Quick update on MQTT and message routing in IoT Hub and IoT Edge

I’ve made an update to my posts on using a standard MQTT client and using Node-Red to connect to IoT Edge via MQTT.

One thing I missed in the original post, that I needed to figure out today for a customer, is that if you want to route messages in IoT Edge based on the message body, you need to use the contentType and contentEncoding fields to tell edgeHub that the content is, in fact, JSON, and also what encoding it is (i.e. utf-8, utf-16, etc). This is not unique to IoT Edge, but a requirement for IoT Hub itself as well.

The way you do that is by appending ‘properties’ to the end of your MQTT topic to indicated the content type and the content encoding. In MQTT, those are the $.ct and $.ce properties, respectively.. So, regardless of your client, you do that by appending the values $.ct=application/json and $.ce=utf-8, after URL encoding them, to your topic, like this:

devices/[device_id]/messages/events/$.ct=application%2Fjson&$.ce=utf-8

where [device_id] is obviously the device id in iot hub of your sending device. This allows you to do things like this in your IoT Edge routing (pretend we are sending a message like {“messageType”:”alert”, ……})

{
"routeToAlertHandler":"FROM /messages/* WHERE $body.messageType='alert' INTO ........
}

Enjoy! and, as always, if you have any questions, hit me up in the comments section or twitter/etc (links in my ‘about me’ page)

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