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Azure IoT Hub Device Twin

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The Azure IoT Hub Device Twin allows to store and retrieve metadata about an IoT device in the cloud. This metadata, also known as "twin". This information can be used to configure and manage the device, as well as to monitor its status and troubleshoot issues. In this blog, we will take a closer look at Azure IoT Hub Device Twin and provide an example of how it can be used in a real-world scenario.

Each device twin contains the following set of properties that describe the current state of the device, as well as a set of desired properties that specify the desired state of the device.

  • Desired Properties
  • Reported Properties
  • Twin Tags

Desired Properties: 

The desired Properties in the Azure IoT Hub Device twins are the configurations and settings that the IoT solution or the application wants the device to have. These are the configurations and settings that the application wants the device to have, and it can update them by sending an update request to the IoT hub. The device can read these desired properties and apply the changes to its behaviour accordingly.

  • Desired properties are used to configure and control the behaviour of the device remotely.
  • Desired properties can be used to send new configuration settings or commands to the device.
  • The device can read the desired properties and take actions based on the new settings.
  • The application can update desired properties at any time, and the device will receive the updates and act accordingly.
  • Desired properties can be used to implement a simple form of device management, such as reboot or reset commands.
  • Desired properties can be used to update firmware, change device configuration, or update a device's behavior.
  • Desired properties are updated by the application and can be accessed by the device via the Azure IoT SDK.

Reported Properties: 

The reported properties in an Azure IoT Hub Device Twins are the device's current state and configurations.These are the properties that the device is reporting back to the IoT hub, and they can be used to monitor the status and health of the device.

  • Reported properties are used to report the device's current state and configurations.
  • Reported properties can be used to report sensor data, device status, error messages, and other information.
  • The IoT solution or the application can read reported properties to monitor the status and health of the device.
  • The device can update reported properties at any time, and the IoT hub will receive the updates and act accordingly.
  • Reported properties can be used to implement a simple form of device monitoring, such as checking if the device is online or offline.
  • Reported properties can be used to report the device's location, temperature, battery level, and other telemetry data.
  • Reported properties are updated by the device and can be accessed by the IoT solution or the application via the Azure IoT SDK.

Twin Tags:

Azure IoT Hub Device Twins also support a feature called "Twin Tags," which allows you to add metadata to the Device Twin properties, including the desired and reported properties. These tags can be used to filter, query, and identify the devices in your solution.

  • Twin tags are used to add metadata to the device's twin properties.
  • Twin tags can be used to filter, query, and identify the devices in your solution.
  • The IoT solution or the application can add tags to the Device Twin to filter and group the devices according to specific characteristics.
  • Twin tags can be used to add information such as the device's location, manufacturer, and model.
  • Twin tags can be used to filter devices based on their characteristics, such as device type, location, or firmware version.
  • Twin tags can be used to segment devices into different groups, such as test devices, production devices, or devices in a specific region.
  • Twin tags can be used to filter devices based on specific criteria, and then apply different policies or rules to each group of devices.
  • Twin tags are stored in the Azure IoT hub and can be accessed by the IoT solution or the application via the Azure IoT SDK.

Example:

Consider a smart thermostat device that is connected to the Azure IoT platform. The device twin for this device might contain the following properties:

  • Desired temperature: The temperature that the user wants the thermostat to be set to.
  • Reported temperature: The temperature that the thermostat is currently set to.
  • Battery level: The current battery level of the thermostat.
  • Firmware version: The version of the firmware currently installed on the thermostat.
  • Twin tags: Location (e.g. "Living Room"), Device type (e.g. "Thermostat")
The device twin uses the JSON structure, which contains two main sections:
  • Desired
  • Reported
The "desired" section contains the desired properties that are set by the cloud, and the "reported" section contains the reported properties that are reported by the device.

{
  "desired": {
    "temperature": 72,
    "thermostatMode": "Heat"
  },
  "reported": {
    "temperature": 68,
    "thermostatMode": "Cool",
    "batteryLevel": 0.75,
    "firmwareVersion": "1.0.0"
  },
  "tags": {
    "location": "Living Room",
    "deviceType": "Thermostat"
  }
}

In the preceding example, the desired temperature is set to 72 degrees and the thermostat mode is set to "heat," while the reported temperature is 68 degrees, the thermostat mode is "cool," the battery level is 0.75, and the firmware version is 1.0.0. The device twin also has two tags: location and device type.

Key Points: IoT Hub Device Twin

  • Azure IoT Hub Device Twins are JSON documents that store metadata and configurations for devices connected to an IoT hub.
  • The size of the Device Twin JSON document is limited to 8 KB.
  • The property depth is limited to 32 levels, meaning that the Device Twin JSON document can only have 32 nested levels of properties.
  • If the size of the device twin or property depth exceeds these limits, the update will fail and an error will be returned.
  • The device twin can be used to store and retrieve configuration information, such as desired properties and reported properties.
  • The desired properties are updated by the application, and the reported properties are updated by the device.
  • Desired and reported properties can also be used to implement a simple form of device management, such as reboot or reset commands.
  • The device twin is stored in the Azure IoT hub, so it will be highly available and automatically backed up.
  • The device twin is specific to the device, so it can store device-specific information like version, device model, and firmware.
  • The device twin can also be used to store the device's location, which can be useful for tracking and monitoring.

Summary

I hope the preceding explanations helped you understand that Azure IoT Device Twins are a powerful feature that allows developers to synchronise the state of their IoT devices with the cloud. Each device twin contains a set of properties that describe the current state of the device, as well as a set of desired properties that specify the desired state of the device. The desired and reported properties can be updated at any time, making it easy to keep track of the state of your devices and take appropriate actions.

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