Real-Time Delivery: Polling → SSE → WebSockets

"Make it real-time" hides a ladder of escalating tradeoffs. Climb it: short polling, long polling, Server-Sent Events, and WebSockets — when each is the right tool, what they cost to scale, and the operational details that bite (reconnection with backoff, message ordering, missed-message recovery, presence, fan-out to thousands of clients, and proxy/load-balancer behavior). Grounded in the real browser APIs (EventSource, WebSocket) and Node server patterns.