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SUMMARYTrends in networking and distributed computing are creating a new generation of applications that must adapt as the environment within which they execute changes. Examples of adaptation include switching protocols to overcome a security exposure or failure mode seen only in certain settings, changing data rates to accommodate a slow link, or adapting the behavior of a high level application to match the set of participants using the application. We describe the Ensemble system, a tool for building adaptive distributed programs.

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The Horus system supports a communication architecture that treats protocols as instances of an abstract data type. This approach encourages developers to partition complex protocols into simple microprotocols, each of which is implemented by a protocol layer. Protocol layers can be stacked on top of each other in a variety of ways, at run-time.First, we describe the classes of protocols that can be supported this way.Next, we present the Horus object model that we designed for this technology, and the interface between the layers that makes it all work.We then present an example layer that implements a group membership protocol. Next, we show how, given a set of required properties, an appropriate stack can be constructed. We look at an example stack of protocols, which provides fault-tolerant, totally ordered communication between a group of processes. The work contributes a standard framework for protocol development and experimentation, provides a high performance implementation of the virtual synchrony model, and introduces a methodology for increasing the robustness of the protocol development process.