Business processes often requires to execute a task multiple time in series or in parallel. In some workflow management systems this possibility is already supported and called “multiple instantiation”. Usually the term “iteration” is used to define multiple executions in series. Nevertheless, the existing solutions impose many constraints for workflow designers and decrease flexibility. Almost all of them use new operators to represent multiple instances that are integrated in the workflow as any other workflow basic operators. This way of representation encumbers and complicates the workflow so that it's unreadable for the end user. In this article, we propose a new way of defining multiple instantiations in a workflow without using exotic operators, nor complicating the workflow itself. Our approach is based on defining sets of tasks in a dynamic workflow process. Each set contains activities that must be executed multiple times. Each set is governed by constraints making it possible to supervise the multiple executions. These sets can be nested or even overlap. We use two types of sets in this work: “parallel instance's set” for those activities that are executed multiple times in parallel. And the second type is “iterative instance's set” for those that are executed multiple times in sequence. The number of instantiations to do and the condition to iterate could be evaluated at run-time. In this paper, we also show on a real process executed in an experience how this model could have been used to ease its definition.

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