UML23 — remove StructuredActivities::ActivityGroup

The fact that StructuredActivityNode specializes ExecutableNode but does not specialize Action in figure 12.21 is intentional. The StructuredActivities package is merged into L2, and at this level it is intended that “simple” structured nodes be usable on activities without being actions themselves, without pins, etc. This is to support a “procedural” style of the use of actions (along with the use of variables, sequence nodes and value pins).

It is only in CompleteStructuredActivites (figure 12.22) that StructuredActivityNode specializes Action. CompleteStructuredActivities is merged into L3. As a result, at this level, StructuredActivityNode ends up specializing both ExecutableNode and Action, even though this is redundant. But I don’t think it is avoidable if the structured nodes are to be kept as non-actions in StructuredActivities and L2.

Finally, the extra ActivityGroup element that appears on figure 12.21 seems to be completely irrelevant. ActivityGroup already has an association with Activity as originally defined in FundamentalActivities, so the merge increment doesn’t seem to add anything. And, as you note, StructuredActivityNode doesn’t even specialize it. My (unresearched) guess is that there was a change at some point to have StructuredActivityNode specialize FundamentalActivities::ActivityGroup rather than the StructuredActivities::ActivityGroup merge increment, and then the latter was just never removed (and remained unnoticed in its little corner of the diagram!).

So, I think the class description text is OK, but there should be an issue filed to remove StructuredActivities::ActivityGroup – it is probably more than should be done “editorially”. But this is not particularly urgent, since the extra ActivityGroup merge increment on figure 12.21 doesn’t actually hurt anything in the end.

See issue 15264 for disposition