System design interview question
"Design a toll system for a ride-hailing fleet. When a cab passes through a toll plaza, the toll must be detected, charged to the correct trip, and appear on the rider's fare. Hundreds of plazas, hundreds of thousands of cabs, and the detection signal is unreliable."
matching noisy real-world events to business records, plus financial correctness. This is a deceptively practical question: the hard parts are attribution (which trip was this vehicle on?), idempotency (the same toll must not be charged twice), and reconciliation against an external authority whose data arrives late and sometimes wrong.
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