Test Component

Create a test component



executable("my_test_bin") {
  testonly = true
  output_name = "my_test"

  sources = [

test_package("my_test_pkg") {
  deps = [

  tests = [
      name = "my_test"

test_package will expect that there is a corresponding cmx file in the meta folder. So for above example there should be a my_test.cmx file in meta/.


    "program": {
        "binary": "test/my_test"
    "sandbox": {
        "services": [...]

Running the tests

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

fx test TEST_NAME

For more information on running Fuchsia tests, see Running tests as components.

Isolated Storage

  • By default, the test component is launched in a new hermetic environment.
  • The generated environment name is of form test_env_XXXXX, where XXXXX is a randomly generated number.
  • Each test component receives a new isolated storage directory.
  • The directory is deleted after the text exits, regardless of the test's outcome.

Keep storage for debugging

If you need to keep test storage for the debugging after the test ends, use run-test-component in the Fuchsia shell and pass --realm-label flag.

The --realm-label flag defines the label for environment that your test runs in. When the test ends, the storage won't be deleted automatically - it'll be accessible at a path under /data. Assuming you:

  • gave your test component (in package mypackage with component manifest myurl.cmx) access to the "isolated-persistent-storage" feature
  • passed --realm-label=foo to run-test-component
  • wrote to the file /data/bar from the test binary
  • can connect to the device via fx shell

You should see the written file under the path /data/r/sys/r/<REALM>/fuchsia.com:<PACKAGE>:0#meta:<CMX>/<FILE>, e.g. /data/r/sys/r/foo/fuchsia.com:mypackage:0#meta:myurl.cmx/bar

Assuming you can connect to the device via ssh, you can get the data off the device with the in-tree utility fx scp.

When you're done exploring the contents of the directory, you may want to delete it to free up space or prevent it from interfering with the results of future tests.

Ambient Services

All test components are started in a new hermetic environment. By default, this environment only contains a few basic services (ambient):


Tests can use these services by mentioning them in their sandbox > services.

Logger Service

Tests and the components launched in a hermetic environment will have access to system's fuchsia.logger.LogSink service if it is included in their sandbox. For tests to inject Logger, the tests must use injected-services (see below). Then, the injected Logger service takes precedence.

Run external services

If your test needs to use (i.e. its sandbox includes) any services other than the ambient and logger services above, you must perform either, both or none:

  • Inject the services by starting other components that provide those services in the hermetic test environment
  • Request non-hermetic system services be included in the test environment, when a service cannot be faked or mocked, see Other system services.

To inject additional services, you can add a injected-services clause to the manifest file's facets:

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

fx test will start component_url1 and component_url2 and the test will have access to service_name1 and service_name2. Note that this makes the injected services available in the test environment, but the test component still needs to "use" them by including the service in its sandbox > services.

Network access

Currently we cannot run an instance of netstack inside a hermetic environment, because it conflicts with the real netstack. If your test needs to talk to netstack, it may only talk to the real netstack outside the test environment. To enable this workaround you need to allow some system services:

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

Other system services

There are some services, such as network, that cannot be faked or mocked. However, you can connect to real system versions of these services by mentioning these services in system-services. Services that cannot be faked:


Depending on your use case you can include one or more of the services above. However, services that are not listed here are not supported.

This option would be replaced once we fix CP-144 (in component manager v2).