Hundreds of millions of persistent connections, the Signal protocol's X3DH and Double Ratchet, per-user inbox queues, sender keys for groups, and multi-device sync without the server reading anything.
Online Chess
Sub-100ms move sync over binary WebSockets, authoritative bitboard validation, clock fairness under network lag, Glicko matchmaking, engine-assistance detection, and spectating a million-viewer game.
LeetCode
Sandboxing hostile code with microVMs and seccomp, absorbing contest submission spikes, deterministic time and memory measurement with CPU pinning, and real-time contest leaderboards.