WebSockets provide a persistent, full-duplex communication channel over a single TCP connection. After an HTTP upgrade handshake, both client and server can send messages at any time — no polling. Ideal for: chat, live collaboration, real-time dashboards, multiplayer games. Challenges: stateful connections complicate horizontal scaling (use sticky sessions or a pub/sub layer like Redis). Socket.IO adds fallback transports, rooms, namespaces, and reconnection on top of raw WebSockets.
Event-based messaging with Socket.IO rooms.
Without a Redis adapter, broadcasting to 'room-42' only reaches clients on the same server. Socket.IO Redis adapter or Pub/Sub adapter lets any server broadcast to any room globally.
The WebSocket handshake URL can include a token. But also validate the token in the first message after connect — it's easier to pass tokens in the message body than in query strings where they get logged.
WebSocket connections drop. Mobile clients move between networks, servers restart, load balancers time out. Clients must re-authenticate and re-subscribe to rooms on every reconnection. Server state (rooms, presence) must handle this.
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