How to implement realtime Communication between Device and Azure Function over IoT Hub

Question

Hello,

i would like to implement a low-delay (near-realtime) control loop over Azure IoT Hub.
(I am aware of the problems and challenges of the time relevant control via the cloud)

Control Loop:
The microcontroller sends the current position of an actuator to the IoT Hub.

Input:
{"deviceId":"MyDevice","position_x":14.00,"position_y":24.00,"position_z":75.00}

The cloud service automatically execute a short calculation and send a response to the microcontroller with the new velocity.

Return:
{"deviceId":"MyDevice","velocity":3.00}

The service should run through this loop with a minimum of delay.

Current status:

As a first Step i created an IoT Hub and ran the given Quickstart-Examples (in python).
(https://learn.microsoft.com/en-us/azure/iot-hub/quickstart-send-telemetry-python)

In the next stage I used the MSP432E401Y Microcontroller from Texas Instruments instead of the SimulatedDevice.py.

After that I added Eventgrid + Azure Function.

Overview:

The Azure Function is in python and looks like:

function.json:

{  
    "scriptFile": "init.py",  
    "bindings": [  
    {  
         "type": "eventGridTrigger",  
         "name": "eventGridEvent",  
         "direction": "in"  
    }  
    ]  
}  

init.py:

 import json  
 import logging  
 import azure.functions as func  
 import datetime  
  
 import random  
 import sys  
 from azure.iot.hub import IoTHubRegistryManager  
      
 MESSAGE_COUNT = 1  
 AVG_WIND_SPEED = 10.0  
 MSG_TXT = "{\"service client sent a message\": %.2f}"  
      
 CONNECTION_STRING = "HostName=XXXX.azure-devices.net;SharedAccessKeyName=service;SharedAccessKey=XXXXX"  
 DEVICE_ID = "XXXX"  
      
 def iothub_messaging_sample_run():  
     try:  
         # Create IoTHubRegistryManager  
         registry_manager = IoTHubRegistryManager(CONNECTION_STRING)  
      
         for i in range(0, MESSAGE_COUNT):  
             print ( 'Sending message: {0}'.format(i) )  
             data = MSG_TXT % (AVG_WIND_SPEED + (random.random() * 4 + 2))  
      
             props={}  
             # optional: assign system properties  
             props.update(messageId = "Return_message_%d" % i)  
             props.update(correlationId = "correlation_%d" % i)  
             props.update(contentType = "application/json")  
      
             # optional: assign application properties  
             prop_text = "PropMsg_%d" % i  
             props.update(testProperty = prop_text)  
      
             registry_manager.send_c2d_message(DEVICE_ID, data, properties=props)  
      
     except Exception as ex:  
         print ( "Unexpected error {0}" % ex )  
         return  
     except KeyboardInterrupt:  
         print ( "IoT Hub C2D Messaging service sample stopped" )  
      
 def main(eventGridEvent: func.EventGridEvent) -> None:  
      
     result = json.dumps({  
         'id': eventGridEvent.id,  
         'data': eventGridEvent.get_json(),  
         'topic': eventGridEvent.topic,  
         'subject': eventGridEvent.subject,  
         'event_type': eventGridEvent.event_type,  
     })  
      
     logging.info('Python EventGrid trigger processed an event: %s', result)  
      
     iothub_messaging_sample_run()  
      
     msg_data = eventGridEvent.get_json()  
     logging.info(msg_data)  
          
     if 'properties' in msg_data and 'position' in msg_data['properties']:  
         logging.info(msg_data['properties']['position'])  

It works! But i am sure there is an easier or more efficient way to go (like Input + Output Binding in EventGrid). I tried it but didn't get it to work.

Question:
After I used the Provision Service to connect the device to cloud and used the authentication type "X.509 CA Signed" a "Connection String" isn't longer available.

Therefore my workaround over the connection string no longer work.

Could somebody explain me, how to fix it.

With best regards
Patrick

Answer 1

Hi @Patrick Schneider ,

I have limited experience with Python too.

I was able to get a Python Azure Function triggered:

the function.json looks like this:

{  
 "scriptFile": "__init__.py",  
 "bindings": [  
   {  
   "type": "eventHubTrigger",  
   "name": "events",  
   "direction": "in",  
   "eventHubName": "samples-workitems",  
   "connection": "mybeerliftconnection",  
   "consumerGroup": "pfa"  
   }  
 ]  
}  

in the portal:

The mybeerliftconnection references the public eventhub-compatible endpoint of the IoT hub:

Endpoint=sb://iothub-ns-[hidden].servicebus.windows.net/;SharedAccessKeyName=iothubowner;SharedAccessKey=[secret]=;EntityPath=beerlift-weu-ih  

It is stored in the application settings of the Azure Function App:

Answer 2

Hello @Patrick Schneider ,

I see two questions:

1. is this the least viable architecture?
2. How do I connect using a Device Provisioning Service

Regarding question 1:

You make use of this solution: IoT Hub -> EventGrid -> Azure Function.

Although this is a correct use of EventGrid, you can remove the EventGrid from the solution and connect an Azure Function directly to the IoT Hub (built-in eventhub compatible endpoint).

Just select the iot hub (event hub) trigger:

Connect to your iot hub:

Note the Consumer group I added for the built-in endpoint in the IoT Hub (named 'fa').

The 'Event Hub name' can be ignored.

This is a bit less flexible but takes away the cost of the eventgrid.

Question 2:

In the code example, you use a symmetric key.

There is an equivalent security mechanism in a DPS, check the individual enrollment with symmetric keys:

Note: symmetric keys are considered less secure compared to the other options. Therefore, only use it in DEV and Test environment. The Group enrollment equivalent is also interesting.

Answer 3

Hello @Sander van de Velde ,

it works!

Thank you very much.

Answer 4

Hello @Sander van de Velde ,

thank you very much. This helped!

I created a function app on windows and .net as shown in your description. It works :-)

However, I still have two questions:

1)

Problem:
I would like to use python (-> Linux) instead of .net + windows.
But editing functions in the Azure portal is not supported for Linux Consumption Function Apps.

Therefore i created a python function in Visual Studio Code (with the same function.json):

function.json:

{  
  "scriptFile": "__init__.py",  
  "bindings": [  
    {  
      "type": "eventHubTrigger",  
      "name": "myIoTHubMessage",  
      "direction": "in",  
      "eventHubName": "samples-workitems",  
      "connection": "<iothub_connection>",  
      "consumerGroup": "$Default"  
    }  
  ]  
}  

But the python function doesn't work. I don't understand the difference and why it's not working.

2)

Now it's possible to send data from the iot device to the function app. What is the correct way to send a response to the iot device?

Thank you very much! I hope you could help me farther.

With best regards
Patrick

Answer 5

For question 2)

Because this doesn't work:

function.json:

{  
  "bindings": [  
    {  
      "type": "eventHubTrigger",  
      "name": "myIoTHubMessage",  
      "direction": "in",  
      "eventHubName": "samples-workitems",  
      "connection": "XXXXX_events_IOTHUB",  
      "consumerGroup": "consumergroup-001"  
    },  
    {  
      "name": "outputEventHubMessage",  
      "connection": "XXXXX_events_IOTHUB",  
      "eventHubName": "outeventhub",  
      "direction": "out",  
      "type": "eventHub"  
    }  
  ]  
}  

run.csx:

using System;  
  
public static void Run(string myIoTHubMessage, out string outputEventHubMessage, ILogger log)  
{  
    log.LogInformation($"C# IoT Hub trigger function processed a message: {myIoTHubMessage}");  
    outputEventHubMessage = $"HelloWorld";  
}  

I found this https://learn.microsoft.com/en-us/azure/azure-functions/functions-bindings-event-iot-output?tabs=csharp but i am not sure how to use the examples.