Scale

• 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', worker.id);
}
if (cluster.isMaster) {
  require('os').cpus().forEach(() => {
    startWorker();
  });
  // 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.', worker.id);
  });
  // 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)', worker.id, code, signal);
    startWorker();
  });
} else {
    // start our app on worker;
    require('./meadowlark.js')();
}

• 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 req.secure 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