Is the UML sequence diagram correct?

Question

The following sequence diagram is rendered by Sparx Enterprise Architect. Is the start of the new activation (blue) correct? Shouldn't it rather continue with the existing activation on the Object2?

Please note that the flow starts with the asynchronous (red) call.

enter image description here

If the diagram is correct, why the new activation is not started in the following (almost identical) case?

enter image description here

• gnat: please don't cross-post : stackoverflow.com/questions/76306648/… "Cross-posting is frowned upon as it leads to fragmented answers splattered all over the network..."
• Petr B: Sorry, didn't know the sites are interconnected as it required to login again.
• CPlus: The fact that you have to login again is probably due to the fact that stackoverflow.com is a different domain from stackexchange.com. This is a bug that they have not fixed.
• candied_orange: Traditionally sequence diagrams progress from left to right not right to left. This is because we use a language that is read from left to right.

Answer 1

Both sequence diagrams are correct from an UML perspective, but they show slightly different sequences.

In the first diagram, with the new activation on object2, the call from object2 to object6, with the new activation, is not the result of the asynchronous call from object4 to object2. Rather it is initiated independently by a trigger that is not shown in the sequence diagram.

In the second diagram, with one continuous activation on object2, the call from object2 to object6 does happen in response to the asynchronous call to object2.

Based on how you draw your diagrams, a tool like Sparx Enterprise Architect will make some assumptions regarding what you mean and create the diagrams according to those assumptions. Typically you can do things like extending an activation box to tell the tool that a second outgoing arrow belongs to the same activation.

• Petr B: It's worth mentioning that I didn't use anything like extending activations etc. I just drew the lines in the sequence order and wonder why EA has drawn it this way. Do I understand it correctly that EA just guessed that in the first example I would like to start new activation and in the second not? In other words, different toll might draw it differently and I would need to tweak it manually?