There is disclosed a system for flattening hierarchically structured flows using a breadth-first approach. At each level of hierarchy of a hierarchically structured source flow, complex nodes are flattened by one level across the entire breadth of the flow. The results of this flattening are placed in a target flow, and any connections that existed in the source flow are re-established in the target flow in such a way that any data input into the target flow will be processed as if it had been input into the source flow. After a processing iteration, if there are still complex nodes remaining in the target flow, the target flow becomes the next source flow, and the process is repeated until the flow has been completely flattened.