Event Structure Semantics of Orc

One challenge in developing wide-area distributed applications is analyzing the system's non-functional properties, including timing constraints and internal dependencies that can affect quality of service. Analysis of non-functional properties requires a precise formal semantics for the language in which the system is written; but labelled transition systems and trace semantics, which are commonly used for this purpose, do not facilitate this kind of analysis. Event structures provide an explicit representation of the the causal dependencies between events in the execution of a system. But event structures are difficult to construct compositionally, because they cannot easily represent fragments of a computation. In this paper we present a partial-order semantics based on heaps (an explicitly encoded form of occurrence nets with read arcs), which naturally represent fragments of behavior. Heaps are then easily translated into asymmetric event structures. The semantics is developed for Orc, an orchestration language in which concurrent services are invoked to achieve a goal while managing time-outs, exceptions, and priority. Orc, and this new semantics, are being used to study quality of service (QoS) for wide area orchestrations