Before we move on and take a look at how we can implement highly available virtual machines that are protected in the event of a disaster in the Microsoft Azure cloud, we need to briefly discuss the application design and tiers that are found in typical application designs. Most systems and applications are divided into multiple tiers with each tier performing a different role in the system. These tiers allow you to deliver an available, scalable, fault-tolerant system to your business. As we've discussed, this is the objective of any highly available system, including cloud-based ones. Here, I'm going to look at the three basic tiers of simple web applications, which are as follows: