Cluster Module

A single instance of Node.js runs in a single thread. To take advantage of multi-core systems, the user will sometimes want to launch a cluster of Node.js processes to handle the load.

The cluster module allows easy creation of child processes that all share server ports.

const cluster = require('cluster');
const http = require('http');
const numCPUs = require('os').cpus().length;

if (cluster.isMaster) {
    console.log(`Master ${process.pid} is running`);

    // Fork workers. (Spawn a new worker process)
    for (let i = 0; i < numCPUs; i++) {
        cluster.fork();
    }

    cluster.on('exit', (worker, code, signal) => {
        console.log(`worker ${worker.process.pid} died`);
        cluster.fork();
    });

} else {
    // Workers can share any TCP connection
    // In this case it is an HTTP server
    http.createServer((req, res) => {
        res.writeHead(200);
        res.end('hello world\n');
    }).listen(8000);

    console.log(`Worker ${process.pid} started`);
}

Cluster VS Worker Threads

Cluster (multi-processing):

One process is launched on each CPU and can communicate via IPC.
Each process has it's own memory with it's own Node (v8) instance. Creating tons of them may create memory issues.
Great for spawning many HTTP servers that share the same port b/c the master main process will multiplex the requests to the child processes.

Worker Threads (multi-threading):

One process total
Creates multiple threads with each thread having one Node instance (one event loop, one JS engine). Most Node API's are available to each thread except a few. So essentially Node is embedding itself and creating a new thread.
Shares memory with other threads (e.g. SharedArrayBuffer)
Great for CPU intensive tasks like processing data or accessing the file system. Because NodeJS is single threaded, synchronous tasks can be made more efficient with workers

1.9 KiB Raw Blame History

Cluster Module

Cluster VS Worker Threads

1.9 KiB

Raw Blame History