DDS and MQTT are two commonly used technologies in IoT, Robotics, V2X, Aerospace, etc., that take diametrically opposite approaches to implement the pub/sub abstraction. DDS embraces a peer-to-peer model, in which applications communicate directly with each other. On the other hand, MQTT adopts a hub-and-spoke architecture in which a central broker mediates communication between applications. Both technologies have shown scale-up/down and scale-out limitations. This presentation will explain the challenges faced when trying to scale-out or scale-down DDS and MQTT in various application domains, including Robotics, IoT, and V2X, and illustrate how Eclipse Zenoh can be used, w/o any changes to your existing applications to overcome these challenges.