Inspect Quickstart

This quickstart guides you through the basics of using Component Inspection. You will learn how to integrate Inspect into your component using the language-specific libraries and review the data using ffx inspect.

For a more detailed walkthrough of Inspect concepts, see the Inspect codelab.

Project setup

See below for the quick start guide in your language of choice:

{C++}

This section assumes you are writing an asynchronous component and that some part of your component (typically main.cc) looks like this:

async::Loop loop(&kAsyncLoopConfigAttachToCurrentThread);
auto context_ = sys::ComponentContext::CreateAndServeOutgoingDirectory();
// ...
loop.Run();

This sets up an async loop, creates a ComponentContext wrapping handles provided by the runtime, and then runs that loop following some other initialization work.

Add the Inspect library dependencies to your BUILD.gn file:

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/diagnostics/inspect/cpp/BUILD.gn" region_tag="inspect_libs" adjust_indentation="auto" %}

Add the following includes:

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/diagnostics/inspect/cpp/example_server_app.h" region_tag="inspect_imports" adjust_indentation="auto" %}

Add the following code to initialize Inspect:

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/diagnostics/inspect/cpp/example_server_app.cc" region_tag="initialization" adjust_indentation="auto" %}

You are now using Inspect! Create properties in the Inspect tree by attaching them to the root node:

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/diagnostics/inspect/cpp/example_server_app.cc" region_tag="properties" adjust_indentation="auto" %}

Note: For a complete working example, see //examples/diagnostics/inspect/cpp.

See Supported Data Types for a full list of data types you can try.

Health checks

The health check subsystem provides a standardized inspection metric for component health. You can use the health node to report the overall status of your component:

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/diagnostics/inspect/cpp/example_server_app.cc" region_tag="health_check" adjust_indentation="auto" %}

Note: For more details on health metrics, see Health check.

Testing

To test your inspect code, you can use //sdklib/inspect/testing/cpp/inspect.h:

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/diagnostics/inspect/cpp/example_unittests.cc" region_tag="test_imports" adjust_indentation="auto" %}

This library includes a full set of matchers to validate the contents of the Inspect tree.

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/diagnostics/inspect/cpp/example_unittests.cc" region_tag="inspect_test" adjust_indentation="auto" %}

{Rust}

This section assumes you are writing an asynchronous component and that some part of your component (typically main.rs) looks similar to this:

async fn main() -> Result<(), Error> {
  // ...
  let mut service_fs = ServiceFs::new();
  // ...
  service_fs.take_and_serve_directory_handle().unwrap();
  service_fs.collect::<()>().await;
  Ok(())
}

Add the Inspect library dependencies to your BUILD.gn file:

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/diagnostics/inspect/rust/BUILD.gn" region_tag="inspect_libs" adjust_indentation="auto" %}

Add the following code to initialize Inspect:

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/diagnostics/inspect/rust/src/echo_server.rs" region_tag="initialization" adjust_indentation="auto" %}

You are now using Inspect! Create properties in the Inspect tree by attaching them to the root node:

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/diagnostics/inspect/rust/src/echo_server.rs" region_tag="properties" adjust_indentation="auto" %}

Note: For a complete working example, see //examples/diagnostics/inspect/rust.

See Supported Data Types for a full list of data types you can try.

Health checks

The health check subsystem provides a standardized inspection metric for component health. You can use the health node to report the overall status of your component:

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/diagnostics/inspect/rust/src/echo_server.rs" region_tag="health_check" adjust_indentation="auto" %}

Note: For more details on health metrics, see Health check.

Testing

To test your Inspect code, you can use assert_data_tree to validate the contents of the Inspect tree:

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/diagnostics/inspect/rust/src/echo_server.rs" region_tag="inspect_test" adjust_indentation="auto" %}

Note: To learn more about the Rust library, see the complete reference for fuchsia_inspect.

Inspect Libraries

Now that you have a root_node you may start building your hierarchy. This section describes some important concepts and patterns to help you get started.

A Node may have any number of key/value pairs called Properties.
The key for a Value is always a UTF-8 string, the value may be one of the supported types below.
A Node may have any number of children, which are also Nodes.
{C++}
The code above gives you access to a single node named “root”. hello_world_property is a Property that contains a string value (aptly called a StringProperty).
- Values and Nodes are created under a parent Node.
Class Node has creator methods for every type of supported value. hello_world_property was created using CreateStringProperty. You could create a child under the root node by calling root_node.CreateChild("child name"). Note that names must always be UTF-8 strings.
- Values and Nodes have strict ownership semantics.
hello_world_property owns the Property. When it is destroyed (goes out of scope) the underlying Property is deleted and no longer present in your component's Inspect output. This is true for child Nodes as well.
If you are creating a value that doesn't need to be modified, use a ValueList to keep them alive until they are no longer needed.
- Inspection is best-effort.
Due to space limitations, the Inspect library may be unable to satisfy a Create request. This error is not surfaced to your code: you will receive a Node/Property object for which the methods are no-ops.
- Pattern: Pass in child Nodes to child objects.
It is useful to add an inspect::Node argument to the constructors for your own classes. The parent object, which should own its own inspect::Node, may then pass in the result of CreateChild(...) to its children when they are constructed:
```
class Child {
  public:
    Child(inspect::Node my_node) : my_node_(std::move(my_node)) {
      // Create a string that doesn't change, and emplace it in the ValueList
      my_node_.CreateString("version", "1.0", &values_);
      // Create metrics and properties on my_node_.
    }

  private:
    inspect::Node my_node_;
    inspect::StringProperty some_property_;
    inspect::ValueList values_;
    // ... more properties and metrics
};

class Parent {
  public:
    // ...

    void AddChild() {
      // Note: inspect::UniqueName returns a globally unique name with the specified prefix.
      children_.emplace_back(my_node_.CreateChild(inspect::UniqueName("child-")));
    }

  private:
    std::vector<Child> children_;
    inspect::Node my_node_;
};
```
{Rust}
Refer to C++ Library Concepts, as similar concepts apply in Rust.
The Rust library provides two ways of managing nodes and properties: creation and recording.
With the create_* methods, the ownership of the property or node object to the caller. When the returned object is dropped, it is removed. For example:
```
{
    let property = root.create_int("name", 1);
}
```
In this example, the property went out of scope so a drop on the property is called. Readers won't see this property.
With the record_* methods, the lifetime of the node the method is called on is entangled with the resulting property. When the node the method was called is deleted, the recorded property is deleted.
```
{
    let node = root.create_child("name");
    {
      node.record_uint(2); // no return
    }
    // The uint property will still be visible to readers.
}
```
In this example, neither the name node nor the uint property is visible to readers after node is dropped.

Dynamic values

This section describes support in the Inspect libraries for nodes that are inflated lazily at read-time. The methods accept a callback function instead of a value. The callback function is invoked when the property value is read.

{C++}

The C++ library has two property creators for dynamic values: CreateLazyNode and CreateLazyValues.

Both of these methods take a callback returning a promise for an inspect::Inspector, the only difference is how the dynamic values are stored in the tree.

root->CreateLazyNode(name, callback) creates a child node of root with the given name. The callback returns a promise for an inspect::Inspector whose root node is spliced into the parent hierarchy when read. The example below shows that a child called “lazy” exists with the string property “version” and has an additional child that is called “lazy.”

root->CreateLazyValues(name, callback) works like root->CreateLazyNode(name, callback), except all properties and child nodes on the promised root node are added directly as values to the original root. In the second output of this example, the internal lazy nodes do not appear and their values are flattened into properties on root.

root->CreateLazy{Node,Values}("lazy", [] {
  Inspector a;
  a.GetRoot().CreateString("version", "1.0", &a);
  a.GetRoot().CreateLazy{Node,Values}("lazy", [] {
    Inspector b;
    b.GetRoot().CreateInt("value", 10, &b);
    return fpromise::make_ok_promise(std::move(b));
  }, &a);

  return fpromise::make_ok_promise(std::move(a));
});

Output (CreateLazyNode):

root:
  lazy:
    version = "1.0"
    lazy:
      val = 10

Output (CreateLazyValues):

root:
  val = 10
  version = "1.0"

Warning: It is the developer's responsibility to ensure that names flattened from multiple lazy value nodes do not conflict. If they do, output behavior is undefined.

The return value of CreateLazy{Node,Values} is a LazyNode that owns the passed callback. The callback is never called once the LazyNode is destroyed. If you destroy a LazyNode concurrently with the execution of a callback, the destroy operation is blocked until the callback returns its promise.

If you want to dynamically expose properties on this, you may simply write the following:

class Employee {
  public:
    Employee(inspect::Node node) : node_(std::move(node)) {
      calls_ = node_.CreateInt("calls", 0);

      // Create a lazy node that populates values on its parent
      // dynamically.
      // Note: The callback will never be called after the LazyNode is
      // destroyed, so it is safe to capture "this."
      lazy_ = node_.CreateLazyValues("lazy", [this] {
        // Create a new Inspector and put any data in it you want.
        inspect::Inspector inspector;

        // Keep track of the number of times this callback is executed.
        // This is safe because the callback is executed without locking
        // any state in the parent node.
        calls_.Add(1);

        // ERROR: You cannot modify the LazyNode from the callback. Doing
        // so may deadlock!
        // lazy_ = ...

        // The value is set to the result of calling a method on "this".
        inspector.GetRoot().CreateInt("performance_score",
                                      this->CalculatePerformance(), &inspector);

        // Callbacks return a fpromise::promise<Inspector>, so return a result
        // promise containing the value we created.
        // You can alternatively return a promise that is completed by
        // some asynchronous task.
        return fpromise::make_ok_promise(std::move(inspector));
      });
    }

  private:
    inspect::Node node_;
    inspect::IntProperty calls_;
    inspect::LazyNode lazy_;
};

{Rust}

Refer to C++ Dynamic Value Support, as similar concepts apply in Rust.

Example:

root.create_lazy_{child,values}("lazy", [] {
    async move {
        let inspector = Inspector::new();
        inspector.root().record_string("version", "1.0");
        inspector.root().record_lazy_{node,values}("lazy", || {
            let inspector = Inspector::new();
            inspector.root().record_int("value", 10);
            // `_value`'s drop is called when the function returns, so it will be removed.
            // For these situations `record_` is provided.
            let _value = inspector.root().create_int("gone", 2);
            Ok(inspector)
        });
        Ok(inspector)
    }
    .boxed()
});

Output (create_lazy_node):
root:
  lazy:
    version = "1.0"
    lazy:
      val = 10

Output (create_lazy_values):
root:
  val = 10
  version = "1.0"

String references

{C++}

You can use inspect::StringReference to reduce the memory footprint of an Inspect hierarchy that has a lot of repeated data. For instance,

using inspect::Inspector;

Inspector inspector;

for (int i = 0; i < 100; i++) {
  inspector.GetRoot().CreateChild("child", &inspector);
}

Will include 100 copies of the string "child" in your inspect output.

Alternatively,

using inspect::Inspector;
using inspect::StringReference;

namespace {
  const StringReference kChild("child");
}

Inspector inspector;
for (int i = 0; i < 100; i++) {
  inspector.GetRoot().CreateChild(kChild, &inspector)
}

Will generate only one copy of "child" which is referenced 100 times.

This pattern is recommended anywhere a global constant key would be used.

{Rust}
You can use fuchsia_inspect::StringReference to reduce the memory footprint of an Inspect hierarchy that has a lot of repeated data. For instance,
```
use fuchsia_inspect::Inspector;

let inspector = Inspector::new();
for _ in 0..100 {
  inspector.root().record_child("child");
}
```
Will generate 100 copies of "child".
Alternatively,
```
use fuchsia_inspect::{Inspector, StringReference}

lazy_static! {
  static ref CHILD: StringReference<'static> = "child".into();
}

let inspector = Inspector::new();
for _ in 0..100 {
  inspector.root().record_child(&*CHILD);
}
```
Will generate only 1 copy of "child" which is referenced 100 times.
This pattern is recommended anytime a global constant key would be used.
Note: It isn't necessary for the StringReference to be static. It can also take a String, owning it.

Viewing Inspect Data

You can use the ffx inspect command to view the Inspect data you exported from your component.

This section assumes you have SSH access to your running Fuchsia system and that you started running your component. We will use the name my_component.cmx as a placeholder for the name of your component.

Find your Inspect endpoint

The command below prints all available components that expose inspect:

ffx inspect list

The command below prints all fuchsia.diagnostics.ArchiveAccessor paths:

ffx inspect list-accessors

Your component's endpoint is listed as <path>/my_component.cmx/<id>/out/diagnostics/fuchsia.inspect.Tree. However, in some languages (without dynamic value support) and in drivers, the data is placed in VMO files instead. In that case, the endpoint is listed as <path>/my_component.cmx/<id>/out/diagnostics/root.inspect.

An accessor path listed by ffx inspect list-accessors can later be used by ffx inspect show and ffx inspect selectors using the --accessor-path flag.

The command below prints all available selectors for a component (for example, my_component.cmx):

ffx inspect selectors my_component.cmx

Read your Inspect data

The command below prints the inspect hierarchies of all components running in the system:

ffx inspect show

Using the output from ffx inspect list, you can specify a single component (for example, my_component.cmx) as input to ffx inspect show:

ffx inspect show my_component.cmx

Or specify multiple components (for example, core/font_provider and my_component.cmx):

ffx inspect show core/font_provider my_component.cmx

You can also specify a node and property value (for example, my_component.cmx:root.inspect) from ffx inspect selectors as input to ffx inspect show:

ffx inspect show my_component.cmx:root

This will print out the following if you followed the suggested steps above:

root:
  hello = world

Supported Data Types

Type	Description	Notes
IntProperty	A metric containing a signed 64-bit integer.	All Languages
UIntProperty	A metric containing an unsigned 64-bit integer.	Not supported in Dart
DoubleProperty	A metric containing a double floating-point number.	All Languages
BoolProperty	A metric containing a double floating-point number.	All Languages
{Int,Double,Uint}Array	An array of metric types, includes typed wrappers for various histograms.	Same language support as base metric type
StringArray	An array of strings. Represented as a StringReference.	Not supported in Dart.
StringProperty	A property with a UTF-8 string value.	All Languages
ByteVectorProperty	A property with an arbitrary byte value.	All Languages
Node	A node under which metrics, properties, and more nodes may be nested.	All Languages
LazyNode	Instantiates a complete tree of Nodes dynamically.	C++, Rust