So you've done a web app, you tested it, deployed it, and your first customers begin to use your product and share it to their friends. That's great! Now, how do you know how are they using your application? Are you sure of the impacts of your deployments? What if you doubled the response time by fixing a bug and didn't see it?
In order to watch your application behavior, you need real-time metrics.
What are exactly real-time metrics? What can you use to store and watch them? What are exactly RRDtool, Graphite, StatsD? Can we improve existing solutions?