Adaptive distributed and fault-tolerant systems

An adaptive computing system is one that modifies its behavior based on changes in the environment. Since sites connected by a local-area network inherently have to deal with network congestion and the failure of other sites, distributed systems can be viewed as an important subclass of adaptive systems. As such, use of adaptive methods in this context has the same potential advantages of improved efficiency and structural simplicity as for adaptive systems in general. This paper describes a model for adaptive systems that can be applied in many scenarios arising in distributed and fault-tolerant systems. This model divides the adaptation process into three different phases-change detection, agreement, and action-that can be used to describe existing algorithms that deal with change, as well as to develop new adaptive algorithms. In addition to clarifying the logical structure of such algorithms, this model can also serve as a unifying implementation framework. Several adaptive algorithms are given as examples, including an adaptive network transmission protocol and a group membership protocol. A technique for implementing the model in a distributed system using small segments of code called micro-protocols and an event-driven execution paradigm is also presented.