• Scale-out is more cost efficient than scale-up
  • Persist with filesystems only read-only data like logging and backups

Scale-out with App Clusters

  • single-server form of scaling out
  • create on independent server for each core (CPU)
  • don't create more than one cluster per core otherwise the extra servers will be useless

The Pros

  • Maximize server performance (hardware or virtual machine)
  • low overhead to test your app under parallel conditions
// Web development with Node & Express pg. 134
var cluster = require('cluster');

function startWorker() {
  var worker = cluster.fork();
  console.log('CLUSTER: Worker %d started',;

if (cluster.isMaster) {
  require('os').cpus().forEach(() => {

  // log any workers that disconnect; if a worker disconnects, it
  // should then exit so we'll wait for the exit event to spawn
  // a new worker to replace it
  cluster.on('disconnect', (worker) => {
    console.log('CLUSTER: WOrker % disconnected from the cluster.',;

  // when a worker dies (exits), create a worker to replace it
  cluster.on('exit', (worker, code, signal) {
    console.log('CLUSTER: Worker %d died with exit code %d (%s)',, code, signal);
} else {
    // start our app on worker;
  • when a script is run directly, require.main === module will be true.
  • if false, it means your script has been loaded from another script using require.
  • cluster
  • virtual machines default to a single core.

Scaling Out with Multiple Servers

  • To scale out you need a proxy server (often called a reverse proxy or forward-facing proxy.)
  • Most popular: Nginx and HAProxy
  • Dev proxies: proxy and node-http-proxy

Working with proxies

  • if you use a proxy server, make sure to tell Express to trust it with app.enable('trust proxy');
  • this will ensure that req.ip, req.protocol, and reflect details about the connection client <> proxy, and not client<>server
  • req.ips will be an array of ips with original client and the names or IP addresses of intermediate proxies

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