How do you use the cluster module to scale Node.js across CPUs?

Question

Accepted Answer

**cluster** module creates child processes (workers) that share a server port.

**How it works:**
- Master process forks workers.
- Workers share the same server socket.
- Round-robin scheduling (default on Linux).

```javascript
const cluster = require("cluster");
const http = require("http");
const numCPUs = require("os").cpus().length;

if (cluster.isPrimary) {
    console.log(`Primary ${process.pid} is running`);
    for (let i = 0; i < numCPUs; i++) {
        cluster.fork();
    }
    cluster.on("exit", (worker) => {
        console.log(`Worker ${worker.process.pid} died`);
        cluster.fork(); // Restart
    });
} else {
    http.createServer((req, res) => {
        res.writeHead(200);
        res.end("Hello from worker " + process.pid);
    }).listen(8000);
    console.log(`Worker ${process.pid} started`);
}
```

**Key points:**
- Each worker is a separate process.
- Share port 8000.
- Worker crashes are handled by the primary.

Node.js: How do you use the cluster module to scale Node.js across CPUs?