Ride Matching Interview Questions

The core loop is: rider requests a trip, we find nearby idle drivers, offer, accept, then track. Drivers send GPS every few seconds. I would not scan all drivers. I bucket locations into geohash or S2 cells in Redis and query neighboring cells. Matching ranks by ETA, rating, and maybe destination. The dangerous bug is double-booking a driver. I would take a lock or compare-and-set on driver status so one trip wins. Trip is a state machine: requested, matched, en route, ongoing, completed. ETA comes from a routing service. Surge is demand versus supply per cell. I would say payments and fraud exist but I would not design the whole payments ledger in the first 20 minutes unless they ask. Consistency matters on 'one driver, one trip' more than on the last GPS ping.

I keep drivers in a geo index — geohash or a quadtree / Redis GEO. When a ride is requested I query a small radius, filter by availability and vehicle type, then offer to the best few. I expand radius if nobody accepts. I would not run SQL LIKE on lat long for every request. Surge and ETA are computed on that candidate set. The geo index is the heart of matching.

The offer must be transactional. I would hold the driver with a short lease in Redis or a compare-and-set on driver state. The second request sees the driver as busy and moves to the next. If the driver rejects, the lease expires and matching retries. I would not assign in memory without a lock. Double-assign is the classic race. I mention it before they ask.

ETA is a model: distance from map services plus traffic plus the driver's recent speed. I would cache map tiles and traffic, not call a third party on every UI poll. I would update ETA as the driver moves. I would tell the interviewer that ETA is approximate and that over-promising is a product bug. Matching nearby does not equal a good ETA if the driver is on a one-way flyover.

I compute demand versus supply in a geohash cell over a short window. Surge is a multiplier with a cap. I would show it before the user confirms. I would not change the price after they accept unless policy says so. I store the quoted multiplier with the trip. Internally I still need the geo demand counters. I would mention fairness and that I would not surge a stalled GPS glitch from one driver.