eval $([ -f .env ] && cat .env) node cluster.js

cluster.js

var cluster = require('cluster')
  , express = require('express')
  , app = express()
  , cpus = require('os').cpus().length;

app.get('/', function(req, res, next){
  res.json({ "env":process.env.NODE_ENV || "development", "pid":cluster.worker.process.pid });
});

if (cluster.isMaster) {
  for (var i = 0; i < cpus; i++) {
    cluster.fork();
  }
  cluster.on('exit', function(worker, code, signal) {
    console.log('worker %s died', worker.process.pid);
    cluster.fork();
  });
} else {
  app.listen(process.env.PORT || 3000, function(){
    console.log("port: %s | pid: %s", process.env.PORT || 3000, cluster.worker.process.pid);
  });
}