Get the code here! https://gist.github.com/learncodeacademy/954568155105f4ff3599
Many Node.js developers don't know that Node by default only uses one core of your processor. It's intentional and it's a great move. It just means that you have to run cluster to get the full capacity out of your server.
When cluster.js runs, it spins up one copy (fork) of your application for each core on your machine, and balances the traffic between them. It also listens to each fork, so if one dies, it spins up a new one in it's place.
Clustering is a great way to add stability to your application and drastically increase it's load capacity.