Realm builder

The realm builder library exists to facilitate integration testing of v2 components by allowing for the run-time construction of realms and mocked components specific to individual test cases.

If a test wishes to launch a child component, then realm builder is likely a good fit for assisting the test.

If a test does not benefit from having either realms tailor made to each test case or realms containing mocked components unique to each test case, then the test can likely be made simpler to implement, understand, and maintain by using static component manifests. If a test does call for either (or both) of these things, then realm builder is a good fit for assisting the test.

The realm builder library is available in multiple languages, and the exact semantics and abilities available in each language may vary. This document uses Rust in its example code, but the concepts shown exist in all versions of the library. For a comprehensive list of features and which languages they are supported in, see the feature matrix at the end of this document.

Setting up realm builder

When the realm builder library is to be used in a test, the realm builder CML shard must be included in the test's manifest. The shard brings important configuration needed to set up the collection in which the constructed realms will run. The library will not work without it.

    include: [

Constructing a realm

The best practice is to create a new RealmBuilder instance (by calling the library constructor) for each test case in your test. This will create a unique, isolated, child realm that ensures that the side-effects of one test case do not affect the others.

Within this realm, you can create any number of child components, route capabilities between them, and even add child components that are implemented directly within the test controller code.

Adding a static component as a child

Once a new RealmBuilder instance has been created, components can be added to it. Each added component needs two things: a name it will exist under in the realm (just like it would when defining static components in a .cml file), along with a source which defines how the component is created when the realm is built.

For static components, the provided source should be a ComponentSource::url. Any valid component URL is a suitable here, if it would work in the children section of a component manifest then it will work with realm builder.

In the example below two components are added to a new realm with the names a and b. Both of these components are children of the realm's root, and once the realm is created the component framework will attempt to load component a from fuchsia-pkg://, and b from fuchsia-pkg://

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="src/lib/fuchsia-component-test/tests/src/" region_tag="add_a_and_b_example" adjust_indentation="auto" %}
  /      \
 a        b

Adding capability routes

By default there are no capability routes in the new realm, neither to nor from any components in the realm. Unless some are added, none of the components in the realm will be able to access any capabilities.

Routes between components in the realm

In our example above where we add components a and b to our realm, by default neither of them can access anything from each other. If b needs to access a capability from a, then we need to add a capability route between the two components.

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="src/lib/fuchsia-component-test/tests/src/" region_tag="route_from_a_to_b_example" adjust_indentation="auto" %}

Now, component b can open fidl.examples.routing.echo.Echo in its namespace (presuming it has a suitable use declaration) and the request will be routed to component a. Component a must then serve and expose fidl.examples.routing.echo.Echo.

Routes from the realm to the test controller

There are often capabilities available inside the created realm which the test would like to access. For example, the test may wish to verify that component b is behaving as expected by making assertions on state exposed by the protocol fidl.examples.routing.echo.EchoClientStats.

These routes can be added by using a route endpoint of “above root”. When a route has a target of RouteEndpoint::above_root() the created realm will automatically expose the capability to its parent. This allows the realm builder instance to access the exposed capability.

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="src/lib/fuchsia-component-test/tests/src/" region_tag="route_to_above_root_example" adjust_indentation="auto" %}

Routes from outside the realm #{routes-from-outside}

This section shows how capabilities can be routed from the test controller's realm to within the generated realm. In order to serve a capability from the test controller directly and inject it into the realm, jump to mock components."

In order for a capability to be routed to components within any realm built by realm builder, you must:

  • offer the capability from the test controller's realm to the #fuchsia_component_test_collection collection, and
  • add a route from RouteEndpoint::above_root() to the target component.

Some capabilities are offered automatically by virtue of including the realm builder CML shard. The set of automatically offered capabilities can be seen by looking at the relevant offer declarations in realm builder's CML shard:

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="src/lib/fuchsia-component-test/meta/fuchsia_component_test.shard.cml" region_tag="collection_offers" adjust_indentation="auto" %}

As an example, to make thefuchsia.logger.LogSink protocol from the parent's realm available to components a and b, write the following code:

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="src/lib/fuchsia-component-test/tests/src/" region_tag="route_logsink_example" adjust_indentation="auto" %}

To route a capability that isn't in the realm builder shard, you must offer it directly.

    include: [
    children: [
            name: "some-child",
            url: "...",
    offer: [
            protocol: "",
            from: "#some-child",
            to: [ "#fuchsia_component_test_collection" ],

Adding a mock component

Mock components allow tests to supply a local function to behave as a dedicated component, instead of relying on a component implemented elsewhere. The local function can hold state specific to the test case it is being used in, allowing each constructed realm to have a mock geared for its specific use case.

Revisiting our above example where we have two components, a and b, where a is a static component and serves fidl.examples.routing.echo.Echo to b, we could alternatively serve fidl.examples.routing.echo.Echo from a mock component directly. This mock component could be set up to pass a message back to the test when its echo implementation is accessed, allowing the test to confirm that b is using this protocol as expected.

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="src/lib/fuchsia-component-test/tests/src/" region_tag="mock_component_example" adjust_indentation="auto" %}

The echo_server_mock function used above can be implemented just like any standalone component would be, by creating a new ServiceFs to handle incoming FIDL connections. When the echo_string function on the protocol is exercised, the oneshot is consumed to send a message back to the test, allowing the test to proceed to completion.

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="src/lib/fuchsia-component-test/tests/src/" region_tag="echo_server_mock" adjust_indentation="auto" %}

The above example is identical to the one detailed in the static components section, with one key difference: component a has a manifest constructed by realm builder and when run its implementation is provided by a local Rust function, instead of by an executable loaded from a package.

Under the hood, this works because realm builder implements the correct protocols to enable the Component Framework to treat the local function just like any other real component. The framework generates valid handles for the component, and informs the component's runner (realm builder, in this case) when the component should start and stop running, just like every other component running on the system.

It is with those framework-generated handles that the function provides capabilities into the realm or consumes capabilities from it. Running a ServiceFs on the mock‘s outgoing directory handle to provide a FIDL protocol is identical from the framework’s perspective to other components, most of which do the same thing in their implementations.

Modifying generated manifests

The .add_route function of realm builder addresses the majority of capability routing needs, but does not handle some edge cases. Installing custom environments, or setting the subdir field on a directory capability, are both currently unsupported.

To address use cases such as these, once the components and routes have been provided, the individual component manifests that realm builder has assembled can be manually tweaked.

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="src/lib/fuchsia-component-test/tests/src/" region_tag="mutate_generated_manifest_example" adjust_indentation="auto" %}

Note that the manifests for components with a source of url can not be fetched or set, as the contents of these components are not directly available to realm builder.

Running a realm

Once realm builder contains all the components and routes needed for the test case, the real realm can be created, and its components made ready to execute. This is done by calling realm.create().

Under the hood, realm builder generates a series of component manifests, acts as a Resolver to serve the manifests using generated URLs, and then adds the generated root URL to a [component collection].

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="src/lib/fuchsia-component-test/tests/src/" region_tag="create_realm" adjust_indentation="auto" %}

At this point, the constructed realm is no longer mutable. All further interactions must be performed using the realm_instance returned by create(). Any eager components will immediately begin execution, and any capabilities that were routed to above_root can be accessed by the test.

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="src/lib/fuchsia-component-test/tests/src/" region_tag="connect_to_protocol" adjust_indentation="auto" %}

Realm destruction

When the test no longer needs the realm, it can be destroyed by destroying the realm instance returned by create():

// As per rust semantics, this also happens when `realm_instance` goes out of
// scope.

This action instructs Component Manager to destroy the realm and all its children.

Advanced configuration of the test realm collection

If a test author wants more control over the test realm‘s environment, they can declare a new collection in the test driver’s manifest. Any such collection must be set up appropriately for realm builder, so copying the contents of realm builder's shard into the test driver manifest (and renaming the environment and collection) is a good starting point for setting up a new collection. For a real-world example of this.


Invalid capability routes

The .add_route function of realm builder is powerful, but it's not always able to ensure that the routes are fully valid.

One common source of invalid capability routes comes from attempting to route capabilities with a source of above_root: Unless the test's manifest has been modified to offer the capabilities to the #fuchsia_component_test_collection collection, then requests to open the capability will not resolve. When a component attempts to access the capability, Component Manager will follow the offers to the root of the created realm and then error out, due to a missing offer. Users can know when this happens by watching for log messages from component manager, such as:

[86842.196][klog][E] [component_manager] ERROR: Failed to route protocol `fidl.examples.routing.echo.Echo` with target component `/core:0/test_manager:0/tests:auto-10238282593681900609:4/test_wrapper:0/test_root:0/fuchsia_component_test_
[86842.197][klog][I] collection:auto-4046836611955440668:16/echo-client:0`: An `offer from parent` declaration was found at `/core:0/test_manager:0/tests:auto-10238282593681900609:4/test_wrapper:0/test_root:0/fuchsia_component_test_colle
[86842.197][klog][I] ction:auto-4046836611955440668:16` for `fidl.examples.routing.echo.Echo`, but no matching `offer` declaration was found in the parent

See the routes coming from outside the realm section for more information on how to set up these routes correctly.

Language feature matrix

CFv2 componentsY
CFv1 componentsN
mock componentsY
strong capability routesY
weak capability routesN
custom environmentsN
setting subdirectoriesN