Manage IoT Plug and Play digital twins

מאמר
02/06/2024

IoT Plug and Play supports Get digital twin and Update digital twin operations to manage digital twins. You can use either the REST APIs or one of the service SDKs.

Update a digital twin

An IoT Plug and Play device implements a model described by Digital Twins Definition Language (DTDL). Solution developers can use the Update Digital Twin API to update the state of component and the properties of the digital twin.

The IoT Plug and Play device used as an example in this article implements the Temperature Controller model with Thermostat components.

The following snippet shows the response to a Get digital twin request formatted as a JSON object. To learn more about the digital twin format, see Understand IoT Plug and Play digital twins:

{
    "$dtId": "sample-device",
    "serialNumber": "alwinexlepaho8329",
    "thermostat1": {
        "maxTempSinceLastReboot": 25.3,
        "targetTemperature": 20.4,
        "$metadata": {
            "targetTemperature": {
                "desiredValue": 20.4,
                "desiredVersion": 4,
                "ackVersion": 4,
                "ackCode": 200,
                "ackDescription": "Successfully executed patch",
                "lastUpdateTime": "2020-07-17T06:11:04.9309159Z"
            },
            "maxTempSinceLastReboot": {
                "lastUpdateTime": "2020-07-17T06:10:31.9609233Z"
            }
        }
    },
    "$metadata": {
        "$model": "dtmi:com:example:TemperatureController;1",
        "serialNumber": {
            "lastUpdateTime": "2020-07-17T06:10:31.9609233Z"
        }
    }
}

Digital twins let you update an entire component or property using a JSON Patch.

For example, you can update the targetTemperature property as follows:

[
    {
        "op": "add",
        "path": "/thermostat1/targetTemperature",
        "value": 21.4
    }
]

The previous update sets the desired value of a property in the corresponding component-level $metadata as shown in the following snippet. IoT Hub updates the desired version of the property:

"thermostat1": {
    "targetTemperature": 20.4,
    "$metadata": {
        "targetTemperature": {
            "desiredValue": 21.4,
            "desiredVersion": 5,
            "ackVersion": 4,
            "ackCode": 200,
            "ackDescription": "Successfully executed patch",
            "lastUpdateTime": "2020-07-17T06:11:04.9309159Z"
        }
    }
}

Add, replace, or remove a component

Component level operations require an empty object $metadata marker within the value.

An add or replace component operation sets the desired values of all the provided properties. It also clears desired values for any writable properties not provided with the update.

Removing a component clears the desired values of all writable properties present. A device eventually synchronizes this removal and stops reporting the individual properties. The component is then removed from the digital twin.

The following JSON Patch sample shows how to add, replace, or remove a component:

[
    {
        "op": "add",
        "path": "/thermostat1",
        "value": {
            "targetTemperature": 21.4,
            "anotherWritableProperty": 42,
            "$metadata": {}
        }
    },
    {
        "op": "replace",
        "path": "/thermostat1",
        "value": {
            "targetTemperature": 21.4,
            "$metadata": {}
        }
    },
    {
        "op": "remove",
        "path": "/thermostat2"
    }
]

Add, replace, or remove a property

An add or replace operation sets the desired value of a property. The device can synchronize state and report an update of the value along with an ack code, version, and description.

Removing a property clears the desired value of property if it's set. The device can then stop reporting this property and it's removed from the component. If this property is the last one in the component, then the component is removed as well.

The following JSON Patch sample shows how to add, replace, or remove a property within a component:

[
    {
        "op": "add",
        "path": "/thermostat1/targetTemperature",
        "value": 21.4
    },
    {
        "op": "replace",
        "path": "/thermostat1/anotherWritableProperty",
        "value": 42
    },
    {
        "op": "remove",
        "path": "/thermostat2/targetTemperature",
    }
]

Rules for setting the desired value of a digital twin property

Name

The name of a component or property must be valid DTDL name.

Allowed characters are a-z, A-Z, 0-9 (not as the first character), and underscore (not as the first or last character).

A name can be 1-64 characters long.

Property value

The value must be a valid DTDL Property.

All primitive types are supported. Within complex types, enums, maps, and objects are supported. To learn more, see DTDL Schemas.

Properties don't support array or any complex schema with an array.

A maximum depth of a five levels is supported for a complex object.

All field names within complex object should be valid DTDL names.

All map keys should be valid DTDL names.

Troubleshoot update digital twin API errors

The digital twin API throws the following generic error message:

ErrorCode:ArgumentInvalid;'{propertyName}' exists within the device twin and is not digital twin conformant property. Please refer to aka.ms/dtpatch to update this to be conformant.

If you see this error, make sure the update patch follows the rules for setting desired value of a digital twin property.

When you update a component, make sure that the empty object $metadata marker is set.

Updates can fail if a device's reported values don't conform to the IoT plug and play conventions.

Next steps

Now that you've learned about digital twins, here are some more resources:

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