Design Google's Advertising Auction Infrastructure — billions of bids per day, in milliseconds.

The real-time-bidding-at-enormous-scale problem: how billions of ad auctions per day are conducted within a millisecond budget, how targeting signals get applied at the moment of auction, how the billing system accurately tracks spend across billions of auctions, and how advertiser budget depletion is enforced in real time so no advertiser overspends.