Chat: ten million open sockets, and two ticks

One strong interview answer to "design a chat system": stateful WebSocket gateways in front of a stateless chat service, a per-conversation sequence number that orders every device, a sent tick only after the message is stored, client message IDs that make retries harmless, a session registry that finds Bob's gateways, catch-up by sequence, and how to restart 200 stateful servers without a reconnect storm.