Machine status for university dorm laundry rooms

Your company runs the laundry rooms in 150 university dorms, about 2,400 washers and dryers in all. Students pay for and start a machine in your app. Each machine has a small controller on the dorm Wi-Fi. It reports the machine's state every minute and whenever it changes: free, running with the minutes left, or done with the door still closed.

Before carrying a basket down several floors, students check the app. It shows each room's machines, no more than 15 seconds out of date. A student also gets a push notification when their machine is done.

About 20,000 loads run on a busy Sunday, and up to 3,000 students may have the app open at once.

How do the room view and the push notification get their data, and what does the server keep for each machine?