Event Grid Namespace with MQTT Enabled: Event Subscription processes event where data key is null

Question

We have created an Event Grid Namespace with the MQTT feature enabled.

From there we already route the data to an Event-Hubs (as in this tutorial) which triggers an Azure Function. This way works as expected.

Now we want to directly trigger an Azure Function directly from the Event Grid for specific Mqtt topics. Therefore we created a new Event Subscription inside the Event Grid Topic with a prefix filter that matches our Mqtt topic. The filtering works. We are receiving messages, using the Python Azure functions library.

Problem: Data field of event messages is null.

Expected behaviour: The data field of the event message has the value of the json payload of the mqtt message.

We are able to receive messages with the python libary when we send Mqtt Events but the 'data' field is null also the event_type is null.

Here is an example payload we receive in the Azure fuction: tested with azure-functions 1.17.0 https://pypi.org/project/azure-functions/ and azure-functions 1.18.0b3 https://pypi.org/project/azure-functions/1.18.0b3/

def main(event: func.EventGridEvent, signalRMessages: func.Out[str]) -> None:
    event_dict = {
        'id': event.id,
        'data': event.get_json(),
        'topic': event.topic,
        'subject': event.subject,
        'event_type': event.event_type,
    }

{"id": "1572f624-9581-4210-8bf4-6e121c6bd649", "data": null, "topic": null, "subject": "dockerhost02/Kathrein-RTLS/test1", "event_type": null}

Answer 1

Hello Jonathan,

welcome to this moderated Azure community forum.

The Data field is only filled when the message is marked as being JSON / UTF-8.

For MQTT v5 messages of content type= “application/json; charset=utf-8” or of a payload format indicator=1, the payload will be forwarded in the data object, and the message will be serialized as a JSON

Can you check if the "data_base64" field is filled?

If so, this field has the same value but encoded as Bas64. You just need to decode it.

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