How would you scale a Node.js application across CPU cores, and what is the cluster module?

A single Node process uses one core for JavaScript, so on an eight-core machine you are using an eighth of the hardware. There are three answers, and the best one depends on where you deploy.

• The cluster module forks one worker process per core. The primary process holds the listening socket and distributes incoming connections; the workers each run your full application. Because they are separate processes they share nothing, so in-memory state and sessions must move to Redis or a database.
• A process manager such as PM2 in cluster mode does the same thing with restarts, zero-downtime reloads, and monitoring included.
• Container replicas. In Kubernetes or ECS the usual approach is one Node process per container and let the orchestrator run several containers — it gives you the same parallelism plus scheduling and health checks.

Worker threads are a different tool. Cluster scales I/O-bound work across processes; worker_threads moves a CPU-bound task off the main thread within one process, and can share memory through SharedArrayBuffer.

Note: Say explicitly that clustering does not fix a blocked event loop. If one request spends 500ms hashing, clustering just gives you eight event loops to block.