Developing wide-area distributed applications requires jointly analyzing functional and Quality of Service (QoS) aspects, such as timing properties. Labelled transition systems and sequential trace semantics - the common semantic domains - do not facilitate this kind of analysis because they do not precisely express the causal relationships between events. Asymmetric Event Structures (AES) provide an explicit representation of the causal dependencies between events in the execution of a system and allow for an elegant coding of preemption. Event structures are, however, difficult to construct compositionally, because they cannot easily represent fragments of a computation. The heaps we develop here allow for such a representation, and easily generate AES. In this paper, we develop a partial-order semantics in terms of heaps, for Orc, an orchestration language used to describe distributed computations over the internet. We briefly show how Orc, and this new semantics, are used for QoS studies of wide area orchestrations.
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