How does EC2 reserve capacity to ensure availability?

The concept of Reserved Instances in Amazon EC2 is designed specifically to reserve capacity and ensure availability for users who commit to using instances over a longer term, generally one or three years. By purchasing Reserved Instances, customers are guaranteed the capacity they have reserved within a designated Availability Zone. This is particularly beneficial for workloads with predictable usage patterns, allowing organizations to better manage costs and ensure the necessary resources are available when needed.

When customers choose Reserved Instances, they essentially make a financial commitment to AWS, which not only gives them a significant discount compared to on-demand instance pricing but also assures them that the required capacity will be available when they launch their instances. This is particularly important for applications that cannot tolerate interruptions or require consistent performance levels.

The other options presented serve different functions. For instance, Spot Instances cater to a completely different model where users bid for unused capacity, but this does not guarantee availability, as the instances can be interrupted when AWS needs the capacity back. Running instances on-demand allows for flexible usage but doesn't ensure that the needed capacity will be available at peak times. Auto Scaling groups focus on dynamically adjusting capacity based on demand, rather than reserving it ahead of time. Each of these methods supports different scenarios, but only Reserved Instances specifically ensure capacity reserve and