Google Maps - the case study

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Before you start. A website that makes you wait for an answer is showing you latency - the time between asking and getting a reply. That’s the only idea this lesson needs already in your head; every other word it uses, it explains the moment that word shows up.

The idea in 30 seconds

Almost everything Google Maps shows you was worked out long before you tapped anything; answering you is mostly a lookup, not a live calculation.
A continent-sized road network doesn’t get searched fresh each time - shortcuts are built once, so crossing it becomes a handful of hops.
Roads barely change; travel times change by the minute. Only the numbers on the shortcuts get refreshed, not the shortcuts themselves.
The map picture you scroll through is a stack of pictures drawn once and handed to you, not painted live for your phone.
“Nearby” is cheap because the globe was pre-divided into small patches; you only ever check your own patch and its neighbours.
An arrival time is a prediction about the future, not a sum of the present - the one thing here that has to be worked out live, because the future can’t be precomputed.

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