As IoT networks continue to grow, they are becoming increasingly heterogeneous. Different types of sensors, actors, user clients, and services driven by different programming platforms are gradually added to an existing installation. Also, installed components have to be replaced by improved solutions. These kinds of environments need a communication protocol to handle the data flow. MQTT is an extremely lightweight publish/subscribe M2M/IoT connectivity protocol. This session is about a complete MQTT-driven IoT stack with several sensors, remote-controlled switches, and different client platforms (desktop, embedded and mobile devices). In the demo part, I will show an example of a seamless replacement of a Raspberry Pi-driven sensor host by a low-energy Microcontroller-based solution without any needs for code changes. Session schedule: - Introduction of the IoT network architecture and the involved components - MQTT protocol stack in a nutshell - Introducing MQTT.fx, a graphical tool for testing, simulating, and debugging MQTT communications - Live demo