Inspect C++ codelab

This document contains the codelab for Inspect using C++.

The code is available at //src/diagnostics/examples/inspect/cpp.

This codelab is organized into several parts, each with their own subdirectory. The starting point for the codelab is part 1, and the code for each part contains the solution for the previous parts.

When working on this codelab, you may continue adding your solutions to "part_1", or you may skip around by building on the existing solutions.


Set up your development environment.

This codelab assumes you have completed Getting Started and have:

  1. A checked out and built Fuchsia tree.
  2. A device or emulator (fx emu) that runs Fuchsia.
  3. A workstation to serve components (fx serve) to your Fuchsia device or emulator.

To build and run the examples in this codelab, add the following arguments to your fx set invocation:

fx set core.x64 \
--with //src/diagnostics/examples/inspect/cpp \
--with //src/diagnostics/examples/inspect/cpp:tests

Part 1: A buggy component

There is a component that serves a protocol called Reverser:

// Implementation of a string reverser.
protocol Reverser {
    // Returns the input string reversed character-by-character.
    Reverse(string:1024 input) -> (string:1024 response);

This protocol has a single method, called "Reverse," that simply reverses any string passed to it. An implementation of the protocol is provided, but it has a critical bug. The bug makes clients who attempt to call the Reverse method see that their call hangs indefinitely. It is up to you to fix this bug.

Run the component

There is a client application called inspect_cpp_codelab_client that will launch the Reverser component and send the rest of its command line arguments as strings to Reverse:

  1. See usage

    fx shell run inspect_cpp_codelab_client
  2. Run part 1 code, and reverse the string "Hello"

    fx shell run inspect_cpp_codelab_client 1 Hello

    This command hangs.

  3. Press Ctrl+C to stop the client and try running with more arguments:

    fx shell run inspect_cpp_codelab_client 1 Hello World

    This command also hangs.

You are now ready to look through the code to troubleshoot the issue.

Look through the code

Now that you can reproduce the problem, take a look at what the client is doing:

// Repeatedly send strings to be reversed to the other component.
for (int i = 2; i < argc; i++) {
  printf("Input: %s\n", argv[i]);

  std::string output;
  if (ZX_OK != reverser->Reverse(argv[i], &output)) {
    printf("Error: Failed to reverse string.\nPerhaps %s was not found?\n",

  printf("Output: %s\n", output.c_str());

In this code snippet, the client calls the Reverse method but never seems to get a response. There doesn't seem to be an error message or output.

Take a look at the server code for this part of the codelab. There is a lot of standard component setup:

  • Logging initialization

    InitLogger({"inspect_cpp_codelab", "part1"});
  • Creating an asynchronous executor

    async::Loop loop(&kAsyncLoopConfigAttachToCurrentThread);
    auto context = sys::ComponentContext::Create();
  • Serving a public service


See what the reverser definition is:

class Reverser final : public fuchsia::examples::inspect::Reverser {
  // Implementation of Reverser.Reverse().
  void Reverse(std::string input, ReverseCallback callback) override;

  // Return a request handler for the Reverser protocol that binds
  // incoming requests to new Reversers.
  static fidl::InterfaceRequestHandler<fuchsia::examples::inspect::Reverser>

This class implements the Reverser protocol. A helper method called CreateDefaultHandler constructs an InterfaceRequestHandler that creates new Reversers for incoming requests.

Add Inspect

Now that you know the code structure, you can start to instrument the code with Inspect to find the problem.

You may have previously debugged programs by printing or logging. While this is often effective, asynchronous Components that run persistently often output numerous logs about their internal state over time. This codelab shows how Inspect provides snapshots of your component's current state without needing to dig through logs.

  1. Include Inspect dependencies in

    source_set("lib") {
    public_deps = [
  2. Initialize Inspect in

    async::Loop loop(&kAsyncLoopConfigAttachToCurrentThread);
    auto context = sys::ComponentContext::Create();
    // Create an inspector for this component.
    sys::ComponentInspector inspector(context.get());

    You are now using Inspect.

  3. Add a simple "version" property to show which version is running:

    inspector.root().CreateString("version", "part1", &inspector);

This snippet does the following:

  1. Obtain the "root" node of the Inspect hierarchy.

    The Inspect hierarchy for your component consists of a tree of Nodes, each of which contains any number of properties.

  2. Create a new property using CreateString.

    This adds a new StringProperty on the root. This StringProperty is called "version", and its value is "part1".

  3. Emplace the new property in the inspector.

    The lifetime of a property is tied to an object returned by Create, and destroying the object causes the property to disappear. The optional third parameter emplaces the new property in inspector rather than return it. As a result, the new property lives as long as the inspector itself (the entire execution of the component).

Reading Inspect Data

Now that you have added Inspect to your component, you can read what it says:

  1. Rebuild and push the component:

    fx build-push inspect_cpp_codelab
    fx build && fx update
  2. Run the client (should still hang):

    fx shell run inspect_cpp_codelab_client 1 Hello
  3. Use iquery (Inspect query) to view your output:

    fx iquery

    This dumps all of the Inspect data for the entire system, which may be a lot of data.

  4. Since iquery supports regex matching, run:

    $ fx iquery inspect_cpp_codelab_part_1
      version = part1
  5. You can also view the output as JSON:

    $ fx iquery -f json inspect_cpp_codelab_part_1
            "contents": {
                "root": {
                    "version": "part1"
            "path": "/hub/r/codelab/1234/c/inspect_cpp_codelab_part_1.cmx/1234/out/diagnostics/root.inspect"

Instrumenting the code to find the bug

Now that you have initialized Inspect and know how to read data, you are ready to instrument your code and uncover the bug.

The previous output shows you how the component is actually running and that the component is not hanging completely. Otherwise the Inspect read would hang.

Add new information per-connection to observe if the connection is even being handled by your component.

  1. Add a new child to your root node to contain statistics about the reverser service:

  2. Update the definition of CreateDefaultHandler to accept this node in reverser.h and

    fidl::InterfaceRequestHandler<fuchsia::examples::inspect::Reverser> Reverser::CreateDefaultHandler(
        inspect::Node node) { ...
  3. Add a property to keep track of the number of connections:

    return [connection_count = node.CreateUint("connection_count", 0),
            node = std::move(node),
            binding_set =
               fidl::InterfaceRequest<ReverserProto> request) mutable {

    This snippet demonstrates creating a new UintProperty (containing a uint64_t) called connection_count and setting it to 0. In the handler (which runs for each connection), the property is incremented by 1.

  4. Run iquery:

    $ fx iquery inspect_cpp_codelab_part_1
    "contents": {
        "root": {
            "reverser_service": {
                "connection_count": 1,
            "version": "part1"

The output above demonstrates that the client successfully connected to the service, so the hanging problem must be caused by the Reverser implementation itself. In particular, it will be helpful to know:

  1. If the connection is still open while the client is hanging.

  2. If the Reverse method was called.

Exercise: Create a child node for each connection, and record "request_count" inside the Reverser.

  • Hint: There is a utility function for generating unique names:

    auto child = node.CreateChild(node.UniqueName("connection-"));

    This will create unique names starting with "connection-".

  • Hint: You will find it helpful to create a constructor for Reverser that takes inspect::Node. Part 3 of this codelab explains why this is a useful pattern.

  • Hint: You will need to create a member on Reverser to hold the request_count property. Its type will be inspect::UintProperty.

  • Follow up: Does request count give you all of the information you need? Add response_count as well.

  • Advanced: Can you add a count of all requests on all connections? The Reverser objects must share some state. You may find it helpful to refactor arguments to Reverser into a separate struct (See solution in part 2 for this approach).

After completing this exercise, you should see something like this:

$ fx iquery inspect_cpp_codelab_part_1
"contents": {
    "root": {
        "reverser_service": {
            "connection-0x0": {
                "request_count": 1,
            "connection_count": 1,
        "version": "part1"

The output above shows that the connection is still open and it received one request.

If you added "response_count" as well, you may have noticed the bug. The Reverse method receives a callback, but it is never called with the value of output.

  1. Add a call to callback to fix the bug:

    // At the end of Reverser::Reverse
  2. Run the client again:

    fx shell run inspect_cpp_codelab_client 1 hello
    Input: hello
    Output: olleh
    Done. Press Ctrl+C to exit

    The component continues running until Ctrl+C is pressed to give you a chance to run iquery and observe your output.

This concludes part 1. You may commit your changes so far:

git commit -am "solution to part 1"

Part 2: Diagnosing inter-component problems

You received a bug report, the "FizzBuzz" team is saying they are not receiving data from your component.

In addition to serving the Reverser protocol, the component also reaches out to the "FizzBuzz" service and prints the response:

// Send a request to the FizzBuzz service and print the response when it arrives.
fuchsia::examples::inspect::FizzBuzzPtr fizz_buzz;
fizz_buzz->Execute(30, [](std::string result) { FX_LOGS(INFO) << "Got FizzBuzz: " << result; });

If you use fx log --tag inspect_cpp_codelab, you will see that this log is never printed. You will need to diagnose and solve this problem.

Diagnose the issue with Inspect

  1. Run the component to see what is happening:

    $ fx shell run inspect_cpp_codelab_client 2 hello

    Fortunately the FizzBuzz team instrumented their component using Inspect.

  2. Read the FizzBuzz Inspect data:

    $ fx iquery -f json inspect_cpp_codelab_fizzbuzz
    "contents": {
        "root": {
            "fizzbuzz_service": {
                "closed_connection_count": 0,
                "incoming_connection_count": 0,
                "request_count": 0,

    This output confirms that FizzBuzz is not receiving any connections.

  3. Add Inspect to identify the problem:

    // Send a request to the FizzBuzz service and print the response when it arrives.
    fuchsia::examples::inspect::FizzBuzzPtr fizz_buzz;
    // Create an error handler for the FizzBuzz service.
    fizz_buzz.set_error_handler([&](zx_status_t status) {
      // CODELAB: Add Inspect here to see if there is an error
    fizz_buzz->Execute(30, [&](std::string result) {
      // CODELAB: Add Inspect here to see if there is a response.
      FX_LOGS(INFO) << "Got FizzBuzz: " << result;

Exercise: Add Inspect to the FizzBuzz connection to identify the problem

  • Hint: Use the snippet above as a starting point, it provides an error handler for the connection attempt.

  • Follow up: Can you store the status somewhere? You can convert it to a string using zx_status_get_string(status).

  • Advanced: inspector has a method called Health() that announces overall health status in a special location. Since our service is not healthy unless it can connect to FizzBuzz, can you incorporate this:

    "fuchsia.inspect.Health": {
        "status": "STARTING_UP"
    "fuchsia.inspect.Health": {
        "status": "OK"
    "fuchsia.inspect.Health": {
        "status": "UNHEALTHY",
        "message": "Something went wrong!"
    inspector.Health().Unhealthy("Something went wrong!");

Once you complete this exercise, you should see that the connection error handler is being called with a "not found" error. Inspect output showed that FizzBuzz is running, so maybe something is misconfigured. Unfortunately not everything uses Inspect (yet!) so look at the logs:

$ fx log --only FizzBuzz
... Component fuchsia-pkg://
is not allowed to connect to fuchsia.examples.inspect.FizzBuzz...

Sandboxing errors are a common pitfall that are sometimes difficult to uncover.

Looking at the sandbox, you can see it is missing the service:

"sandbox": {
    "services": [

Add "fuchsia.examples.inspect.FizzBuzz" to the services array, rebuild, and run again. You should now see FizzBuzz in the logs and an OK status:

$ fx log --tag inspect_cpp_codelab
[inspect_cpp_codelab, part3] INFO: Got FizzBuzz: 1 2 Fizz
4 Buzz Fizz 7 8 Fizz Buzz 11 Fizz 13 14 FizzBuzz 16 17 Fizz 19 Buzz Fizz
22 23 Fizz Buzz 26 Fizz 28 29 FizzBuzz

This concludes Part 2.

You can now commit your solution:

git commit -am "solution for part 2"

Part 3: Unit Testing for Inspect

All code on Fuchsia should be tested, and this applies to Inspect data as well.

While Inspect data is not required to be tested in general, you need to test Inspect data that is depended upon by other tools such as Triage or Feedback.

Reverser has a basic unit test called

Run the unit test:

fx run-test inspect_cpp_codelab_unittests

The unit test ensures that Reverser works properly (and doesn't hang!), but it does not check that the Inspect output is as expected.

Passing Nodes into constructors is a form of Dependency Injection, which allows you to pass in test versions of dependencies to check their state.

The code to open a Reverser looks like the following:


// Alternatively

A default version of the Inspect Node is passed into the Reverser. This allows the reverser code to run properly in tests, but it does not support asserting on Inspect output.

Exercise: Change OpenReverser to take the dependency for Reverser as an argument and use it when constructing Reverser.

  • Hint: Create an inspect::Inspector in the test function. You can get the root using inspector.GetRoot().

  • Hint: You will need to create a child on the root to pass in to OpenReverser.

  • Follow up: Create multiple reverser connections and test them independently.

Following this exercise, your unit test will set real values in an Inspect hierarchy.

Add code to test the output in Inspect:

#include <lib/inspect/cpp/reader.h>

fit::result<inspect::Hierarchy> hierarchy = inspect::ReadFromVmo(inspector.DuplicateVmo());

The snippet above reads the underlying virtual memory object (VMO) containing Inspect data and parses it into a readable hierarchy.

You can now read individual properties and children as follows:

// Get the property on root called "request_count"
auto* global_count =
// Ensure it is valid.
// Check its value.
EXPECT_EQ(3u, global_count->value());

// Get connection 0 by path.
auto* connection_0 = hierarchy.value().GetByPath({"connection_0x0"});
// Check it is valid and obtain the request_count.
auto* requests_0 =
// Check that is valid, and check its value.
EXPECT_EQ(2u, requests_0->value());

Exercise: Add assertions for the rest of your Inspect data.

This concludes Part 3.

You may commit your changes:

git commit -am "solution to part 3"

Part 4: Integration Testing for Inspect

Integration testing is an important part of the software development workflow for Fuchsia. Integration tests allow you to observe the behavior of your actual component when it runs on the system.

Running integration tests

You can run the integration tests for the codelab as follows:

$ fx run-test inspect_cpp_codelab_integration_tests

View the code

Look at how the integration test is setup:

  1. View the component manifest (cmx):

        "facets": {
            "fuchsia.test": {
                "injected-services": {
                    "fuchsia.diagnostics.Archive": "fuchsia-pkg://"
        "program": {
            "binary": "test/integration_part_4"
        "sandbox": {
            "services": [

    The important parts of this file are:

    • Injected services

    The fuchsia.test facet includes configuration for tests.

    In this file, the fuchsia.diagnostics.Archive service is injected and points to a component called observer.cmx. The observer collects information from all components in your test environment and provides a reading interface. You can use this information to look at your Inspect output.

    • Sandbox services

    Integration tests need to start other components in the test environment and wire them up. For this you need fuchsia.sys.Loader and fuchsia.sys.Environment.

  2. Look at the integration test itself. The individual test cases are fairly straightforward:

    TEST_F(CodelabTest, StartWithFizzBuzz) {
      auto ptr = StartComponentAndConnect({.include_fizzbuzz_service = true});
      bool error = false;
      ptr.set_error_handler([&](zx_status_t unused) { error = true; });
      bool done = false;
      std::string result;
      ptr->Reverse("hello", [&](std::string value) {
        result = std::move(value);
        done = true;
      // Run until either the error handler reports an error or the result is set.
      RunLoopUntil([&] { return done || error; });
      EXPECT_EQ("olleh", result);

    StartComponentAndConnect is responsible for creating a new test environment and starting the codelab component inside of it. The include_fizzbuzz_service option instructs the method to optionally include FizzBuzz. This feature tests that your Inspect output is as expected in case it fails to connect to FizzBuzz as in Part 2.

  3. Add the following method to your test fixture to read from the Archive service:

    #include <rapidjson/document.h>
    #include <rapidjson/pointer.h>
    std::string GetInspectJson() {
        // Connect to the Archive.
        fuchsia::diagnostics::ArchivePtr archive;
        // Open a Reader for each type of data the Archive contains.
        fuchsia::diagnostics::ReaderPtr reader;
        archive->ReadInspect(reader.NewRequest(), {} /* selectors */,
                             [](auto res) { ASSERT_FALSE(res.is_err()) << "Failed to get reader"; });
        // Since components are asynchronous, you do not know if the observer has seen the test
        // components yet. You will need to repeatedly read the data.
        while (true) {
          std::vector<fuchsia::diagnostics::FormattedContent> current_entries;
          // Get a new snapshot in JSON format.
          fuchsia::diagnostics::BatchIteratorPtr iterator;
          reader->GetSnapshot(fuchsia::diagnostics::Format::JSON, iterator.NewRequest(),
                              [](auto res) { ASSERT_FALSE(res.is_err()) << "Failed to get snapshot"; });
          // Get individual batches from the iterator.
          bool done;
          iterator->GetNext([&](auto result) {
            auto res = fit::result<ContentVector, fuchsia::diagnostics::ReaderError>(std::move(result));
            if (res.is_ok()) {
              current_entries = res.take_value();
            done = true;
          RunLoopUntil([&] { return done; });
          // Find the returned value that contains the name of your component, this is the
          // JSON you want.
          for (const auto& content : current_entries) {
            std::string json;
            fsl::StringFromVmo(content.formatted_json_hierarchy(), &json);
            if (json.find("sys/inspect_cpp_codelab_part_5.cmx") != std::string::npos) {
              return json;
          // Retry with delay until the data appears.
        return "";
  4. Use rapidjson to parse the returned data in your tests:

    rapidjson::Document document;

Exercise: Add assertions on the returned JSON data.

  • Hint: It may help to print the JSON output to view the schema.

  • Hint: You can read values by path as follows:

      document, "/contents/root/fuchsia.inspect.Health/status", ""));
  • Hint: You can EXPECT_EQ by passing in the expected value as a rapidjson::Value: rapidjson::Value("OK").

Your integration test will now ensure your inspect output is correct.

This concludes Part 4.

You may commit your solution:

git commit -am "solution to part 4"

Part 5: Feedback Selectors

This section is under construction.

  • TODO: Writing a feedback selector and adding tests to your integration test.

  • TODO: Selectors for Feedback and other pipelines