Cab Booking System

Start by separating the flow: driver availability and location updates, rider request, candidate lookup, atomic assignment, then trip lifecycle. Store current driver state separately from an append-only trip/event record; query nearby available drivers through a geospatial index or cell buckets, then rank by ETA and constraints. The critical race is two riders selecting one driver, so make the transition AVAILABLE to RESERVED conditional on its version/status and retry on conflict. Specify idempotency keys for request retries, expiration for unanswered offers, and an outbox/event stream for notifications. Ask whether the goal is nearest driver, lowest ETA, or fairness, because that changes ranking. Candidate retrieval is approximately O(log n + k); assignment must be atomic, not a read-then-write sequence.