docs/concepts/testing/v2/test_runner_framework.md - fuchsia - Git at Google

 # The Fuchsia Test Runner Framework

 <<../../components/_v2_banner.md>>

 ## Integrating testing frameworks with the Component Framework

 The Fuchsia [Component Framework][cf] allows developers to create components in
 a variety of languages and runtimes. Fuchsia's own code uses a diverse mix of
 programming languages for components, including C/C++, Rust, Dart, and Go.

 The Test Runner Framework uses Component Framework [runners][runners] as an
 integration layer between various testing runtimes and a common Fuchsia protocol
 for launching tests and receiving their results. This makes for an [inclusive]
 design that on one hand allows developers to bring their language and testing
 framework of choice, and on the other hand allows building and testing Fuchsia
 on a variety of systems and targeting different hardware.

 ## The Test Manager

 The `test_manager` component is responsible for running tests on a Fuchsia
 device. Test manager exposes the
 [`fuchsia.test.manager.RunBuilder`][fidl-test-manager] protocol, which allows
 launching test suites.

 Each test suite is launched as a child of test manager. Test suites are offered
 capabilities by test manager that enable them to do their work while
 maintaining isolation between the test and the rest of the system. For instance
 hermetic tests are given the capability to log messages, but are not given the
 capability to interact with real system resources outside of their sandbox.
 Test manager uses only one capability from the test realm, a controller protocol
 that test suites expose. This is done to ensure hermeticity (test results aren't
 affected by anything outside of their intended sandbox) and isolation (tests
 don't affect each other or the rest of the system).

 The test manager controller itself is offered to other components in the system
 in order to integrate test execution with various developer tools. Tests can
 then be launched with such tools as [`fx test`][fx-test] and [`ffx`][ffx].

 ## The test suite protocol {#test-suite-protocol}

 The test suite protocol, [`fuchsia.test.Suite`][fidl-test-suite], is used by the
 test manager to control tests, such as to invoke test cases and to collect their
 results.

 Test authors typically don't need to implement this protocol. Instead, they rely
 on a [test runner](#test-runners) to do this for them. For instance, you might
 write a test in C++ using the GoogleTest framework, and then use
 [`gtest_runner`](#gtest-runner) in your [component manifest][component-manifest]
 to integrate with the Test Runner Framework.

 ## Test runners {#test-runners}

 ### A language and runtime-inclusive framework

 Test runners are reusable adapters between the Test Runner Framework and common
 languages & frameworks used by developers to write tests. They implement the
 [`fuchsia.test.Suite`][fidl-test-suite] protocol on behalf of the test author,
 allowing developers to write idiomatic tests for their language and framework of
 choice.

 Component manifests for simple unit tests can be [generated][unit-tests]
 by the build rules. Generated component manifests for v2 tests will include the
 appropriate test runner based on their build definition. For instance a test
 executable that depends on the GoogleTest library will include the
 [GoogleTest runner](#gtest-runner) in its generated manifest.

 ### Inventory of test runners

 The following test runners are currently available for general use:

 #### GoogleTest runner {#gtest-runner}

 A runner for tests written in C/C++ using the GoogleTest framework.
 Use this for all tests written using GoogleTest.

 Common GoogleTest features are supported, such as disabling tests, running
 only specified tests, running the same test multiple times, etc'.
 Standard output, standard error, and logs are captured from the test.

 In order to use this runner, add the following to your component manifest:

 ```json5
 {
     include: [ "//src/sys/test_runners/gtest/default.shard.cml" ]
 }
 ```

 By default GoogleTest test cases run serially (one test case at a time).

 #### Rust runner {#rust-runner}

 A runner for tests written in the Rust programming language and following Rust
 testing idioms.
 Use this for all idiomatic Rust tests (i.e. tests with modules that set the
 attribute `[cfg(test)]`).

 Common Rust testing features are supported, such as disabling tests, running
 only specified tests, running the same test multiple times, etc'.
 Standard output, standard error, and logs are captured from the test.

 In order to use this runner, add the following to your component manifest:

 ```json5
 {
     include: [ "//src/sys/test_runners/rust/default.shard.cml" ]
 }
 ```

 By default Rust test cases run in parallel, at most 10 cases at a time.

 #### Go test runner {#gotest-runner}

 A runner for tests written in the Go programming language and following Go
 testing idioms.
 Use this for all tests written in Go using `import "testing"`.

 Common Go testing features are supported, such as disabling tests, running
 only specified tests, running the same test multiple times, etc'.
 Standard output, standard error, and logs are captured from the test.

 In order to use this runner, add the following to your component manifest:

 ```json5
 {
     include: [ "//src/sys/test_runners/gotests/default.shard.cml" ]
 }
 ```

 By default Go test cases run in parallel, at most 10 cases at a time.

 #### ELF test runner {#elf-test-runner}

 The simplest test runner - it waits for your program to terminate, then reports
 that the test passed if the program returned zero or that it failed for any
 non-zero return value.

 Use this test runner if your test is implemented as an ELF program (for instance
 an executable written in C/C++) but it does not use a common testing framework
 that's supported by existing runners and you'd rather not implement a bespoke
 test runner.

 In order to use this runner, add the following to your component manifest:

 ```json5
 {
     include: [ "//src/sys/test_runners/elf/default.shard.cml" ]
 }
 ```

 There is no notion of parallelism since tests that use this runner don't have a
 notion of multiple test cases.

 ### Controlling parallel execution of test cases

 When using `fx test` to launch tests, they may run each test case in sequence or
 run multiple test cases in parallel up to a given limit. The default
 parallelism behavior is determined by the test runner. To manually control the
 number of test cases to run in parallel use test spec:

 ```gn
 fuchsia_test_package("my-test-pkg") {
   test_components = [ ":my_test_component" ]
   test_specs = {
     # control the parallelism
     parallel = 10
   }
 }
 ```

 To override the value specified in the test spec, pass the parallel option when
 invoking fx test:

 ```posix-terminal
 fx test --parallel=5 <test_url>
 ```

 ### Running test multiple times

 To run a test multiple times use:

 ```posix-terminal
  fx test --count=<n> <test_url>
 ```

 If an iteration times out, no further iteration will be executed.

 ### Passing arguments

 Custom arguments to the tests can be passed using `fx test`:

 ```posix-terminal
 fx test <test_url> -- <custom_args>
 ```

 Individual test runners have restrictions on these custom flags:

 #### GoogleTest runner {#gtest-runner-custom-arg}

 Note the following known behavior change:

 **--gtest_break_on_failure**: As each test case is executed in a different process,
 this flag will not work.

 The following flags are restricted and the test fails if any are passed as
 fuchsia.test.Suite provides equivalent functionality that replaces them.

 - **--gtest_filter** - Instead use:

 ```posix-terminal
  fx test --test-filter=<glob_pattern> <test_url>
 ```

 `--test-filter` may be specified multiple times. Tests that match any of the
 given glob patterns will be executed.

 - **--gtest_also_run_disabled_tests** - Instead use:

 ```posix-terminal
  fx test --also-run-disabled-tests <test_url>
 ```

 - **--gtest_repeat** - See [Running test multiple times](#running_test_multiple_times).
 - **--gtest_output** - Emitting gtest json output is not supported.
 - **--gtest_list_tests** - Listing test cases is not supported.

 #### Rust runner {#rust-runner-custom-arg}

 The following flags are restricted and the test fails if any are passed as
 fuchsia.test.Suite provides equivalent functionality that replaces them.

 - **--nocapture** - Output is printed by default.
 - **--list** - Listing test cases is not supported.

 #### Go test runner {#gotest-runner-custom-arg}

 Note the following known behavior change:

 **-test.failfast**: As each test case is executed in a different process, this
 flag will only influence sub-tests.

 The following flags are restricted and the test fails if any are passed as
 fuchsia.test.Suite provides equivalent functionality that replaces them

 - **-test.run** - Instead use:

 ```posix-terminal
  fx test --test-filter=<glob_pattern> <test_url>
 ```

 `--test-filter` may be specified multiple times. Tests that match any of the
 given glob patterns will be executed.

 - **-test.count** - See [Running test multiple times](#running_test_multiple_times).
 - **-test.v** - Output is printed by default.
 - **-test.parallel** - See [Controlling parallel execution of test cases](#controlling_parallel_execution_of_test_cases).

 ### A runtime-agnostic, runtime-inclusive testing framework {#inclusive}

 Fuchsia aims to be [inclusive][inclusive], for instance in the sense that
 developers can create components (and their tests) in their language and runtime
 of choice. The Test Runner Framework itself is language-agnostic by design, with
 individual test runners specializing in particular programming languages or test
 runtimes and therefore being language-inclusive. Anyone can create and use new
 test runners.

 Creating new test runners is relatively easy, with the possibility of sharing
 code between different runners. For instance, the GoogleTest runner and the Rust
 runner share code related to launching an ELF binary, but differ in code for
 passing command line arguments to the test and parsing the test's results.

 ## Temporary storage

 To use temporary storage in your test, add the following to your component manifest:

 ```json5
 {
     include: [ "//src/sys/test_runners/tmp_storage.shard.cml" ]
 }
 ```

 At runtime, your test will have read/write access to `/tmp`.
 The contents of this directory will be empty when the test starts, and will be
 deleted after the test finishes.

 [Tests that don't specify a custom manifest][component-unit-tests] and instead
 rely on the build system to generate their component manifest can add the
 following dependency:

 ```gn
 fuchsia_unittest_package("foo-tests") {
   deps = [
     ":foo_test",
     "//src/sys/test_runners:tmp_storage",
   ]
 }
 ```

 ## Hermeticity

 A test is *hermetic* if it [uses][manifests-use] or [offers][manifests-offer] no
 capabilities from the [test root's](#tests-as-components) parent. As a rule of
 thumb, tests should be hermetic, but sometimes a test requires a capability that
 cannot be injected in the test realm.

 In the context of hermetic tests, a capability that originates from outside of
 the test's realm is called a *system capability*.

 There are some capabilities which all tests can use which do not violate test
 hermeticity:

 | Protocol | Description |
 | -----------| ------------|
 | `fuchsia.boot.WriteOnlyLog` | Write to kernel log |
 | `fuchsia.logger.LogSink` | Write to syslog |
 | `fuchsia.process.Launcher` | Launch a child process from the test package |
 | `fuchsia.sys2.EventSource` | Access to event protocol |

 To use these capabilities, there should be a use declaration added to test's
 manifest file:

 ```json5
 // my_test.cml
 {
     use: [
         ...
         {
             protocol: [
               "{{ '<var label="protocol">fuchsia.logger.LogSink</var>' }}"
             ],
         },
     ],
 }
 ```

 Tests are also provided with some default storage capabilities which are
 destroyed after the test finishes execution.

 | Storage Capability | Description | Path |
 | ------------------ | ----------- | ---- |
 |  `data` | Isolated data storage directory | `/data` |
 |  `cache` | Isolated cache storage directory | `/cache` |
 |  `temp` | Isolated in-memory [temporary storage directory](#temporary_storage) | `/tmp` |

 Add a use declaration in test's manifest file to use these capabilities.

 ```json5
 // my_test.cml
 {
     use: [
         ...
         {
             storage: "{{ '<var label="storage">data</var>' }}",
             path: "{{ '<var label="storage path">/data</var>' }}",
         },
     ],
 }
 ```

 The framework also provides some [capabilities][framework-capabilities] to all the
 components and can be used by test components if required.

 ## Performance

 When writing a test runner that launches processes, the runner needs to
 provide a [library loader][loader-service] implementation.

 Test runners typically launch individual test cases in separate processes to
 achieve a greater degree of isolation between test cases. However this can come
 at a significant performance cost. To mitigate this, the test runners listed
 above use a [caching loader service][caching-loader-service] which reduces the
 extra overhead per process launched.

 ## Test roles {#test-roles}

 Components in the test realm may play various roles in the test, as follows:

 -   Test driver: The component that actually runs the test, and implements
     (either directly or through a [test runner](#test-runners)) the
     [`fuchsia.test.Suite`][test-suite-protocol] protocol. This role may be, but
     is not necessarily, owned by the [test root](#tests-as-components).
 -   Capability provider: A component that provides a capability that the test
     will exercise somehow. The component may either provide a "fake"
     implementation of the capability for test, or a "real" implementation that
     is equivalent to what production uses.
 -   Component under test: A component that exercises some behavior to be tested.
     This may be identical to a component from production, or a component written
     specifically for the test intended to model production behavior.

 ## Further reading

 - [Complex topologies and integration testing][integration-testing]: testing
   interactions between multiple components in isolation from the rest of the
   system.

 [cf]: /docs/concepts/components/v2/
 [component-manifest]: /docs/concepts/components/v2/component_manifests.md
 [component-unit-tests]: /docs/development/components/build.md#unit-tests
 [fidl-test-manager]: /sdk/fidl/fuchsia.test.manager/test_manager.fidl
 [fidl-test-suite]: /sdk/fidl/fuchsia.test/suite.fidl
 [ffx]: /docs/development/tools/ffx/overview.md
 [fx-test]: https://fuchsia.dev/reference/tools/fx/cmd/test
 [inclusive]: /docs/concepts/principles/inclusive.md
 [integration-testing]: /docs/concepts/testing/v2/integration_testing.md
 [manifests-offer]: /docs/concepts/components/v2/component_manifests.md#offer
 [manifests-use]: /docs/concepts/components/v2/component_manifests.md#use
 [runners]: /docs/concepts/components/v2/capabilities/runners.md
 [test-suite-protocol]: /docs/concepts/components/v2/realms.md
 [unit-tests]: /docs/development/components/build.md#unit_tests_with_generated_manifests
 [loader-service]: /docs/concepts/booting/program_loading.md#the_loader_service
 [caching-loader-service]: /src/sys/test_runners/src/elf/elf_component.rs
 [framework-capabilities]: /docs/concepts/components/v2/component_manifests.md#framework-protocols
	# The Fuchsia Test Runner Framework

	<<../../components/_v2_banner.md>>

	## Integrating testing frameworks with the Component Framework

	The Fuchsia [Component Framework][cf] allows developers to create components in
	a variety of languages and runtimes. Fuchsia's own code uses a diverse mix of
	programming languages for components, including C/C++, Rust, Dart, and Go.

	The Test Runner Framework uses Component Framework [runners][runners] as an
	integration layer between various testing runtimes and a common Fuchsia protocol
	for launching tests and receiving their results. This makes for an [inclusive]
	design that on one hand allows developers to bring their language and testing
	framework of choice, and on the other hand allows building and testing Fuchsia
	on a variety of systems and targeting different hardware.

	## The Test Manager

	The `test_manager` component is responsible for running tests on a Fuchsia
	device. Test manager exposes the
	[`fuchsia.test.manager.RunBuilder`][fidl-test-manager] protocol, which allows
	launching test suites.

	Each test suite is launched as a child of test manager. Test suites are offered
	capabilities by test manager that enable them to do their work while
	maintaining isolation between the test and the rest of the system. For instance
	hermetic tests are given the capability to log messages, but are not given the
	capability to interact with real system resources outside of their sandbox.
	Test manager uses only one capability from the test realm, a controller protocol
	that test suites expose. This is done to ensure hermeticity (test results aren't
	affected by anything outside of their intended sandbox) and isolation (tests
	don't affect each other or the rest of the system).

	The test manager controller itself is offered to other components in the system
	in order to integrate test execution with various developer tools. Tests can
	then be launched with such tools as [`fx test`][fx-test] and [`ffx`][ffx].

	## The test suite protocol {#test-suite-protocol}

	The test suite protocol, [`fuchsia.test.Suite`][fidl-test-suite], is used by the
	test manager to control tests, such as to invoke test cases and to collect their
	results.

	Test authors typically don't need to implement this protocol. Instead, they rely
	on a [test runner](#test-runners) to do this for them. For instance, you might
	write a test in C++ using the GoogleTest framework, and then use
	[`gtest_runner`](#gtest-runner) in your [component manifest][component-manifest]
	to integrate with the Test Runner Framework.

	## Test runners {#test-runners}

	### A language and runtime-inclusive framework

	Test runners are reusable adapters between the Test Runner Framework and common
	languages & frameworks used by developers to write tests. They implement the
	[`fuchsia.test.Suite`][fidl-test-suite] protocol on behalf of the test author,
	allowing developers to write idiomatic tests for their language and framework of
	choice.

	Component manifests for simple unit tests can be [generated][unit-tests]
	by the build rules. Generated component manifests for v2 tests will include the
	appropriate test runner based on their build definition. For instance a test
	executable that depends on the GoogleTest library will include the
	[GoogleTest runner](#gtest-runner) in its generated manifest.

	### Inventory of test runners

	The following test runners are currently available for general use:

	#### GoogleTest runner {#gtest-runner}

	A runner for tests written in C/C++ using the GoogleTest framework.
	Use this for all tests written using GoogleTest.

	Common GoogleTest features are supported, such as disabling tests, running
	only specified tests, running the same test multiple times, etc'.
	Standard output, standard error, and logs are captured from the test.

	In order to use this runner, add the following to your component manifest:

	```json5
	{
	include: [ "//src/sys/test_runners/gtest/default.shard.cml" ]
	}
	```

	By default GoogleTest test cases run serially (one test case at a time).

	#### Rust runner {#rust-runner}

	A runner for tests written in the Rust programming language and following Rust
	testing idioms.
	Use this for all idiomatic Rust tests (i.e. tests with modules that set the
	attribute `[cfg(test)]`).

	Common Rust testing features are supported, such as disabling tests, running
	only specified tests, running the same test multiple times, etc'.
	Standard output, standard error, and logs are captured from the test.

	In order to use this runner, add the following to your component manifest:

	```json5
	{
	include: [ "//src/sys/test_runners/rust/default.shard.cml" ]
	}
	```

	By default Rust test cases run in parallel, at most 10 cases at a time.

	#### Go test runner {#gotest-runner}

	A runner for tests written in the Go programming language and following Go
	testing idioms.
	Use this for all tests written in Go using `import "testing"`.

	Common Go testing features are supported, such as disabling tests, running
	only specified tests, running the same test multiple times, etc'.
	Standard output, standard error, and logs are captured from the test.

	In order to use this runner, add the following to your component manifest:

	```json5
	{
	include: [ "//src/sys/test_runners/gotests/default.shard.cml" ]
	}
	```

	By default Go test cases run in parallel, at most 10 cases at a time.

	#### ELF test runner {#elf-test-runner}

	The simplest test runner - it waits for your program to terminate, then reports
	that the test passed if the program returned zero or that it failed for any
	non-zero return value.

	Use this test runner if your test is implemented as an ELF program (for instance
	an executable written in C/C++) but it does not use a common testing framework
	that's supported by existing runners and you'd rather not implement a bespoke
	test runner.

	In order to use this runner, add the following to your component manifest:

	```json5
	{
	include: [ "//src/sys/test_runners/elf/default.shard.cml" ]
	}
	```

	There is no notion of parallelism since tests that use this runner don't have a
	notion of multiple test cases.

	### Controlling parallel execution of test cases

	When using `fx test` to launch tests, they may run each test case in sequence or
	run multiple test cases in parallel up to a given limit. The default
	parallelism behavior is determined by the test runner. To manually control the
	number of test cases to run in parallel use test spec:

	```gn
	fuchsia_test_package("my-test-pkg") {
	test_components = [ ":my_test_component" ]
	test_specs = {
	# control the parallelism
	parallel = 10
	}
	}
	```

	To override the value specified in the test spec, pass the parallel option when
	invoking fx test:

	```posix-terminal
	fx test --parallel=5 <test_url>
	```

	### Running test multiple times

	To run a test multiple times use:

	```posix-terminal
	fx test --count=<n> <test_url>
	```

	If an iteration times out, no further iteration will be executed.

	### Passing arguments

	Custom arguments to the tests can be passed using `fx test`:

	```posix-terminal
	fx test <test_url> -- <custom_args>
	```

	Individual test runners have restrictions on these custom flags:

	#### GoogleTest runner {#gtest-runner-custom-arg}

	Note the following known behavior change:

	--gtest_break_on_failure: As each test case is executed in a different process,
	this flag will not work.

	The following flags are restricted and the test fails if any are passed as
	fuchsia.test.Suite provides equivalent functionality that replaces them.

	- --gtest_filter - Instead use:

	```posix-terminal
	fx test --test-filter=<glob_pattern> <test_url>
	```

	`--test-filter` may be specified multiple times. Tests that match any of the
	given glob patterns will be executed.

	- --gtest_also_run_disabled_tests - Instead use:

	```posix-terminal
	fx test --also-run-disabled-tests <test_url>
	```

	- --gtest_repeat - See [Running test multiple times](#running_test_multiple_times).
	- --gtest_output - Emitting gtest json output is not supported.
	- --gtest_list_tests - Listing test cases is not supported.

	#### Rust runner {#rust-runner-custom-arg}

	The following flags are restricted and the test fails if any are passed as
	fuchsia.test.Suite provides equivalent functionality that replaces them.

	- --nocapture - Output is printed by default.
	- --list - Listing test cases is not supported.

	#### Go test runner {#gotest-runner-custom-arg}

	Note the following known behavior change:

	-test.failfast: As each test case is executed in a different process, this
	flag will only influence sub-tests.

	The following flags are restricted and the test fails if any are passed as
	fuchsia.test.Suite provides equivalent functionality that replaces them

	- -test.run - Instead use:

	```posix-terminal
	fx test --test-filter=<glob_pattern> <test_url>
	```

	`--test-filter` may be specified multiple times. Tests that match any of the
	given glob patterns will be executed.

	- -test.count - See [Running test multiple times](#running_test_multiple_times).
	- -test.v - Output is printed by default.
	- -test.parallel - See [Controlling parallel execution of test cases](#controlling_parallel_execution_of_test_cases).

	### A runtime-agnostic, runtime-inclusive testing framework {#inclusive}

	Fuchsia aims to be [inclusive][inclusive], for instance in the sense that
	developers can create components (and their tests) in their language and runtime
	of choice. The Test Runner Framework itself is language-agnostic by design, with
	individual test runners specializing in particular programming languages or test
	runtimes and therefore being language-inclusive. Anyone can create and use new
	test runners.

	Creating new test runners is relatively easy, with the possibility of sharing
	code between different runners. For instance, the GoogleTest runner and the Rust
	runner share code related to launching an ELF binary, but differ in code for
	passing command line arguments to the test and parsing the test's results.

	## Temporary storage

	To use temporary storage in your test, add the following to your component manifest:

	```json5
	{
	include: [ "//src/sys/test_runners/tmp_storage.shard.cml" ]
	}
	```

	At runtime, your test will have read/write access to `/tmp`.
	The contents of this directory will be empty when the test starts, and will be
	deleted after the test finishes.

	[Tests that don't specify a custom manifest][component-unit-tests] and instead
	rely on the build system to generate their component manifest can add the
	following dependency:

	```gn
	fuchsia_unittest_package("foo-tests") {
	deps = [
	":foo_test",
	"//src/sys/test_runners:tmp_storage",
	]
	}
	```

	## Hermeticity

	A test is hermetic if it [uses][manifests-use] or [offers][manifests-offer] no
	capabilities from the [test root's](#tests-as-components) parent. As a rule of
	thumb, tests should be hermetic, but sometimes a test requires a capability that
	cannot be injected in the test realm.

	In the context of hermetic tests, a capability that originates from outside of
	the test's realm is called a system capability.

	There are some capabilities which all tests can use which do not violate test
	hermeticity:

	\| Protocol \| Description \|
	\| -----------\| ------------\|
	\| `fuchsia.boot.WriteOnlyLog` \| Write to kernel log \|
	\| `fuchsia.logger.LogSink` \| Write to syslog \|
	\| `fuchsia.process.Launcher` \| Launch a child process from the test package \|
	\| `fuchsia.sys2.EventSource` \| Access to event protocol \|

	To use these capabilities, there should be a use declaration added to test's
	manifest file:

	```json5
	// my_test.cml
	{
	use: [
	...
	{
	protocol: [
	"{{ '<var label="protocol">fuchsia.logger.LogSink</var>' }}"
	],
	},
	],
	}
	```

	Tests are also provided with some default storage capabilities which are
	destroyed after the test finishes execution.

	\| Storage Capability \| Description \| Path \|
	\| ------------------ \| ----------- \| ---- \|
	\| `data` \| Isolated data storage directory \| `/data` \|
	\| `cache` \| Isolated cache storage directory \| `/cache` \|
	\| `temp` \| Isolated in-memory [temporary storage directory](#temporary_storage) \| `/tmp` \|

	Add a use declaration in test's manifest file to use these capabilities.

	```json5
	// my_test.cml
	{
	use: [
	...
	{
	storage: "{{ '<var label="storage">data</var>' }}",
	path: "{{ '<var label="storage path">/data</var>' }}",
	},
	],
	}
	```

	The framework also provides some [capabilities][framework-capabilities] to all the
	components and can be used by test components if required.

	## Performance

	When writing a test runner that launches processes, the runner needs to
	provide a [library loader][loader-service] implementation.

	Test runners typically launch individual test cases in separate processes to
	achieve a greater degree of isolation between test cases. However this can come
	at a significant performance cost. To mitigate this, the test runners listed
	above use a [caching loader service][caching-loader-service] which reduces the
	extra overhead per process launched.

	## Test roles {#test-roles}

	Components in the test realm may play various roles in the test, as follows:

	- Test driver: The component that actually runs the test, and implements
	(either directly or through a [test runner](#test-runners)) the
	[`fuchsia.test.Suite`][test-suite-protocol] protocol. This role may be, but
	is not necessarily, owned by the [test root](#tests-as-components).
	- Capability provider: A component that provides a capability that the test
	will exercise somehow. The component may either provide a "fake"
	implementation of the capability for test, or a "real" implementation that
	is equivalent to what production uses.
	- Component under test: A component that exercises some behavior to be tested.
	This may be identical to a component from production, or a component written
	specifically for the test intended to model production behavior.

	## Further reading

	- [Complex topologies and integration testing][integration-testing]: testing
	interactions between multiple components in isolation from the rest of the
	system.

	[cf]: /docs/concepts/components/v2/
	[component-manifest]: /docs/concepts/components/v2/component_manifests.md
	[component-unit-tests]: /docs/development/components/build.md#unit-tests
	[fidl-test-manager]: /sdk/fidl/fuchsia.test.manager/test_manager.fidl
	[fidl-test-suite]: /sdk/fidl/fuchsia.test/suite.fidl
	[ffx]: /docs/development/tools/ffx/overview.md
	[fx-test]: https://fuchsia.dev/reference/tools/fx/cmd/test
	[inclusive]: /docs/concepts/principles/inclusive.md
	[integration-testing]: /docs/concepts/testing/v2/integration_testing.md
	[manifests-offer]: /docs/concepts/components/v2/component_manifests.md#offer
	[manifests-use]: /docs/concepts/components/v2/component_manifests.md#use
	[runners]: /docs/concepts/components/v2/capabilities/runners.md
	[test-suite-protocol]: /docs/concepts/components/v2/realms.md
	[unit-tests]: /docs/development/components/build.md#unit_tests_with_generated_manifests
	[loader-service]: /docs/concepts/booting/program_loading.md#the_loader_service
	[caching-loader-service]: /src/sys/test_runners/src/elf/elf_component.rs
	[framework-capabilities]: /docs/concepts/components/v2/component_manifests.md#framework-protocols