Hi,
Opening a coffee shop in Brooklyn NY, and I'm installing a commercial espresso machine, doing most of the plumbing myself.
Does anybody know proper installation on a backflow preventer for the machine? One told me that the backflow has to be above the machine, while another said it just has to be above where it drains into.

If anybody could shed some light that would be appreciated.
Thank you,
James Reina

Since you want to be compliant with local plumbing codes I'd highly recommend you find a LOCAL plumber friend who likes espresso and ask him.

Backflow preventers are usually installed on the supply side line so any pressure or even water that has passed into the machine doesn't bleed back out to the utility water line.
There are different regulations on drain lines all over the country, so please check with your local plumber or even the city planning should be able to tell you what is legally required for a drain. (Often the drain can't be a continuous feed from you to the drain in many cities. Sometimes you have to create a drain catch and have the machine drain hose sit in that catch, but I suppose a backflow preventer on the drain side is possible although with the particulates you'll flush I doubt it would be a good thing.)

Take anything I wrote with a grain of salt. I'm not a plumber; I only play one at home.

More than likely it will need to drain into an indirect drain (air gap) or a floor sink. Get the right info from the building department before you plumb it. Finding out it's not in compliance during pre-opening inspection can be costly.

Many if not most commercial and even prosumer espresso machines already have built in check valves.