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.