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Defined in fuchsia.component.runner/component_runner.fidl

A protocol for binding and controlling the lifetime of a component instance started using ComponentRunner.Start(). The component manager is the intended direct client of this protocol.

When the controlled component instance terminates or becomes inaccessible for any reason, the server closes the connection with an epitaph.


A component may exist in one of two states: Started, or Stopped. The component is Started from the time ComponentRunner.Start() is called until the ComponentRunner closes the ComponentController handle. The component then transitions to Stopped.

Component manager uses ComponentController to terminate a component in two steps:

  1. Component manager calls Stop() to indicate that the ComponentRunner should stop a component's execution and close this connection with an epitaph.
  2. If after some time the ComponentController is not closed, component manager calls Kill() to indicate that the ComponentRunner must halt a component's execution immediately, and then close this connection with an epitaph. The component manager may wait some period of time after calling Kill() before closing the ComponentController channel, but makes no guarantees it will wait or for how long.

Component manager first waits for the ComponentController to close, and then tears down the namespace it hosts for the stopped component. Component manager may call Kill() without first having called Stop().

Before stopping, a component can optionally use OnEscrow to store some state in the framework, to receive those state again the next time it is started.


This protocol sends a FIDL epitaph to indicate that the component instance has been terminated. The component runner is expected to clean up all resources attributed to the component before closing the connection.

The following epitaphs may be sent by the server on error:

  • ZX_OK: The component exited successfully, typically because the component was asked to stop or it decided independently to exit.
    • start_info.resolved_url is not supported by this runner;
    • start_info contains missing or invalid arguments.
  • INSTANCE_CANNOT_START: The runner could not start the component. For example, a critical part of the program could not be found or loaded, or the referenced binary was invalid for this runner.
  • RESOURCE_UNAVAILABLE: The component could not be launched due to lack of resources.
  • INTERNAL: An unexpected internal runner error was encountered.
  • INSTANCE_DIED: The component instance was started but subsequently terminated with an error.
  • Other status codes (e.g. ZX_ERR_PEER_CLOSED) may indicate a failure of the component runner itself. The component manager may respond to such failures by terminating the component runner's job to ensure system stability.


Stop this component instance immediately.

The ComponentRunner must immediately kill the component instance, and then close this connection with an epitaph. After the connection closes, component manager considers this component instance to be Stopped and the component's namespace will be torn down.

In some cases Kill() may be issued before Stop(), but that is not guaranteed.




Store some of the component's state in the framework, to be redelivered to the component the next time it's started (a practice called "escrowing").

When the framework receives this event, it will wait until the current execution of the component has finished, then start the component again when the ZX_CHANNEL_READABLE signal is observed on outgoing_dir.

Repeated calls will replace the old escrowed value. This is discouraged.

Handles escrowed via OnEscrow are always delivered to the next execution of the component.

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payload ComponentControllerOnEscrowRequest


Event for runners to publish diagnostics to the platform.

This event signals to the platform that the runner for this component is publishing diagnostics about the runtime of the component. The component manager may optionally expose this data to clients.


payload fuchsia.diagnostics.types/ComponentDiagnostics


Request to stop the component instance.

After stopping the component instance, the server should close this connection with an epitaph. After the connection closes, component manager considers this component instance to be Stopped and the component's namespace will be torn down.




Defined in fuchsia.component.runner/component_runner.fidl

A protocol used for running components.

This protocol is implemented by components which provide a runtime environment for other components.

Note: The component manager is the only intended direct client of this interface.


Start running a component instance described by start_info.

Component manager binds and uses controller to control the lifetime of the newly started component instance.

Errors are delivered as epitaphs over the ComponentController protocol. In the event of an error, the runner must ensure that resources are cleaned up.



start_info ComponentStartInfo
controller server_end<ComponentController>


ComponentControllerOnEscrowRequest resource

Defined in fuchsia.component.runner/component_runner.fidl

outgoing_dir server_end<fuchsia.io/Directory>

Escrow the outgoing directory server endpoint. Whenever the component is started, the framework will return this channel via ComponentStartInfo.outgoing_dir.

escrowed_dictionary fuchsia.component.sandbox/Dictionary

Escrow some user defined state. Whenever the component is started, the framework will return these handles via ComponentStartInfo.escrowed_dictionary.

The framework will not wait for any signals on these objects.


Let's say a component needs to escrow an event pair that represents the result of some expensive calculation. It can create a dictionary, put the event pair inside with an appropriate key (e.g. "my_event_pair"), then check for that entry on startup.

ComponentNamespaceEntry resource

Defined in fuchsia.component.runner/component_runner.fidl

A single component namespace entry, which describes a namespace mount point (path) and the directory backing it (directory). This type is usually composed inside a vector. See ComponentStartInfo.ns for more details.

path string[4095]

The mount point for the directory, including a leading slash. For example: "/pkg", "/svc", or "/config/data".

directory fuchsia.io/Directory

The directory mounted at the above path.

ComponentStartInfo resource

Defined in fuchsia.component.runner/component_runner.fidl

Parameters for starting a new component instance.

resolved_url fuchsia.url/Url

The resolved URL of the component.

This is the canonical URL obtained by the component resolver after following redirects and resolving relative paths.

program fuchsia.data/Dictionary

The component's program declaration. This information originates from ComponentDecl.program.

ns vector<ComponentNamespaceEntry>[32]

The namespace to provide to the component instance.

A namespace specifies the set of directories that a component instance receives at start-up. Through the namespace directories, a component may access capabilities available to it. The contents of the namespace are mainly determined by the component's use declarations but may also contain additional capabilities automatically provided by the framework.

By convention, a component's namespace typically contains some or all of the following directories:

  • "/svc": A directory containing services that the component requested to use via its "import" declarations.
  • "/pkg": A directory containing the component's package, including its binaries, libraries, and other assets.

The mount points specified in each entry must be unique and non-overlapping. For example, [{"/foo", ..}, {"/foo/bar", ..}] is invalid.

outgoing_dir server_end<fuchsia.io/Directory>

The directory this component serves.

runtime_dir server_end<fuchsia.io/Directory>

The directory served by the runner to present runtime information about the component. The runner must either serve it, or drop it to avoid blocking any consumers indefinitely.

numbered_handles vector<fuchsia.process/HandleInfo>[128]

The numbered handles that were passed to the component.

If the component does not support numbered handles, the runner is expected to close the handles.

encoded_config fuchsia.mem/Data

Binary representation of the component's configuration.


The first 2 bytes of the data should be interpreted as an unsigned 16-bit little-endian integer which denotes the number of bytes following it that contain the configuration checksum. After the checksum, all the remaining bytes are a persistent FIDL message of a top-level struct. The struct's fields match the configuration fields of the component's compiled manifest in the same order.

break_on_start handle<eventpair>

An eventpair that debuggers can use to defer the launch of the component.

For example, ELF runners hold off from creating processes in the component until ZX_EVENTPAIR_PEER_CLOSED is signaled on this eventpair. They also ensure that runtime_dir is served before waiting on this eventpair. ELF debuggers can query the runtime_dir to decide whether to attach before they drop the other side of the eventpair, which is sent in the payload of the DebugStarted event in fuchsia.component.events.

component_instance handle<event>

An opaque token that represents the component instance.

The fuchsia.component/Introspector protocol may be used to get the string moniker of the instance from this token.

Runners may publish this token as part of diagnostics information, to identify the running component without knowing its moniker.

The token is invalidated when the component instance is destroyed.

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escrowed_dictionary fuchsia.component.sandbox/Dictionary

A dictionary containing data and handles that the component has escrowed during its previous execution via ComponentController.OnEscrow.

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