Use the Socket.io Redis adapter for multi-instance broadcasting, PM2 cluster mode for multiple Node.js workers, Nginx with ip_hash for sticky WebSocket sessions, MongoDB compound indexes for efficient message queries, and Redis for caching online status and unread counts.
Backend: Message model (MongoDB), Socket.io server, chat REST API, socket events (message:send, message:received, typing), JWT auth, room management. Frontend: chat list, chat window, message input, typing indicator, online status, unread badge.
User A sends a message via Socket.io emit, server saves to MongoDB, server broadcasts to the room, User B receives instantly via Socket.io. If User B is offline, server sends an email notification. Message history is loaded via REST API when opening a chat.
Both. Use REST for loading message history (GET /chat/:userId with pagination) and chat list. Use Socket.io for real-time events (new messages, typing indicators, online status). REST handles initial load, Socket.io handles live updates.
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