WhatsApp

Signature move

Millions of live connections, a tiny team

WhatsApp keeps every phone's connection held open rather than polling, so a message is pushed the instant it arrives — which means the front door manages millions of long-lived connections, not a churn of short requests. Built on Erlang, made for exactly this kind of massive concurrency, it famously served hundreds of millions of users with a remarkably small engineering team: it scaled by doing less, very carefully, rather than more.

Build it yourself

Now try building one of these architectures from a blank canvas — you're scored on the services you place and how you wire them.