UMLR — ControlNodes in ActivityPartitions

ControlNodes, like any other ActivityNode or ActivityEdge can be contained in an ActivityPartition. Containment in a partition can be designated either using swimlanes or by denoting the partition names in parenthesis above the ActivityNode name.

This is generally fine, but can result in significant layout problems when swimlanes are used to designate activity partitions. UML2 does not define any semantics for ControlNodes or ActivityEdges belonging to an ActivityPartition, so they generally do not belong to any partition. When swimlanes are used, the nodes that do not belong in any partition have to be displayed someplace. One convention is to create a default swimlane for these nodes that for example could appear as a partition with the same name as the activity. This results in extreme layout issues as all the control nodes end up being in the same partition resulting in very ugly diagrams with flow lines crossing partitions everywhere. If a user attempts to layout the diagram by moving the control nodes, this results in edits to the underlying model, something that the user may not have intended, or many not have permission to do. Some tools allow editing diagrams on read only models as long as the edits don't result in any changes to the model. This is especially common for diagrams that include views of other models in order to establish a context and show referenced elements.

Another problem is activity edges which cross many swimlanes while connecting their source and target. It is unclear from the notation in which of these partitions the edge should be included: none of them, all of them, or only the partitions at their connection points.

UML2 should consider a presentation option or notation convention that ControlNodes and ActivityEdges do not belong to ActivityPartitions unless explicitly shown using the partition name in parenthesis above the node or edge name. Putting a control node or edge in a swimlane should not imply that the node or edge is in the corresponding partitions. The notation should require that these element explicitly state the partitions they belong to.