Sending cloud-to-device message Azure IoT Hub with REST api, endpoint

Question

I'm trying to send a message to my device from the cloud, using Azure IoT Hub and REST api (not using the Azure IoT hub python SDK).

I can successfully send a message (POST request) to the hub from the device with uri https://<myiothub>.azure-devices.net/devices/<devid>/messages/events?api-version=2018-06-30. In the documentation at https://learn.microsoft.com/en-us/azure/iot-hub/iot-hub-devguide-messages-c2d it says there is a service-side endpoint at /messages/devicebound. However, they don't show a complete example, so I'm not entirely sure what the full endpoint I should use is, and how/where to specify which device to send to.

I anyway tried the following:

curl -v POST \
  https://<myhub>.azure-devices.net/messages/devicebound?api-version=2018-06-30 \
  -H 'Authorization: SharedAccessSignature <sas>' \
  -H 'Content-Type: application/json' \
  -d '{
    "payload": {
      "key": "value"
    }
  }'

where is generated via Azure CLI az iot hub generate-sas-token -n <myhub>. I get the error message

{"Message":"ErrorCode:ArgumentInvalid;Request must contain IoTHub custom 'To' header","ExceptionMessage":"Tracking ID:ec98ff8...

where I cut off the end. So I tried adding a "To" header, which regardless of what I put returns the same error message.

I also tried what's suggested here Cloud-to-device Azure IoT REST API, namely to send via endpoint https://main.iothub.ext.azure.com/api/Service/SendMessage/, but without luck.

Answer 1

Here's how to do it:

1. Using a text editor, create a SendCloudToDeviceMessage.py file.

2. Add the following import statements and variables at the start of the SendCloudToDeviceMessage.py file:

import random
import sys
import iothub_service_client
from iothub_service_client import IoTHubMessaging, IoTHubMessage, IoTHubError

OPEN_CONTEXT = 0
FEEDBACK_CONTEXT = 1
MESSAGE_COUNT = 1
AVG_WIND_SPEED = 10.0
MSG_TXT = "{\"service client sent a message\": %.2f}"

3. Add the following code to SendCloudToDeviceMessage.py file. Replace the "{iot hub connection string}" and "{device id}" placeholder values with the IoT hub connection string and device ID you noted previously:

CONNECTION_STRING = "{IoTHubConnectionString}"
DEVICE_ID = "{deviceId}"

4. Add the following function to print feedback messages to the console:

def open_complete_callback(context):
    print ( 'open_complete_callback called with context: {0}'.format(context) )

def send_complete_callback(context, messaging_result):
    context = 0
    print ( 'send_complete_callback called with context : {0}'.format(context) )
    print ( 'messagingResult : {0}'.format(messaging_result) )

5. Add the following code to send a message to your device and handle the feedback message when the device acknowledges the cloud-to-device message:

def iothub_messaging_sample_run():
    try:
        iothub_messaging = IoTHubMessaging(CONNECTION_STRING)

        iothub_messaging.open(open_complete_callback, OPEN_CONTEXT)

        for i in range(0, MESSAGE_COUNT):
            print ( 'Sending message: {0}'.format(i) )
            msg_txt_formatted = MSG_TXT % (AVG_WIND_SPEED + (random.random() * 4 + 2))
            message = IoTHubMessage(bytearray(msg_txt_formatted, 'utf8'))

            # optional: assign ids
            message.message_id = "message_%d" % i
            message.correlation_id = "correlation_%d" % i
            # optional: assign properties
            prop_map = message.properties()
            prop_text = "PropMsg_%d" % i
            prop_map.add("Property", prop_text)

            iothub_messaging.send_async(DEVICE_ID, message, send_complete_callback, i)

        try:
            # Try Python 2.xx first
            raw_input("Press Enter to continue...\n")
        except:
            pass
            # Use Python 3.xx in the case of exception
            input("Press Enter to continue...\n")

        iothub_messaging.close()

    except IoTHubError as iothub_error:
        print ( "Unexpected error {0}" % iothub_error )
        return
    except KeyboardInterrupt:
        print ( "IoTHubMessaging sample stopped" )

6. Add the following main function:

if __name__ == '__main__':
    print ( "Starting the IoT Hub Service Client Messaging Python sample..." )
    print ( "    Connection string = {0}".format(CONNECTION_STRING) )
    print ( "    Device ID         = {0}".format(DEVICE_ID) )

    iothub_messaging_sample_run()

7. Save and close SendCloudToDeviceMessage.py file.

8. Install dependency library: pip install azure-iothub-service-client

9. Run the Application: python SendCloudToDeviceMessage.py

Reference: https://github.com/MicrosoftDocs/azure-docs/blob/master/articles/iot-hub/iot-hub-python-python-c2d.md

Answer 2

To receive a Cloud to Device message from the device end using IoT Hub API you will have to do the following request -

curl -X GET \
  'https://{your hub}.azure-devices.net/devices/{your device id}/messages/deviceBound?api-version=2018-06-30' \
  -H 'Authorization: {your sas token}'