System design interview question
"Design shipment tracking for a logistics company. A parcel moves through pickup, several sorting hubs, line-haul trucks, a local depot, and finally a delivery van. Scans happen at each point, customers check status constantly, and you must predict a delivery date. 10 million parcels in transit at any time."
event-sourced state over a physical network, with an unusually large read:write asymmetry — a parcel generates perhaps ten scan events in its life but is checked dozens of times. The distinguishing content is: scans arrive late and out of order from handheld devices, "where is it" must be answered from an event history, and the ETA is a prediction over a graph of hubs, not a straight-line estimate.
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