Inspect discovery and hosting

Inspect is a powerful diagnostics feature for Fuchsia Components.

This document describes the details about how components host their Inspect data and how various tools discover that data.

Hosting Inspect data

A component may expose Inspect data in the following ways:

• fuchsia.inspect.Tree (C++, Rust)
• VmoFile (Dart)
• fuchsia.inspect.deprecated.Inspect (Go)

Components do not need to reach out to other services in order to expose Inspect data.

Components generally do not need to be concerned with which method is being used, client libraries abstract out the specific mechanism for hosting. Nearly all implementations will eventually use Tree.

Feature	fuchsia.inspect.Tree	VmoFile	fuchsia.inspect.deprecated.Inspect	Description
Non-lazy values	Yes	Yes	Yes	Components may record values such as strings and integers.
Lazy values	Yes	No	Yes	Components may generate values dynamically at read time.
Mutable tree	Yes	Yes	Yes	Components may modify the values stored in their output.
Post-mortem inspection	Yes	Yes	No	The values recorded by a component are available after that component exits.
Low-latency snapshots	Yes	Yes	No	A full snapshot of the data can be obtained with low-latency.
Consistent snapshots	Yes*	Yes	No	The snapshot of the tree is guaranteed to represent its state at one point in time.

(*: Each tree's snapshot is consistent, but inter-tree consistency is not guaranteed)

fuchsia.inspect.Tree

fuchsia.inspect.Tree supports all features of the Inspect API, and it is the recommended way to expose Inspect data from components.

Components host a single service file named fuchsia.inspect.Tree under a subdirectory called “diagnostics”. Multiple such files may exist, but they must be in separate subdirectories under “diagnostics”.

Implementation

The fuchsia.inspect.Tree FIDL File defines the protocol that is hosted by components to expose their Inspect data.

The Tree protocol links together multiple Inspect VMOs in a “tree of trees.”

In the above figure, the tree named “root” handles protocol fuchsia.inspect.Tree for connections on the top-level service hosted under diagnostics/fuchsia.inspect.Tree. Child trees may be enumerated and opened using methods on the protocol. For example, “child B” may not exist in memory until opened and read.

The protocol supports this behavior in the following ways:

• GetContent

  This method obtains the content of the tree, currently in the form of an Inspect VMO. By convention, calling this method on the root tree should return a VMO that will be continually updated with new data. The client should not need to re-read the content of the tree to read new values.

• ListChildNames

  This method accepts an iterator over which the names of children of the tree will be returned. For example, the tree in the figure above will return names “child A” and “child B” when run on the root tree.

• OpenChild

  This method accepts a request for fuchsia.inspect.Tree that will be bound to the tree specified by a given name. Using this method a client may iterate through all trees exposed over the root iterface.

VmoFile

Components may expose any number of Inspect VMOs in their out/diagnostics directory ending in the file extension .inspect. By convention, components expose their “root” inspect tree at diagnostics/root.inspect.

Note: Reading services may not disambiguate the sources of data in a component.

Components may choose to generate the content of the VMO when the file is opened if they choose, however, there exists no mechanism to link multiple trees created this way together. For this reason, lazy values are not supported in the context of a single tree, either the entire tree is generated dynamically or none of it is.

fuchsia.inspect.deprecated.Inspect

This deprecated interface is used by Go to expose Inspect data. While fuchsia.inspect.Tree exposes a “tree of trees,” this interface exposes only a single tree where subtrees may be dynamically instantiated.

This interface is deprecated in favor of the VMO format hosted by Inspect tree for the following reasons:

• VMO format supports low-latency snapshots without communicating with the hosting program.
• VMO format snapshots are always consistent for the whole tree.
• VMO format supports postmortem inspection, all Inspect data using the deprecated interface dies with the component.
• Tree protocol supports the same dynamic features as the deprecated interface.

Reading Inspect data

The two primary ways to read Inspect data are:

1. iquery
2. The Archivist

iquery

iquery (Inspect Query) is the CLI for interacting with Inspect data.

iquery's primary mode of operation takes a list of selectors for Inspect data and prints out the contained information. A selector consists of three parts which are separated by a ::

1. The component selector: This is the moniker in v2 or the realm path plus the component name in v1.
2. The node path: The path to a node in the inspect hierarchy.
3. The property path: The name of the property.

For iquery only (1) is required. If only (1) is provided (for example realm/component.cmx) then iquery will use a selector realm/component.cmx:* to fetch all the inspect data.

iquery includes two utility commands to know what components are available and what selectors can be used:

• list: iquery will print all component selectors that are available, this is all v2 monikers and all v1 realm paths with component names.
• selectors: iquery will print all selectors available under the provided selector locations.

These modes could be used together as follows:

iquery show `iquery list | grep component_name`

Alternatively iquery also allows to print the inspect data at a location. A location consists either of the path to a .inspect file, or the path to a directory containing fuchsia.inspect.Tree. iquery includes a utility command to list all files that contain inspect data (list-files). Note that these will be only data from v1 components given that v2 are not accessible through the filesystem at the moment.

iquery's secondary mode of operation (triggered by list-files) recursively identifies locations for Inspect data from the given directorry path. The two modes may be used together as follows:

iquery show-file `iquery list-files /hub | grep -v system_objects | grep component_name`

In the example above, iquery is run to find a list of Inspect locations that do not contain “system_objects” and that do contain “component_name”. Then, iquery is run on the result of the first filter to recursively list data in the matching locations. You may instead write:

fx iquery show --manifest component_name

Archivist

The Fuchsia Diagnostics Platform, hosted by the Archivist, is responsible for monitoring and aggregating Inspect data on demand.

Collection under appmgr

When a component is running under appmgr, diagnostics data is collected from its out/diagnostics directory. A connection to this directory is provided to Archivist by the ComponentEventProvider protocol.

A separate component, called archivist-for-embedding.cmx, serves the same purpose as the Archivist but may be injected into tests. This allows tests to find only their own diagnostics data, helping to make tests hermetic.

Collection under component_manager

When running under component manager, diagnostics data is made available to the Archivist through event capabilities.

The Archivist is offered events by the root realm. Therefore, it sees events from the whole system. The events it is allowed to see and is subscribed to are the following:

• Started: sent to the Archivist when a component is started.
• Running: sent to the Archivist when subscribing for components that were already running.
• Directory ready: with a filter for /diagnostics. Sent to the Archivist when a component exposes a /diagnostics directory to the framework.
• Stopped: sent to the Archivist when a component is stopped.

A component that wishes to expose Inspect needs to expose a /diagnostics directory to the framework. This typically looks as follows:

{
    capabilities: [
        {
            directory: "diagnostics",
            rights: [ "connect" ],
            path: "/diagnostics",
        },
    ],
    expose: [
        {
            directory: "diagnostics",
            from: "self",
            to: "framework",
        },
    ],
}

There's a useful manifest include that simplifies this and is required by Inspect libraries:

{
    include: [
        "inspect/client.shard.cml",
    ]
}

Reading Inspect from the Archivist

The Archivist hosts fuchsia.diagnostics.ArchiveAccessor, which provides the StreamDiagnostics method to obtain Inspect data from running components.