In the last chapter we found restaurants near you. This one looks almost identical — find friends near you — and it is a completely different problem.

Restaurants do not move. A restaurant’s location is written once and read a billion times, which is why that design could precompute an index, cache it globally, and rebuild it overnight.

People move. Every user is emitting a new location every thirty seconds, and every one of those updates has to reach a few hundred other people right now. The index is obsolete before you finish building it.

The news feed we built last chapter can be seconds stale and nobody notices. Chat inverts every one of those assumptions.

A message that arrives thirty seconds late is a broken product. A message that arrives twice is a visible bug. A message that arrives out of order makes a conversation nonsensical — the reply shows up before the question. And unlike a feed, where the user pulls, the server must push, to a client that may be behind a firewall, on a train, or asleep.