The Other Envelope: Cost Estimation

1. The Second Envelope

Every design discussion computes a performance envelope: requests per second, storage, latency. Very few compute the other one, which is what the design costs to run. Leaving it out is the single most reliable difference between a mid-level answer and a senior one, in interviews and in real design reviews alike.

Cost is a design constraint of exactly the same kind as latency. A design that meets every performance target and costs more than the product earns is not a working design