Rider-to-driver matching

Design the rider-to-driver matching service for a ride-hailing platform operating in 200 cities, peaking at 30k ride requests/second globally with ~3M drivers broadcasting GPS updates every 4 seconds. A request must return up to 10 candidate drivers within ~2km in under 150ms p99, and a driver must never be offered to two riders at once. Walk through the geo-index, the dispatch loop, and how you keep consistency under concurrent matches.