Here's a short video about my cloudless, portable, small, low-resource "smart thermostat". It doesn't need an internet connection and uses MQTT. Here, it's directly driving a relay. 
It's running on a Raspberry Pi Zero W, powered by NetBSD, in read-only mode. 
I used it for years and it's time to go back to it, cloudless and local.

 
 
 ʷᵃʳᶦ 