Adding context using EventData properties in Azure Stream Analytics

The Azure IoT Hub is the preferred Azure Cloud gateway for IoT Devices.

The format of messages sent by devices to the IoTHub is described in the EventData class.

There, a regular message is split into three data parts:

1. The actual message body, the data inside
2. System properties, context added by the system (like IoT Hub context)
3. Application (aka User) properties, context added by the user

These properties are key value pairs. these are not encoded opposite to the message body which has to be decoded before it is accessible.

So, we see that the original messages can get context along the way.

As a developer, using Azure IoT, adding application/user context can be done at two levels:

We can add application properties to the device message. This is normally done on the device when the message is constructed.

The Azure IoT Hub also supports message enrichment. These enrichments are also added as application/user properties, as seen in my previous blog post.

There, I showed how to read the properties using an Azure Function.

Consuming message context is a little bit of a challenge as seen in that post.

Here, we dive into this using Azure Stream Analytics.

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Take back control over IoTHub messages in Azure Functions

Azure Functions are a blessing for IoT solutions. To be so flexible executing code whenever messages are arriving, every IoT project is fully depending on it.

But one of the biggest frustrations is the casting of (EventHub) messages towards a string! Only the message body is left! Once a message is passed on to an Azure Function, I only have access to the body of the message. I can not access the (routing) properties anymore.

And before we got Azure Functions, we had to work with Stream Analytics. And I still do! And it’s so nice to have access to the IoT Hub values like the device name of the message. Because I am working with Azure Functions, I have to put it in the Message body first???

It would be great to have access to both the properties and the IoTHub values!

Well, it’s possible now with some clever casting…