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Hermetic testing using OpaqueTest (Components v2)


The OpaqueTest 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 v2 component, OpaqueTest lets you:

  • 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.

Minimum requirements

For the OpaqueTest framework to function correctly, the test cmx manifest must specify (at minimum) the following features and services:

"include": [ "sdk/lib/diagnostics/syslog/client.shard.cmx" ],
"sandbox": {
    "features": [
    "services": [

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


In the simplest case, a test can be started as follows:

let test = OpaqueTest::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 EventSource FIDL service at $out/svc/fuchsia.sys2.EventSource.
  • 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 EventSource FIDL service to subscribe to events and unblock the component manager. The following example shows you how to use the EventSource 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 that receives Stopped events.
  • Component manager’s execution has begun.
  • The root component (and its eager children, if any) will be started soon.

Custom tests

In some cases, you may want to customize OpaqueTest::default. Use an OpaqueTestBuilder 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.

  • The configuration file to be used by component manager.

  • Additional command-line args to be used by component manager.

let test = OpaqueTestBuilder::new("fuchsia-boot:///#meta/")
    .add_dir_handle("/boot", pkg_channel.into())


An EventSource is used to subscribe to system events sent by component manager.

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).

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 an EventSource looks something like this:

// Create an EventSource using ::new_sync() or use the source
// provided by OpaqueTest
let test = OpaqueTest::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 = EventMatcher::ok().expect_match::<Started>(&mut event_stream).await;

// Verify state

// Resume from event

Calling resume() on an event unblocks component manager and allows it to proceed with the event dispatch.

Scoping of events

The EventSource FIDL protocol can be requested by any component instance within the component topology and is served by the component manager.

Events are capailities themselves so they have to be routed as well. Refer to event capabilities for more details on this.

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

    program: {
        binary: "bin/client",
    use: [
            protocol: [
            from: "framework"
          event: [ "started", "stopped" ],
          from: "framework",

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

If a component requests a EventSource 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 =

// Unblock component manager

// Expect 5 components to start
for _ in 1..=5 {
    let event = EventMatcher::ok().expect_match::<Started>(&mut start_event_stream).await;

// Expect a CapabilityRouted event from ./foo:0
let event = EventMatcher::ok()
    .expect_match::<CapabilityRouted>(&mut route_event_stream)

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 route_event_stream = event_source.subscribe(vec![CapabilityRouted::TYPE]).await?;

    // Expect a CapabilityRouted event from ./bar:0
    let event = EventMatcher::ok().moniker("./bar:0").expect_match::<CapabilityRouted>(&mut route_event_stream).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 = EventMatcher::ok().expect_match::<Stopped>(&mut stop_event_stream).await?;
println!("{} was stopped!", event.target_moniker);

Event sequences

When writing tests, it is useful to expect events to occur in some order. Event Sequences allow writers to verify ordering of events:

// This test expects the following events to occur:
// 1. the two trigger components stop in any order
// 2. the parent component stops
// 3. the two trigger components are destroyed in any order
// 4. the parent component is destroyed
let expectation = EventSequence::new()
    .subscribe_and_expect(&mut event_source)

// Start the component tree

// Wait for the event sequence to occur

Capability injection

Several tests need to mock out capabilities that a component connects to in the test. Sometimes, tests may wish to communicate with components directly. 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");

Injectors can automatically install themselves on CapabilityRouted events.

let echo_capability: Arc<EchoCapability> = EchoCapability::new();

// Inject the Echo capability when /foo:0 successfully connects to the Echo service
echo_capability.inject(&event_source, EventMatcher::ok().moniker("/foo:0")).await;


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");

Interposers can automatically install themselves on CapabilityRouted events.

let interposer = EchoInterposer::new();

// Interpose the Echo capability when any component successfully connects to the Echo service
echo_interposer.interpose(&event_source, EventMatcher::ok()).await;


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 = EventLog::record_events(&mut event_source, vec![Started::NAME]).await?;

// Wait for the component to be destroyed
let event = EventMatcher::ok().expect_match::<Destroyed>(&mut event_stream).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()

Debug Mode

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

Component manager's startup behavior can be configued via the --config flag which accepts a path to a JSON configuration file.

To start component manager in debug mode, use the --config flag to pass in a JSON configuration file that has debug set to true. This is exactly what OpaqueTest::default does.

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 EventSource FIDL service at $out/svc/fuchsia.sys2.EventSource.

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

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