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 architecture models reported in [2][5], with a number of refinements to support abstraction and composition. The model is specifically designed to describe open distributed systems independent 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.