Suppose you've built a web application and started getting a few customers. After some feedback and iterations, you are ready with a product that people want. From now, your application is drawing attention to more and more users, with more and more requests per minute (RPM). Suddenly, a wave of new people use your app, and at one point, they can't use it anymore. While testing your app, you realize that it's working correctly. So what happened? You are simply facing a scalability problem.