Google Maps has about a billion daily active users, covers 99% of the world, and takes in something like 25 million updates a day.

We’re going to build a simplified version. Three features:

1. Location updates — the client reporting where you are
2. Navigation — a route from A to B, with an ETA
3. Map rendering — the actual map on your screen

Each one turns out to be a different kind of problem. Rendering is a storage and CDN economics problem. Navigation is a graph algorithms problem, and the graph is far too large to hold in memory. Location updates are a write throughput problem — a million per second at peak.

Every previous chapter optimised for latency, throughput or correctness. This one optimises for money, and that changes which answers are right.

Do the estimate before anything else. Five million daily users watching five videos of 300 MB each, served from a CDN at roughly $0.02/GB:

5,000,000 users x 5 videos x 0.3 GB x $0.02 = $150,000 per day

Fifty-five million dollars a year, in bandwidth alone. No database, no compute, no salaries — just moving bytes to viewers. That single number outweighs every other cost in the system, and it means a design that is elegant but bandwidth-hungry is simply a worse design.