Describing Open Distributed Systems: A Foundation

In this paper we outline a semantic model for open distributed systems which provides a foundation for a corresponding architecture description language. This semantic model is based on reported architecture models, with a number of refinements to support abstraction and composition. The model is specifically designed to describe open distributed systems independently of implementation details such as communication protocols and middleware systems. The modelling concepts in the semantic model are: object (a model of an entity), event (a unit of interaction between an object and its environment), event relationship (a specification of behaviour defining the relationships amongst a set of events), interface (an abstraction of an object's interaction with its environment) and binding (a context for interaction between objects). The binding concept is particularly important because it can describe any kind of interaction in an open distributed system, ranging from remote procedure calls and multicast to more complex, enterprise interactions. Special attention is given to the problem of composition and abstraction of events and behaviour in the model. This is needed to reflect the reuse, evolution and interworking requirements of open distributed systems. Our approach allows for the effective modelling of asynchrony, concurrency and complex flows of information in open distributed systems. We consider a system with the infrastructure for the creation and interconnection of large numbers of distributed persistent objects. This system is exemplified by the Internet: potentially, every appliance and document on the Internet has both persistent state and the ability to interact with large numbers of other appliances and documents on the Internet. This paper elucidates the characteristics of such a system, and proposes the compositional requirements of its corresponding infrastructure. We explore the problems of specifying, composing, reasoning about and implementing applications in such a system. A specific concern of our research is developing the infrastructure to support structuring distributed applications by using sequential, choice and parallel composition, in the anarchic environment where application compositions may be unforeseeable and interactions may be unknown prior to actually occurring. The structuring concepts discussed are relevant to a wide range of distributed applications; our implementation is illustrated with collaborative Java processes interacting over the Internet, but the methodology provided can be applied independent of specific platforms.