Android : How to implement MQTT for one-to-one message distribution

I have been researching MQTT and I am curious how implementation works when trying to create a one-to-one communication system.

What I am trying to do: Set up a system that allows a IOT device to push notifications directly to its user & the user to push notifications to their IOT device via a MQTT broker.

Thoughts on implementation:

1. All devices have a separate topic that each user is subscribed to.


2. Each device publishes to the same topic with a tag that identifies the user to pass the message to.

Below is an image I created of how I see the implementation working.

More Info: I need an system that can scale to include 100k+ devices, but extremely low amount of messages(1 message a day)

Questions:

• How is the MQTT Broker implemented with regard to topics for one-to-one communication when large amounts of users are anticipated?


• Is there a limit on topics?


• What am I missing?