worker_threads vs cluster vs child_process — when do you use each?

Answer: All three add parallelism, but for different needs.

worker_threads — real threads within a single process:

• Each has its own V8 isolate and event loop, but they can share memory via SharedArrayBuffer and pass messages.
• Best for CPU-bound JavaScript (parsing, compression, ML inference) without spinning up whole processes.

const { Worker } = require('worker_threads');
const w = new Worker('./cpu-task.js', { workerData: input });
w.on('message', result => { /* ... */ });

cluster — forks multiple Node processes (workers) that share a listening socket:

• Lets an I/O-bound server use all CPU cores (one process per core), since a single process only uses one core for JS.
• Processes are isolated (no shared memory); the OS/round-robin load-balances connections.
• In production this is often handled by a process manager (PM2) or the platform (containers behind a load balancer).

const cluster = require('cluster');
if (cluster.isPrimary) {
  for (let i = 0; i < os.cpus().length; i++) cluster.fork();
} else {
  http.createServer(handler).listen(3000);
}

child_process — spawn any external program or another Node script:

• spawn (streams, long-running), exec (buffered, shell commands), fork (a Node child with an IPC channel).

const { spawn } = require('child_process');
spawn('ffmpeg', ['-i', 'in.mp4', 'out.webm']);

Choosing: