Scaling Node.js: Horizontal vs. Vertical and State Management

Scale vertically using the cluster module (multi-core) and horizontally using a Load Balancer (multi-server); move all state (sessions, cache) to an external store like Redis.

To scale a Node.js app effectively, it must be stateless. Vertical scaling (adding CPUs) is limited by a single machine's capacity. Horizontal scaling involves spinning up containers (K8s/Docker) behind a Load Balancer (Nginx/AWS ALB). The critical architectural shift is ensuring that if Instance A handles login, Instance B can handle the subsequent request. This requires a shared session store (Redis) and shared file storage (S3).