scale

A full-stack AI system design with scale, money, languages and angry customers. The interviewer wants tiers: cheap triage for everything, an agent where it pays, and humans where it matters, with the cost per day worked out.

At a billion requests a day, a fraction of a cent per call is millions of dollars a month and every millisecond is capacity. Principal answers start from the arithmetic and reshape the feature, the models and the serving to fit.