The RPi (main/broker) acts as the brains and hosts the MQTT broker. What happens is you run a command on Pi, the Pi does what it needs to do to process then sends a message via the broker to the sonoff(aux/client) which then responds. Think of the sonoff as a mini wireless board with 4 gpio...
I have a 10gal kettle. I'm currently in the middle of doing a 13# Belgian Tripel with 9.1gal of strike water and holding 1.2gal back. I'm right at the rim, so <12 and under is easily doable with your kettle and larger grain bills you can just hold a few litres back to add before boil.
Excuse...