Tinder HLD: Mutual Matches, a Swipe Firehose, and a Fresh Deck

"Design Tinder." The swipe is one tap, but two things behind it are the design. First, a match is a mutual like — and when two people swipe right on each other at nearly the same instant, exactly one "It's a match!" must fire, never two and never zero. Second, the deck — the stack of profiles you swipe — must be fresh, nearby, relevant, and never repeat a face, drawn from a pool of millions. Around those sits a firehose: billions of swipes a day, almost all of them one-sided likes that must stay private until reciprocated. Two ideas carry it: reciprocity detection (a like becomes a match only …