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 framework or builtin capabilities into component manager.
  • 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
  • BreakpointSystemClient


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 breakpoint system.
  • 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 BreakpointSystem FIDL service at $out/svc/fuchsia.test.breakpoints.BreakpointSystem.
  • 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.

The BreakpointSystem FIDL service is used to set breakpoints and unblock component manager. This code demonstrates using the BreakpointSystem service:

let breakpoint_system = test.connect_to_breakpoint_system().await?;
let receiver = breakpoint_system.set_breakpoints(vec![StopInstance::TYPE]).await?;

By the end of this code block:

  • A breakpoint receiver has been created which listens to StopInstance 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 breakpoint system addresses the problem of verifying state in component manager and is analogous to a debugger's breakpoint system.

Since the breakpoint system is built on top of system events:

  • A breakpoint 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, the breakpoint system 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 BreakpointSystemClient library looks something like this:

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

// Get a receiver by setting breakpoints
let breakpoint_system = test.connect_to_breakpoint_system().await?;
let receiver = breakpoint_system.set_breakpoints(vec![StartInstance::TYPE]).await?;

// Unblock component manager

// Wait for an invocation
let invocation = receiver.expect_type::<StartInstance>().await?;

// Verify state

// Resume from invocation

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, breakpoints offer the following additional functionality:

Multiple receivers

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

// StartInstance and RouteCapability events can be interleaved,
// so use different receivers.
let start_receiver = breakpoint_system.set_breakpoints(vec![StartInstance::TYPE]).await?;
let route_receiver =

// Expect 5 components to start
for _ in 1..=5 {
    let invocation = start_receiver.expect_type::<StartInstance>().await?;

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

Discardable receivers

It is possible to listen for specific invocations and then discard the receiver, causing future invocations to be ignored:

// Set a breakpoint on StopInstance events
let stop_receiver = breakpoint_system.set_breakpoints(vec![StopInstance::TYPE]).await?;

    // Temporarily set a breakpoint on RouteCapability events
    let use_receiver = breakpoint_system.set_breakpoints(vec![RouteCapability::TYPE]).await?;

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

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

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

Capability injection

Several tests need to communicate with components directly. The simplest way to do this is for a component to connect to a capability offered by the test.

It is possible to listen for a RouteCapability event and inject an external capability provider:

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

// Set a breakpoint on RouteCapability events
let receiver = breakpoint_system.set_breakpoints(vec![RouteCapability::TYPE]).await?;

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

// Inject the EchoService capability
let serve_fn = echo_capability.serve_async();

// Resume from the invocation

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 RouteCapability event:

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

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

Event sinks

It is possible to soak up events of certain types and drain them at a later point in time:

let receiver = breakpoint_system.set_breakpoints(vec![PostDestroyInstance::TYPE]).await?;
let sink = breakpoint_system.soak_events(vec![StartInstance::TYPE]).await?;

// Wait for the root component to be destroyed
let invocation = receiver.expect_exact::<PostDestroyInstance>("/").await?;

// Drain events from the sink
let events = sink.drain().await;

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

Debug Mode

Both BlackBoxTest and BreakpointSystemClient 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 breakpoint system.

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

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

    • The BreakpointSystem FIDL service at $out/svc/fuchsia.test.breakpoints.BreakpointSystem.

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

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