Mock interview: the Noon Drop at Cartwala

A full 50-minute model interview: Arjun splits browsing a price from charging one, proves with numbers that flushing the cache at 12:00 can't work, stores each product's price timeline in the cache so the flip needs no invalidation, pins the deal of the day in every pod's memory, survives a Redis shard dying mid-sale, re-prices every order from the primary through a pricing mistake, and keeps a poisoned price queue from ever causing a wrong charge.