Email is the oldest system in this series by decades. SMTP was specified in 1982. POP and IMAP followed. Those protocols still carry the world’s mail, and they were designed for an internet of a few thousand machines where you downloaded your messages and the server forgot them.

Now Gmail has over 1.8 billion users.

This chapter is about what happens when you keep the interface and replace everything behind it. And it produces the largest numbers we’ve seen — by a wide margin.

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.

Amazon DynamoDB stores hundreds of trillions of items and handles tens of millions of requests per second at peak. Netflix uses it to keep track of what you were watching. Airbnb uses it for availability calendars. Discord uses it for message storage.

What do all of these have in common? They all need to store and retrieve data by a simple key — blazingly fast, at global scale, with near-zero downtime. That’s what a key-value store does.

Imagine you are the infrastructure engineer at a hot social media platform. Your system is humming along with 4 cache servers, each holding about 25% of your data. Life is good.

Then your platform goes viral overnight. You urgently add a 5th server. You restart everything. And suddenly, your database is on fire — every single cache server is getting a tsunami of cache misses. Users experience 10× slower page loads. The whole site is crawling.