Category: Stream Analytics

Count provisioned devices by DPS

This post shows a way to find out how many IoT (Edge) devices have been provisioned by a specific enrolment group within the last x minutes.

The solution could be much simpler if I just wanted to know how many devices are registering themselves. In this case the build in metrics are enough to get that information.

IoT Hub Metrics

The use case required a more sophisticated solution that is able to reflect the tenants, identified by tags.

Solution Architecture

Device Provisioning Service

Different Enrolment Groups separate devices in this scenario into Tenants. To be able to identify the customers, an initial tag CustomerId is added to the enrolment group. It is then applied to the devices that are create by DPS in the IoT Hub.

{
  "tags": {
    "CustomerId": "AnotherCustomer"
  },
  "properties": {
    "desired": {}
  }
}

This tag can then be used for e.g. message enrichment. I’ve written previously about using it: https://www.hezser.de/blog/2020/05/13/properties-for-iot-messages-in-azure-stream-analytics/ (opens in a new tab)

The metrics from DPS did not allow me to distinguish the tags/customers. But IoT Hub will make them available and offers events for newly created devices.

IoT Hub

Within IoT Hub I created an event subscription, that passed on all necessary events to an EventHub.

Event Subscription in IoT Hub

The event will include the device twin, which has been prepopulated with the tags specified in the enrolment group.

Device Twin in IoT Hub

As seen in the architecture diagram, Event Grid has been connected to an Event Hub. #plugandplay 😉 It will fire an event with the documented schema: Azure IoT Hub and Event Grid | Microsoft Docs

Event Hub

Why the additional Event Hub? Event Grid cannot be used as input for an Azure Stream Analytics Job and Event Hub is the universal connector in this case.

You can use the smallest tier (which is Basic) as there is not a lot of events flowing through it. The default 2 partitions is also fine.

Stream Analytics Job

I chose Stream Analytics for the further analysis of the events, because it offers an out-of-the box functionality for queries on time windows: Introduction to Azure Stream Analytics windowing functions | Microsoft Docs

As you can see, the query is pretty simple and can be adjusted easily.

Azure Stream Analytics Job Query

The example uses a blob storage as output, but you can choose to write to an Azure Function or whatever you want to do with the know how that one customer has onboarded lots of devices in a short period of time.

Properties for IoT Messages in Azure Stream Analytics

In this post I want to show how to use properties that are added to messages that IoT devices are sending to Azure IoT Hub in Stream Analytics. And while talking about properties, let’s even use message enrichment 🙂

Stream Analytics Architecture

Sample Message

The green properties will be added by the Message enrichment feature of IoT Hub, as the data is not most likely not known on the IoT device or does not need to be transferred with each message.

{
  "body": {
    "messageId": 2300,
    "temperature": 28,
    "humidity": 66
  },
  "enqueuedTime": "2020-05-08T09:55:24.886Z",
  "properties": {
    "temperatureAlert": "false",
    "CustomerName": "Microsoft Deutschland GmbH",
    "CustomerId": "4711"
  }
}

Sample IoT Device

This message is sent by a sample C# client. I used this one: https://github.com/Azure-Samples/azure-iot-samples-csharp/tree/master/iot-hub/Samples/device/MessageSample

The code that sends the message with the alert property has been adjusted to this:

string dataBuffer = $"{{\"messageId\":{count},\"temperature\":{_temperature},\"humidity\":{_humidity}}}";
using (var eventMessage = new Message(Encoding.UTF8.GetBytes(dataBuffer)))
{
    eventMessage.Properties.Add("temperatureAlert", (_temperature > TemperatureThreshold) ? "true" : "false");

Configure IoT Hub

Device Twin

In most cases the IoT (Edge) device does not know which customer it is associated, as it does not need to know. For further processing of the data – or for device management – this information is relevant. Therefore we add this information to the device twin in Azure IoT Hub.

 "version": 3,
  "tags": {
    "customer": {
      "id": "4711",
      "name": "Microsoft Deutschland GmbH"
    }
  },
  "properties": {

The property names do not need to match the desired properties that will be added via message enrichment. You can choose a structure that fits best.

Message Enrichment

We want to add the customer number and id from the device twins to the message before it is being passed along to an endpoint.

Message Enrichment settings in IoT Hub

As you can see the name of the property that is added does not need to match the name of the twin properties. Make sure you add the message enrichment to the right endpoint(s). You can decide to add different properties to messages that are routed to different endpoints.

Azure Stream Analytics

In the Stream Analytics job we use a SQL like query to filter the incoming message stream and route the messages to endpoints. The query will work fine as long as you use only the columns that are in the body of the messages (like “temperature” or “humidity” in this examle).

To be able to use the values in the properties, we need to use the GetMetadataPropertyValue function. Please take not of the sentence on the docs page: This function cannot be tested on the Azure portal using sample data

Query

SELECT
    GetMetadataPropertyValue([IoTHub-Messaging], '[User].[temperatureAlert]') AS temperaturealert,
    GetMetadataPropertyValue([IoTHub-Messaging], '[User].[CustomerName]') AS customername,
    GetMetadataPropertyValue([IoTHub-Messaging], '[User].[CustomerId]') AS customerid,
    *

The first three columns are our property and message enrichment columns while the other columns are all added as well.

Output

Let’s assume we want to add all message to a storage account where the customer id is part of the path.

Stream Analytics Blob storage output

This will work, as we added the customerid column in the query and it can be used for the path. Remember this is a demo and we only use the customerid as part of the path.

In the architecture diagram at the beginning of the post an Alert route is drawn. You can achieve this by adding a second query to the job which routes certain messages to that output.