Black box testing with component manager


The black box testing framework enables an integration test to observe or influence the behavior of component manager without depending on its internal libraries.

Creating dependencies on component manager's internal libraries is problematic for a number of reasons:

  • A test can set up component manager inconsistently with how component manager is normally started.
  • A test can modify component manager’s behavior in arbitrary ways.
  • Changes to component manager may require changing the test.


To test the behavior of a component or a component manager feature, you must be able to write a black box test that can:

  • Start component manager in a hermetic environment.
  • Communicate with component manager using only FIDL and the hub.
  • Access the hub of the root component.
  • Wait for events to occur in component manager.
  • Halt a component manager task on an event.
  • Inject or mock out capabilities.
  • Interpose between a client and a service.

Note: The black box testing framework covers all of these points.


The testing framework provides two Rust libraries for black box testing:

  • BlackBoxTest
  • EventSource


BlackBoxTest is a Rust library provided by the testing framework. You can use the classes and methods in this library to automate large parts of the setup needed for a black box test.

Minimum requirements

For the BlackBoxTest library to function correctly, the integration test component manifest must specify (at minimum) the following features and services:

"sandbox": {
    "features": [
    "services": [

These services and features ensure that BlackBoxTest can set up a hermetic environment and launch component manager.


In the simplest case, a black box test looks like the following:

let test = BlackBoxTest::default("fuchsia-pkg://").await?;

By the end of this statement:

  • A component manager instance has been created in a hermetic environment.
  • The root component is specified by the given URL.
  • Component manager is waiting to be unblocked by the EventSource.
  • The root component manifest ( has been resolved.
  • No component has been started.
  • Component manager’s outgoing directory is serving:
    • The hub of the root component at $out/hub.
    • The BlockingEventSource FIDL service at $out/svc/fuchsia.sys2.BlockingEventSource.
  • The state of the hub reflects the following:
    • Static children of the root component should be visible.
    • Grandchildren of the root component should not be visible (because they haven't been resolved yet).
    • There should be no exec directories for any component.

Use the BlockingEventSource FIDL service to subscribe to events and unblock the component manager. The following example shows you how to use the BlockingEventSource service:

let event_source = test.connect_to_event_source().await?;
let event_stream = event_source.subscribe(vec![Stopped::TYPE]).await?;

By the end of this code block:

  • An event_stream has been created which receives Stopped events.
  • Component manager’s execution has begun.
  • The root component (and its eager children, if any) will be started soon.

Custom tests and convenience methods

In some cases, you may want to customize BlackBoxTest::default. BlackBoxTest::custom allows you to specify:

  • The component manager manifest to be used for the test.

  • Additional directories to be created in component manager's namespace.

  • A file descriptor to redirect output from components.

let test = BlackBoxTest::custom(
    vec![("my_dir", my_dir_handle)],

The BlackBoxTest library also provides convenience methods for starting up component manager and expecting a particular output:

    "Hippos rule!",


The EventSource addresses the problem of verifying state in component manager and is analogous to a debugger's breakpoint system.

Since the EventSource is built on top of system events:

  • A subscription can only be set on a system event.
  • It supports all system events in component manager.
  • It can be scoped down to a realm of the component hierarchy.
  • It follows the component manager’s rules of event propagation (i.e - an event dispatched at a child realm is also dispatched to its parent).

Note: When component manager is in debug mode, an EventSource is installed at the root. Hence it receives events from all components.

For reliable state verification, a test must be able to:

  • Expect or wait for various events to occur in component manager.
  • Halt the component manager task that is processing the event.

The workflow for the EventSource library looks something like this:

// Create a EventSource using ::new() or use the client
// provided by BlackBoxTest
let test = BlackBoxTest::default("fuchsia-pkg://").await?;

// Get an event stream of the `Started` event.
let event_source = test.connect_to_event_source().await?;
let event_stream = event_source.subscribe(vec![Started::TYPE]).await?;

// Unblock component manager

// Wait for an event
let event = event_stream.expect_type::<Started>().await?;

// Verify state

// Resume from event

Scoping of events

The EventSource can be requested by any component instance within the component topology served by the component manager. The component manager only delivers system events within the realm of the requesting component instance.

A component instance can request a scoped BlockingEventSource in its manifest file as follows:

    "program": {
        "binary": "bin/client",
    "use": [
            "protocol": [
            "from": "framework"

Another component can pass along its scope of system events by passing along the BlockingEventSource capability through the conventional routing operations offer, expose and use.

If a component requests a BlockingEventSource then its children cannot start until it explicitly calls start_component_tree.

Additional functionality

With complex component hierarchies, event propagation is hard to predict and may even be non-deterministic due to the asynchronous nature of component manager. To deal with these cases, EventSource offers the following additional functionality:

Multiple event streams

It is possible to register multiple event streams, each listening to their own set of events:

// Started and CapabilityRouted events can be interleaved,
// so use different event streams.
let start_event_stream = event_source.subscribe(vec![Started::TYPE]).await?;
let route_event_stream =

// Expect 5 components to start
for _ in 1..=5 {
    let event = start_event_stream.expect_type::<Started>().await?;

// Expect a CapabilityRouted event from ./foo:0
let event = route_event_stream.expect_exact::<CapabilityRouted>("./foo:0").await?;

Discardable event streams

It is possible to listen for specific events and then discard the event stream, causing future events to be ignored:

// Subscribe to Stopped events
let stop_event_stream = event_source.subscribe(vec![Stopped::TYPE]).await?;

    // Temporarily subscribe to CapabilityRouted events
    let use_event_stream = event_source.subscribe(vec![CapabilityRouted::TYPE]).await?;

    // Expect a CapabilityRouted event from ./bar:0
    let event = route_event_stream.expect_exact::<CapabilityRouted>("./bar:0").await?;
    println!("/bar:0 used capability -> {}", event.capability_id);

// At this point, the test does not care about CapabilityRouted events, so the
// event stream can be dropped. If the event stream were left instantiated,
// component manager would halt on future CapabilityRouted events.

// Expect a Stopped event
let event = stop_event_stream.expect_type::<Stopped>().await?;
println!("{} was stopped!", event.target_moniker);

Capability injection

Several tests need to communicate with components directly. It is often also desirable to mock out capabilities that a component connects to in the test. The simplest way to do this is to implement an Injector.

/// Client <---> EchoCapability
/// EchoCapability implements the Echo protocol and responds to clients.
struct EchoCapability;

impl Injector for EchoCapability {
    type Marker = fecho::EchoMarker;

    async fn serve(self: Arc<Self>, mut request_stream: fecho::EchoRequestStream) {
        // Start listening to requests from client
        while let Some(Ok(fecho::EchoRequest::EchoString { value: Some(input), responder })) =
            // Respond to the client with the echo string.
            responder.send(Some(&input)).expect("failed to send echo response");

It is possible to listen for a CapabilityRouted event and install the injector:

// Create the server end of EchoService
let echo_capability = EchoCapability::new();

// Subscribe to CapabilityRouted events
let event_stream = event_source.subscribe(vec![CapabilityRouted::TYPE]).await?;

// Wait until ./foo:0 attempts to connect to the EchoService component capability
let event = event_stream.wait_until_component_capability(


// Resume from the event

Alternatively, the BlockingEventSource can automatically install an injector matching the capability requested by any component in the test.

let echo_capability = EchoCapability::new();

// Subscribe to other events.


Capability interposition

Tests may want to silently observe or mutate messages between a client and service. It is possible to interpose a capability and manipulate the traffic over the channel. Consider an interposer for an Echo service that mutates the input from the client before sending it to the service:

/// Client <---> EchoInterposer <---> Echo service
/// The EchoInterposer copies all echo responses from the service
/// and sends them over an mpsc::Channel to the test.
struct EchoInterposer;

impl Interposer for EchoInterposer {
    type Marker = fecho::EchoMarker;

    async fn interpose(
        self: Arc<Self>,
        mut from_client: fecho::EchoRequestStream,
        to_service: fecho::EchoProxy,
    ) {
        // Start listening to requests from client
        while let Some(Ok(fecho::EchoRequest::EchoString { value: Some(input), responder })) =
            // Copy the response from the service and send it to the test
            let modified_input = format!("{} Let there be chaos!", input);

            // Forward the request to the service and get a response
            let out = to_service
                .expect("echo_string failed")
                .expect("echo_string got empty result");

            // Respond to the client with the response from the service
            responder.send(Some(out.as_str())).expect("failed to send echo response");

The test can use EchoInterposer on any CapabilityRouted event:

// Wait for echo_looper to attempt to connect to the Echo service
let event = event_stream
    .wait_until_component_capability("/echo_looper:0", "/svc/fidl.examples.routing.echo.Echo")

// Setup the interposer
let (interposer, mut rx) = EchoInterposer::new();

Alternatively, the BlockingEventSource can automatically install an interposer matching the capability requested by any component in the test.

let (interposer, mut rx) = EchoInterposer::new();

// Subscribe to othere events.


Event logs

It is possible to record events of certain types asynchronously and flush them at a later point in time:

let event_stream = event_source.subscribe(vec![Destroyed::TYPE]).await?;
let event_log = event_source.record_events(vec![Started::TYPE]).await?;

// Wait for the root component to be destroyed
let event = event_stream.expect_exact::<Destroyed>(".").await?;

// Flush events from the log
let events = event_log.flush().await;

// Verify that the 3 components were started in the correct order
assert_eq!(events, vec![
    RecordedEvent {
        event_type: Started::TYPE,
        target_moniker: "./".to_string()
    RecordedEvent {
        event_type: Started::TYPE,
        target_moniker: "./foo:0".to_string()
    RecordedEvent {
        event_type: Started::TYPE,
        target_moniker: "./foo:0/bar:0".to_string()

Note that recording of events will continue until the EventLog object goes out of scope.

Debug Mode

Both BlackBoxTest and EventSource rely on component manager’s debug mode.

To start component manager in debug mode, pass in --debug as an additional argument to the component_manager.cmx component. In fact, this is exactly what BlackBoxTest::default does when setting up a black box test.

When component manager is in debug mode, it does the following:

  1. Creates the root realm and built-in services.

  2. Creates the hub and the EventSource.

  3. Serves the following from component manager's outgoing directory:

    • The hub of the root component at $out/hub.

    • The BlockingEventSource FIDL service at $out/svc/fuchsia.sys2.BlockingEventSource.

  4. Waits to be unblocked by the BlockingEventSource FIDL service.

  5. Starts up the root component (including any eager children).