Routes to relay control

I'm investigating the possibility of getting EMONCMS to control relays both by clicking a button on the dashboard and also via a timer. I've been looking through both building blocks and modules and I'm unsure what the routes are to controlling a relay. Obviously I need the relay and board which I'll have to make but what else do I need to control them? Currently I've got a Pi2 with local EMONCMS and EMONTX V3.4.

Can someone please advise what the routes are to controlling a relay board / module?

For that matter does it make more sense for the relay board to be connected to the Pi2?