python

How To Extend Azure IoT Hub Twins via Azure Blob Storage (python)

Please refer to the root README for a high level introduction of this sample. This document covers the details of the specific python implementation.

Getting Started

Prerequisites

The COMMON_SETUP.md file contains more detailed references on how to provision the required resources (e.g., IoT Hub, device identity, Storage Account, blob container).

The python example also requires

• Azure CLI (>= 2.0.46)
• Azure IoT CLI extension
• Either of the following
  • Docker
  • Python 3.6+

Quickstart

Start the simulated device client

In the device-twin-client directory, you can choose to run the client example as a python app on your own host or inside a Docker container.

In either case

• Retrieve the device connection string for your IoT Hub device
  • One option is to execute the following Azure CLI command az iot hub device-identity show-connection-string --hub-name YourIoTHubName --device-id MyNodeDevice --output table
  • Another option is to retrieve the connection string from the Azure Portal as described in the Create a device identity documentation.

For the Docker experience

• Build the image and tag it: docker build -t localhost:5000/device-twin-client .
• Run the container, being sure to replace the values for the CONNECTION_STRING and DEVICE_ID environment variables: docker run -e "CONNECTION_STRING={deviceConnectionString}" -e "DEVICE_ID={deviceId}" localhost:5000/device-twin-client

To run the python client directly

• Install the dependencies: pip install -r requirements.txt
• Export the CONNECTION_STRING and DEVICE_ID environment variables. When on Windows, use SET instead of export

export CONNECTION_STRING=HostName=yourhub.azure-devices.net;DeviceId=yourdevice;SharedAccessKey=redacted
export DEVICE_ID=yourdevice

• Run the application: python ./sender/device-twin-sample.py

At this point, your client device is connected to IoT Hub and is registered to receive updates for its associated device twin.

Apply a new asset

In the automate-setup directory,

Ensure environment variables are set

If you have followed the COMMON_SETUP and have not closed the shell, the necessary environment variables should already be set. Otherwise, you can manage them by using a .env file.

• Copy .env.sample to .env
• Update the values
• Source the .env file: source .env

Run the script

• Ensure the script is executable: chmod +x new-blob.sh
• Execute the backend script to upload and apply a new blob:

  ./new-blob.sh $RESOURCE_GROUP \
      $AZURE_STORAGE_ACCOUNT \
      $AZURE_STORAGE_CONTAINER \
      $SENSOR_ID \
      ../../sample-files/iotLogo.txt
  

If you left the client running, you should see the new artwork displayed in the console.

Details

Client

• Opens the device client connection (iothub_client_init)
• Registers for device twin updates (client.set_device_twin_callback)
• Enters a loop to send arbitrary sample messages to IoT Hub (client.send_event_async)
• When device twin updates are received (device_twin_callback)
  • Parses the JSON twin (json.loads(payload))
  • Retrieves the blob via the SAS URL (urllib.request.urlopen(url))
  • Prints it to the console (print(text))

Back-end

The automate-setup/new-blob.sh script automates the manual process of

• Uploading a file to blob storage (az storage blob upload)
• Generating a SAS token (az storage blob generate-sas)
• Patching the twin property for a single device identity
  • Obtains the current twin (az iot hub device-twin show)
  • Updates the desired blob properties (jq...)
  • Replaces the twin (az iot hub device-twin replace)

As alternatives, consider

• the nodejs example in this repo similarly uploads the blob and generates a SAS URL. In contrast, it submits a job to IoT Hub, offloading the responsibility of applying the update to each matched twin.
• the dotnet example in this repo uses an Azure Function triggered from updates to the blob container; it iterates over the results of a device query to patch each twin.