heroku-cluster.js

const os = require('os');
const throng = require('throng');
const cluster = require('cluster');

module.exports = ({defaultPort, app}) => {
  const port = process.env.PORT || defaultPort;

  const workers = process.env.WEB_CONCURRENCY ?
    parseInt(process.env.WEB_CONCURRENCY, 10) :
    os.cpus().length;

  throng({
    workers,
    master() {
      console.log(`Starting ${workers} workers`);
    },

    start() {
      app.listen(port, () => console.log(`Worker ${process.pid} listening on port ${port}`));
    },
  });
};