This is an article by Netflix engineers about “auto scaling” with Amazon Web Services. Here’s a brief excerpt:
Since we began migrating our production infrastructure to the cloud in 2010, we have used Amazon's auto scaling groups to manage all of the server pools that we run. We believe auto scaling greatly improves the availability of our services and provides an excellent means of optimizing our cloud costs. With more than two years of experience using the auto scaling service, we thought it a good time to share why and how we use it, as well as some of the lessons that we have learned. For those who aren't familiar with Auto Scaling, it is an automation service Amazon provides as part of their cloud offering. Auto Scaling provides features to manage a running pool of servers, including the capability to replace failed instances and automatically grow and shrink the size of the pool. For a more thorough description, please see the Amazon Documentation.