Zenoh (https://zenoh.io/) is a good alternative to DDS in many cases - it scales down to microcontrollers with most features still intact, but scales up to a routed hybrid mesh if needed.
It does move the schema enforcement out of the middleware layer, so it’s a bit different. But it’s now a first-class alternative middleware implementation for ROS2 as an alternative to DDS.
It does move the schema enforcement out of the middleware layer, so it’s a bit different. But it’s now a first-class alternative middleware implementation for ROS2 as an alternative to DDS.