System design interview question
"Design a subscription grocery delivery service. Customers subscribe to recurring deliveries — 2 litres of milk every morning, bread on alternate days, a weekly vegetable box. Deliveries are generated automatically, customers can pause or skip, and you must plan inventory and routes ahead of time. 5 million subscribers."
scheduled generation of future work at scale, plus the modification semantics that make recurring systems genuinely tricky — pausing, skipping one occurrence, changing quantity from next week, and vacation holds. The distinguishing insight is that you must materialize orders ahead of time for inventory and routing, but customers can change their mind after materialization.
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