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

A Tinder system design: mutual-match detection that fires exactly once, the recommendation deck from a geospatial index, the swipe write firehose, privacy of one-sided likes, and scaling.

By fiveyearsdevJune 14, 202615 min read

"Design Tinder." The swipe is one tap — and that one tap hides the two hardest problems in the room. First, a match is a mutual like, so when two people swipe right on each other in the same instant, exactly one "It's a match!" must fire — never two, never zero, and the two likes may not even sit on the same machine. Second, the deck you thumb through must be fresh, nearby, relevant, and never show a face twice, drawn live from a pool of millions. Wrapped around both is a firehose: billions of swipes a day, almost all of them one-sided likes that must stay secret until the other person likes b…

What’s inside

Let's start nowhere near a computer

Where this exact shape shows up

Step 1 — Functional requirements (sentences first)

Step 2 — Non-functional requirements

Step 3 — Nouns and the data model

Step 4 — The match engine

Step 5 — The deck: who to show next

Step 6 — Trade-offs (each one keeping an NFR)

The complete implementation

Step 7 — Only now, the boxes

Step 8 — Scaling, one bottleneck at a time

Step 9 — When a piece fails: designing for failure

The interview corner

Where to go from here

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