Architectures for adaptation systems

Modern systems need support for adaptation, typically responding to changes in system resources such as available network bandwidth. If an adaptation system is implemented strictly at the system layer, data adaptations can be added within the network or file system. This makes the adaptation system portable across applications, but sacrifices opportunities to change an application's behavior. It's not possible, for example, to first return a low-quality version of an image and later upgrade it should excess network capacity be available. On the flip side, the adaptation logic could be built into each and every application, with the system providing information to the applications in order to help them adapt their behavior. This becomes impractical because many applications will never be written to perform adaptation, and an application writer may not be able to foresee all possible adaptations that may be desirable. We argue that adaptation systems should be centralized, where they can make global observations about system usage and resource availability. We further argue that applications should not be written to perform adaptation. Instead, applications should support an interface where the adaptation system can dynamically modify an application's behavior as it runs.