Grid workflow applications are emerging as one of the most interesting application classes for the Grid. In this paper we present A-GWL, a novel Grid workflow language to describe the workflow of Grid applications at a high level of abstraction. A-GWL has been designed to allow the user to concentrate on describing scientific Grid applications. The user is shielded from details of the underlying Grid infrastructure. A-GWL is XML-based which defines a graph of activities that refers to computational tasks or user interactions. Activities are connected by control- and data-flow links. We have defined A-GWL to support the user in orchestrating Grid workflow applications through a rich set of constructs including sequence of activities, sub-activities, control-flow mechanisms (sequential flow, exclusive choice, and sequential loops), data-flow mechanisms (input/output ports), and data repositories. Moreover, our work differs from most existing Grid workflow languages by advanced workflow constructs such as parallel execution of activities with pre- and post-conditions, parallel loops, event-based synchronization mechanisms, and property-based selection of activities. In addition, the user can specify high-level constraints and properties for activities and data-flow links.
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