TLDR

1. How to use metrics to monitor your application’s health

2. The statistics of web requests is a long tail distribution rather than a normal distribution

i.e. you need to optimise to the right of your distribution rather than the left. The 95th percentile is actually a lot more important than the mean

http://www.confreaks.com/videos/2434-railsconf2013-monitoring-the-health-of-your-app