In most home automation scenarios you usually have a (locally installed) controller software where you act on incoming data from the sensors deployed in your home. With "act" I mean everything from logging data (for graphing) to automatically send out commands to actors. I.e. If movement is detected (by a motion sensor) in a room after sunset then turn on ceiling light for two minutes. So... the controller holds all the "rules" in your building automation setup. You can have a very small MySensors setup where sensors/actors send messages between each other and the "rules" is hardcoded in the nodes. But this will soon get very messy to maintain. You'd need at least one Relay node to get this working (with hard-coded id's in the nodes). The controllers can often run headless on simple hardware like Raspberry. Check out our list of supported controllers.