilities: fuchsia.net.http.Loader

Notice a few of the details reported here:

  1. A unique identifier for the component instance (called a moniker).
  2. The package URL where this component was loaded from.
  3. The execution state of the component.
  4. The current job/process ID where the instance is running.
  5. A set of requested and exposed capabilities for the component.

Trace a capability route

In the previous output, there are three capability groups listed:

  • Incoming Capabilities: Capabilities that the component declares with use. These are provided to the component through its namespace.
  • Outgoing Capabilities: Capabilities the component has published to its outgoing directory.
  • Exposed Capabilities: Capabilities the component declares with expose. These are the component's exposed services.

One of the capabilities exposed by http-client to its parent realm is fuchsia.net.http.Loader. This enables other components to issue HTTP requests.

Use the component select command determine how many components interact with this capability:

ffx component capability fuchsia.net.http.Loader

The command lists all the matching components:

Exposed:
  /core/network/http-client
  /core/network
Used:
  /core/cobalt
  /core/feedback
  /core

This indicates that the cobalt and feedback components use this capability (i.e., have it listed under Incoming Capabilities). The common ancestor between these components is core, which handles the routing of this capability to the necessary children.