node/doc/api/cluster.markdown

Cluster

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

The cluster module allows you to easily create a network of processes all which share server ports.

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

if (cluster.isMaster) {
  // Fork workers.
  for (var i = 0; i < numCPUs; i++) {
    cluster.fork();
  }

  cluster.on('death', function(worker) {
    console.log('worker ' + worker.pid + ' died');
  });
} else {
  // Worker processes have a http server.
  http.Server(function(req, res) {
    res.writeHead(200);
    res.end("hello world\n");
  }).listen(8000);
}

Running node will now share port 8000 between the workers:

% node server.js
Worker 2438 online
Worker 2437 online

cluster.fork()

Spawn a new worker process. This can only be called from the master process.

cluster.isMaster

cluster.isWorker

Boolean flags to determine if the current process is a master or a worker process in a cluster. A process isMaster if process.env.NODE_WORKER_ID is undefined.

cluster.eachWorker(cb)

Synchronously iterates over all of the workers.

cluster.eachWorker(function(worker) {
  console.log("worker pid=" + worker.pid);
});

cluster.workerCount()

Returns the number of workers.

Event: 'death'

When any of the workers die the cluster module will emit the 'death' event. This can be used to restart the worker by calling fork() again.

cluster.on('death', function(worker) {
  console.log('worker ' + worker.pid + ' died. restart...');
  cluster.fork();
});

Different techniques can be used to restart the worker depending on the application.