Test components (Components v1)


Note: see Testing with Components for modern (.cml) component testing.


Test components are components that implement a test. Tests run in a given environment, and then report whether they passed or failed. Typically tests are written using various testing frameworks, and may report more detailed results such as whether individual test cases within a test suite passed or failed.

The Component Framework allows launching tests as components. Most tests are comprised of a single component - these are typically referred to as unit tests. Some tests exercise multiple components working together - these are typically referred to as integration tests.

Creating a test component and package

A test package may contain one or more test components. Test components are components that implement a test suite. Test packages can also contain other components that are not the test itself, but participate in the test. For instance:

  • A package may contain a single test component, which implements a unit test that exercises some business logic.
  • A package may contain a test component and a second component that implements a service. The test component may then act as a client of the second component, which makes for an integration test between client and server code. Both the clients and server are located in the same package in order to ensure that the second component is present and can be launched by the test component.

In order to define your test package and components, you should use the appropriate build rules. Refer to the test packages guide.

Test component manifest

Every component has a manifest. Test components may have manifests that are similar to regular components, or they may use additional special syntax for testing that's covered below.

A component manifest for a simple unit test might be named meta/my_test.cmx and look as follows:

    "include": [ "syslog/client.shard.cmx" ],
    "program": {
        "binary": "bin/my_test"

Component manifests for simple unit tests can be generated by the build rules.

Running your test

To run a Fuchsia test out of your build, execute:

For more information, see Run Fuchsia tests.

Isolated Storage

By default, the test component is launched in a new hermetic environment, isolated from system services and system storage. This keeps the rest of the system from interfering with the results of your test, and vice versa.

Test environments have unique generated names in the form test_env_XXXXXXXX, where the placeholder is 8 random hexadecimal digits.

Test components may use persistent storage as usual, by specifying "isolated-persistent-storage" under sandbox.features in their manifest. However unlike regular components, the test's storage directory will be cleared before the test component is launched and after it terminates.

This is usually the desired behavior since it keeps test runs from interfering with each other via side effects. However, if you need to retain a test's storage for troubleshooting, use run-test-component in the Fuchsia shell and pass the --realm-label flag followed by a name for your test environment.

The --realm-label flag defines the label for an environment that your test runs in. When the test ends, the storage won‘t be deleted automatically. Instead it’ll be accessible at a path under /data. The path will take the following form:


For instance:


You can copy the files from the target device using fx scp. Afterwards, consider deleting the storage directory.

Example commands

Using fx:

fx test my_test_url.cmx --realm my_realm


run-test-component --realm-label=my_realm my_test_url.cmx

Note: run-test-component will be deprecated in a future release, prefer fx test when remove device access is available.


Basic system services

By default, test components may only access a small subset of system services, in order to promote hermeticity. These system services can be used in a test component by specifying them in the test manifest under sandbox.services as usual.


Integration testing

A test component may need to interact with other components, such as in an integration test. One way to do this is to include all components under test in the test‘s package, and then specify in the test’s manifest a mapping between the services that these components offer and their launch URLs.

This is done as follows:

"facets": {
  "fuchsia.test": {
    "injected-services": {
        "service_name1": "component_url1",
        "service_name2": "component_url2"

However, note that all the test executions will run in the same environment. If a service had dirtied state, a subsequent TEST_F execution will inadvertently run against that dirtied state.

See this doc for authoring more sophisticated scenarios (such as graphics and UI tests) in v1.

Additional system services

Tests may request access to additional system services, at the expense of their own hermeticity (as they become subject to elements of the system outside of the test's scope).

This is done as follows:

"facets": {
  "fuchsia.test": {
    "system-services": [

Real system services cannot be accessed by test components unless explicitly allowlisted as shown below:

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="src/sys/run_test_component/test_metadata.cc" region_tag="allowed_system_services" adjust_indentation="auto" %}