websocket-design

Use the @socket.io/redis-adapter package with Redis pub/sub to synchronize messages across server instances. Clients connected to different servers receive broadcasts because events are published through shared Redis channels per room or user.

Socket.IO provides automatic reconnection, long-polling fallback for older browsers, and built-in rooms and namespaces. Native WebSocket is simpler with lower overhead but requires manual reconnection and offers no fallback for unsupported browsers.

Pass the token in socket.handshake.auth and verify it in Socket.IO middleware using io.use() before allowing the connection. Reject invalid tokens by calling next() with an error, and attach the user ID to the socket for later authorization checks.

Enable reconnection with exponential backoff (reconnectionDelay, reconnectionDelayMax) and on reconnect, resubscribe to rooms and request missed messages using a sync event with the last received timestamp. Queue outgoing messages client-side while disconnected.

Track active connections with a gauge, message throughput with a counter labeled by type, and message latency with a histogram (p50, p95, p99). Also monitor reconnection rate and error rate to detect instability or abuse.