System design interview question
"Design ad dayparting. Advertisers specify when their ads may run — weekday evenings only, or 9am to 5pm in the viewer's local timezone — and also set daily budgets that must be spent evenly rather than exhausted by 10am. Millions of campaigns, and the decision must be made in under a millisecond during the ad auction."
a hard real-time constraint combined with budget pacing, which is a control-loop problem. Two distinct hard parts: evaluating time-window eligibility across timezones in microseconds, and spending a budget smoothly when you cannot predict the day's traffic in advance and the feedback is delayed.
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