As Web applications grow, we often worry about identifying and fixing performance bottlenecks — and then scaling further, as our applications become even more popular than before. MasterCard, a major international provider of credit cards, has to deal with a set of applications that need to run very quickly, identify fraud quickly and easily, and be extremely fault tolerant. In this talk, Ted Boehm, who works at MasterCard, describes what they have done to create, test, and maintain systems that need to be so reliable and performant. Even if you are unlikely to create an application as large as MasterCard’s, you will likely learn about scaling (and what not to do) from this talk.