What is the difference between horizontal and vertical scaling in Node.js?

Vertical scaling (scaling up) means increasing the resources of a single server — more CPU cores, more RAM, faster storage. For Node.js, this helps limited because a single Node.js process only uses one core. To utilize multiple cores, you must run multiple processes (cluster mode or PM2). Vertical scaling has a ceiling (you can only buy so much hardware) and involves downtime for hardware changes. Horizontal scaling (scaling out) means adding more servers/instances running the same application, distributing load with a load balancer (Nginx, HAProxy, AWS ALB). This provides near-unlimited scale and high availability — if one instance fails, others continue serving traffic. Node.js is well-suited for horizontal scaling because its stateless design works naturally with load balancing. Challenges of horizontal scaling: (1) Shared state: session data and caches must be in a shared external store (Redis) rather than process memory; (2) File uploads: uploaded files must go to shared storage (S3); (3) WebSocket affinity: sticky sessions or Redis pub/sub needed for Socket.IO; (4) Database bottleneck: scale the database too (read replicas, sharding). Design your app to be stateless from day one.