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.