How do I determine user ID of MQTT clients?

Question

Currently I have an MQTT system where one client publishes, and one subscribes. Now I want a system where many clients publish. The susbcriber must understand which client is sending information.

While one obvious way to do this is to simply append a client ID in the publishing message, I wanted to know if there is a way of getting client ID without explicitly adding it to the message.

To elaborate, suppose the topic is "/hello/world", and client 1 publshes "OK", client 2 publishes "ERR". Is there a way to determine which client sent what message?

Answer 1

I don't believe you can tell who has sent the message without watching the log.

The ways I have overcome this in the past is to either publish the messages with the client id in the topic; (client_id could be anywhere in the topic)

/hello/clientid1/world ok /hello/clientid2/world err /hello/clientid3/world warning

Then subscribe to the topic like this; /hello/+/world Then in your code transform the topic name to get the message and client id.

The other approach I use is to use json in the payload, for example

/hello/world {"msg":"err", "client":"clientid1"}