Abstract
Creol is a high-level, object-oriented language for distributed systems, featuring active objects and asynchronous method calls. In this paper we present a behavioral interface speci cation language over communication trace labels to specify components in terms of traces of observable behavior. In the speci cation, a clean separation of concerns between interaction under the control of the component or coming from the environment is central, which leads to an assumption-commitment style description of a component's behavior. The assumptions schedule the order of inputs, whereas the outputs as commitments are being tested for conformance. To ensure the mentioned separation of responsibilities, we de ne wellformedness conditions which in addition assure that only \meaningful" traces, i.e., those corresponding to actual behavior, can be speci ed. The speci cation language is characterized by two other salient features: it allows to specify freshness of communicated values and furthermore, it respects the asynchronous nature of communication in Creol: the output is tested only up-to an appropriate notion of observability.