Node.js · Chapter 34 of 43
The Cluster Module
Node.js runs JavaScript on a single thread by default, meaning a CPU-heavy or busy process can't use multiple CPU cores directly. The `cluster` module solves this by forking multiple worker processes that share the same server port.
Each worker is a separate Node.js process with its own memory, but the OS load-balances incoming connections between them.
Forking workers
In the master process, `cluster.fork()` spawns worker processes, typically one per CPU core.
Handling worker crashes
Listening for the 'exit' event on workers lets you automatically restart crashed workers to keep the app resilient.
Example 1 (javascript)
const cluster = require('cluster');
const os = require('os');
if (cluster.isPrimary) {
for (let i = 0; i < os.cpus().length; i++) cluster.fork();
} else {
require('./server.js');
}Output
Worker 1 started
Worker 2 started
...The primary process forks one worker per CPU core; each runs the server independently.
Example 2 (javascript)
cluster.on('exit', (worker) => {
console.log('Worker died, restarting...');
cluster.fork();
});Output
Worker died, restarting...Listening for worker exits lets you automatically spawn a replacement.
Key points
- Node.js is single-threaded by default.
- cluster.fork() creates worker processes sharing one port.
- Workers utilize multiple CPU cores for better throughput.
- Restarting crashed workers improves resilience.
💡 Note: For most apps today, tools like PM2 manage clustering automatically without manual cluster module code.