One-to-one Communication in MQTT

Question

I need to set up an MQTT system for a project connecting many clients. These are all different type of clients with different functionalities. Some have information, like say temperature that is being published but at the same time, there are feature states that need to be set like switches, which can be set by other clients.

So, I used MQTT to achieve the following:

• The client publishes information on the temperature it collects like so:

customer/group/device/temperature

• To receive a state for the switch from different clients that it subscribes to, any client connects to a broker like so:

customer/group/device/switch

So far, I have only tried using a websocket based solution but now that MQTT has become such a famous protocol for IoT devices, I also wanted to know to implement one to one communications between clients using MQTT and this was how I tried doing it. 

Just, please take a look and correct me if I've made any mistakes.

Answer 1

Your solution seems to be a decent one. Let's say, for instance, client A publishes to client B by using topic customer/group/B/switch, then client B will reply to customer/group/A/switch.

Alternatively, client A could also publish to customer/group/B/switch and specify the topic where it expects any replies in its payload, like this:

{
    "data":"your message",
    "reply-to":"customer/group/A/segretreply-fromB-1345313"
}

That was just an example of JSON payload. You even can use timestamps, MACs or other unique sources to build the reply-to topic. Client A can subscribe to different reply-to topics for maintaining separate channels for every different client.

However, MQTT is not a one-to-one communication and each client can, potentially, subscribe to # and receive all messages. So, you'll need some sort of authorization in your broker to take a call on whether a client gets to publish/subscribe to a topic or not.

Give the following article a good read: MQTT Security Fundamentals: Authorization. You'll, then, be able to set up topics and authorization rules according to your requirements.