UML,merge "decision node" and "merge node"

Question

In UML,is it legal to use merge node at the same time as merge node and decision node?It seems like this:

I think it may help to avoid complex diagrams.If that is not possible,why it is illegal?

Answer 1

Yes, it is legal

Evidence:

1. chapter "Merge and decision combined" in uml-diagrams.org: UML Activity Diagrams Reference
2. "Figure 15.34 Combined MergeNode/DecisionNode notation" in www.omg.org/spec/UML/2.5/Beta2
3. picture "Decision node and merge node used together, sharing the same symbol" in "Figure 12.76 - Decision node notation" in www.omg.org/spec/UML/2.4.1/Superstructure

Answer 2

Your notation is correct as xmojmr says. However, if you are thinking that it is the same thing as putting a join bar between your nodes and the decision diamond, and therefore represents a simplification, there's a rather subtle difference that you need to be aware of. In your notation, one of the nodes can enter into the decision process before the other activity is completed. If you have a join bar between the nodes and the decision diamond, both activities have to complete before either of them can enter the decision.

So you'll want to be aware that they are not exactly the same.