message body routing from non-SDK leaf devices - azure-iot-edge

Question

the docs say to route messages within Edge properly based on the content of the body of the message, you have to set the content-type and content-encoding system properties, as shown in the sample below that uses the SDK
// Encode message body using UTF-8
var messageBytes = Buffer.from(messageBody, "utf8");
var message = new Message(messageBytes);
// Set message body type and content encoding
message.contentEncoding = "utf-8";
message.contentType = "application/json";
However, for a client that does not use the SDK, and instead uses straight MQTT such as the Paho client (as in this example), how do you specify those system properties? the IoT Hub docs (not edge) say that you can add a property bag of properties to the topic, but that doesn't seem to work for Edge.
So how can I route messages within Edge, based on the body of the messages, from non-SDK leaf devices?

Answer 1

Looking at this code:
https://github.com/Azure/azure-iot-sdk-c/blob/master/iothub_client/src/iothubtransport_mqtt_common.c#L655
you can use $.ce and $.ct to specify respectively the contentEncoding and contentType system properties (i could not find any documentation about this so this must be a hack ;-)). Here is a JavaScript example:
const deviceId = '<your device id>';
const querystring = require('querystring');
const propertyBag = querystring.stringify({
'$.ce': 'utf-8',
'$.ct': 'application/json'
});
const topic = `devices/${deviceId}/messages/events/${propertyBag}`;
Another way to go could be to extract a value you want to filter on and put it in the property bag as an application property. Here we get the value of 'source' from our data and use it to filter the data:
const deviceId = '<your device id>';
const querystring = require('querystring');
let data = ...;
const propertyBag = querystring.stringify({
source: data.source
});
const topic = `devices/${deviceId}/messages/events/${propertyBag}`;
Then when defining the 'message route', you would for instance define the 'routing query' as:
source = 'my_source'
Any data containing a 'source' field with value 'my_source' would then be routed to this message route by IoT Hub.