A command is an event, not a state. And it is absolutely possible to share events across the instances of OH. I’m doing it now with my two OH instances that are federated using an MQTT event bus. The only gotcha is commands can only go one direction, from the “primary” to the “secondary”. Updates (not just changes) flow from the “secondary” to the “primary”.
This is not theoretical stuff. It’s been working since shortly after the release of MQTT 2.4.
HABApp is not openHAB native so until it comes with OH or can be installed as an add-on it can never meet 1. HABApp iteself requires some configuration. The MQTT requires some configuration. That already makes it require more configuration than the MQTT version that I’m trying to replace with something simpler already.
Me too, and mine works both ways Marketplace MQTT Event Bus and has both a Jython and Rules DSL version posted. I plan on doing a JavaScript Rule Template as well so users can import it through PaperUI on the OH 2.x side and the replacement to PaperUI on the OH 3 side without needing to install anything else but set up MQTT.
But a number of people lost their s#@t because “MQTT is too complicated.” I had to add about 2000 words to that post to cover the MQTT basics. So any dependency on MQTT is a non-starter. Any approach that requires anything like manually configuring MQTT Broker Things and Generic MQTT Things is going to fail 2.
One of the justifications for dropping 1.x bindings in OH 3 was the ability to federate openHAB instances. Kai specifically pointed to that link above as the justification. But it is clear too that for some users any dependency on MQTT will put the capability completely out of reach.
It is also why I have 1. as a requirement. This has to be something a user can install, set up, and configure all from the PaperUI and the OH 3 user interface with no other external dependencies, particularly external dependencies that are not a part of the openHAB project itself.
Can it be done using HABApp? Obviously. But if the exact same capability written in native OH rules is already too complicated, HABApp is never going to be viable to solve this particular problem.