Q:

Content-Type alternative in MQTT

I am Working on MQTT communication using Paho and Mosqitto. We have to support both model of serialization - xml and json. So I am looking How to identify the content type or payload type in MQTT. Is there something similar HTTP Content-Type in MQTT to identify it quickly ? 
Content-Type : application/json
Content-Type : application/xml
Thanks

A:

No, MQTT payloads are just byte arrays and there is no space in the headers (because MQTT is designed to be as light weight as possible on the network). Anything else is down to the application to implement with in the payload.
You could use multiple topics to show the difference.
e.g. foo/bar/xml or foo/bar/json and subscribe to foo/bar/+ which will match both and then switch based on the topic.
or just test the first char of the payload, '{' = json  '<' = xml

