System design interview question
"Design the system that counts ad clicks. Advertisers are billed per click and they check near-real-time dashboards, so the numbers must be correct and they must be fresh. A million clicks per second, and clicks arrive late, duplicated, and fraudulent."
stream aggregation where the output is money. It is the sharpest version of the "exactly-once counting" problem, because a miscount is a billing error rather than a cosmetic one. The distinguishing content is: handling late-arriving events without either dropping them or corrupting already-published numbers, deduplication at a million events per second, and reconciling a fast approximate path with a correct one.
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