FUML13 — Input parameter activity nodes should fire first

Consider an activity with one in parameter having multiplicity lower bound of 0. Suppose this activity contains a top-level structured activity node that, in turn, contains an action with an input pin, also having multiplicity lower bound of 0, and an object flow from the activity parameter node for the in parameter to the action input pin. Assume the structured activity node and the action it contains have no other data or control dependencies.

According to the logic specified in ActivityNodeActivationGroup::run, both the activity parameter node and the structured activity node will be enabled when the enclosing activity is executed (because the structured activity node has no incoming edges, even though the action inside it does). The activity parameter node and the structured activity node therefore fire concurrently. Thus, one allowable execution sequence would be for the structured activity node to fire first, resulting in the firing of its contained action, before the activity parameter node fires. Since the activity parameter node has not fired, it does not yet contain any value that might be on the in parameter, but the action will fire anyway, because its input pin has multiplicity lower bound of 0.

The fact that it is allowable for the action in this example to fire with no input, even if there is an input available on the in parameter, would seem to not be what most modelers would expect in this situation. To avoid this, it would be better if, when an activity is run, any input activity parameter nodes are fired first, before any other enabled activity nodes. Note that this is parallel to the case of a structured activity node, in which any input pins of the structure activity node fire before any nodes within the structure activity node fire.

Update ActivityNodeActivationGroup::run

Agreed.