is an event-based concurrent, objectoriented language speci cally designed for prototyping system architectures. Two principle design goals are 1 to provide constructs for de ning executable prototypes of architectures, and 2 to adopt an execution model in which t h e concurrency, synchronization, data ow, and timing properties of a prototype are explicitly represented. This paper describes the partially ordered event set poset execution model and outlines with examples some of the event-based features for de ning communication architectures and relationships between architectures. Various features of Rapide are illustrated by excerpts from a prototype of the X Open distributed transaction processing reference architecture. Keywords| Rapide, architecture de nition languages, partially ordered event sets, architecture, prototyping, concurrency, simulation, formal constraints, constraint-based speci cation, event patterns, causality.

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