Read Azure IoT Hub Telemetry from HttpTrigger Function

Question

Use case

I have an Iot Hub device which sends telemetry data to IoT Hub. I want to process the telemetry, e.g storing to a database, using a Function.

Funtion

I created the following function in VS2019 and published it to Azure:

[FunctionName("HttpTrigger")]
public static async Task<HttpResponseMessage> Run(
[HttpTrigger(AuthorizationLevel.Anonymous, "post")]HttpRequestMessage req,
ILogger log)
{
    log.LogInformation("C# HTTP trigger function processed a request.");

    var messages = await req.Content.ReadAsAsync<JArray>();

    // If the request is for subscription validation, send back the validation code.
    if (messages.Count > 0 && string.Equals((string)messages[0]["eventType"],
        "Microsoft.EventGrid.SubscriptionValidationEvent",
        System.StringComparison.OrdinalIgnoreCase))
    {
        log.LogInformation("Validate request received");
        return req.CreateResponse<object>(new
        {
            validationResponse = messages[0]["data"]["validationCode"]
        });
    }

    // The request is not for subscription validation, so it's for one or more events.
    foreach (JObject message in messages)
    {
        // Handle one event.
        EventGridEvent eventGridEvent = message.ToObject<EventGridEvent>();
        log.LogInformation($"Subject: {eventGridEvent.Subject}");
        log.LogInformation($"Time: {eventGridEvent.EventTime}");
        log.LogInformation($"Event data: {eventGridEvent.Data.ToString()}");
    }

    return req.CreateResponse(HttpStatusCode.OK);
}

Source: https://docs.microsoft.com/en-us/azure/azure-functions/functions-bindings-event-grid#use-an-http-trigger-as-an-event-grid-trigger

Event Subscription

In IoT Hub I created an Event Subscription which triggers the Function, using the Web Hook Endpoint type.

The problem

The body of the event data seems to be encrypted (?):

{{
  "properties": {},
  "systemProperties": {
    "iothub-connection-device-id": "smartmeter",
    "iothub-connection-auth-method": "{\"scope\":\"device\",\"type\":\"sas\",\"issuer\":\"iothub\",\"acceptingIpFilterRule\":null}",
    "iothub-connection-auth-generation-id": "637057961942743477",
    "iothub-enqueuedtime": "2019-10-05T08:09:17.973Z",
    "iothub-message-source": "Telemetry"
  },
  "body": "eyJEYXRlVGltZSI6IjIwMTktMTAtMDVUMTA6MDk6MjkiLCJBY3R1YWxUYXJyaWYiOjEsIkFjdHVhbFBvd2VyRGVsaXZlcmVkIjoyNzEuMCwiVG90YWxFbGVjdHJpY2l0eURlbGl2ZXJlZFRhcnJpZjEiOjYwMTU1NzcuMCwiVG90YWxFbGVjdHJpY2l0eURlbGl2ZXJlZFRhcnJpZjIiOjYwMjc5NTIuMH0="
}}

though in the Cloud Shell I can see the readable data. I can also see the readable data by reading the device to cloud messages with the EventHubClient in .Net.

What am I missing? How can I decrypt the body?

It is not encrypted. It is a base 64 string or is GZIP I do not think it is gzip because a header in the response would indicate that is is gzip. HTML has special characters that cannot be part of the message like the < and >. So I would use Convert.From — jdweng

Roman Kiss Roman Kiss · Accepted Answer · 2019-10-05T15:42:12

your device sent the telemetry data without specifying the content-type and content-encoding, see missing these properties in the systemProperties object.

The device needs to populate those system properties when sending a telemetry data, then you will see in the event message:

 "systemProperties":{
    "iothub-content-type":"application/json",
    "iothub-content-encoding":"utf-8",
    ...

and the data.body of the event will be a json formatted text.

More details here.

Read Azure IoT Hub Telemetry from HttpTrigger Function

2 Answers