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.

Autocomplete looks like a lookup. Type a prefix, return matching strings, sort by popularity. A LIKE 'tr%' query and an ORDER BY.

Two facts destroy that:

It runs on every keystroke. Not once per search — once per character. Typing “dinner” issues six requests. Across 10 million users that is roughly 24,000 queries per second for a feature nobody considers a feature.

The budget is about 100 milliseconds. Facebook’s typeahead team put the threshold there: slower and the suggestions visibly lag your typing, which feels worse than having none at all. That budget covers the network round trip, so the server has perhaps a few tens of milliseconds.