Earlier this year I had an interesting talk about sequence diagrams. The topic was asynchronous messages and how UML models them. As I find the topic interesting, I thought I will write a bit about that.

So how does UML models asynchronous messages, and how can you tell from a sequence diagram if a message is asynchronous? During the talk there were four signs mention that shows that a call is asynchronous:

1. There are dashed arrows showing the return calls.
2. The activity boxes are not continuous on the main lifeline
3. The arrowhead is not filled for the asynchronous message
4. The asynchronous message is not horizontal but diagonal.

My favourite one is the diagonal message, and I first I intended to write only about that, but then I thought better and decided to write bout all four statements above.

I will show you which ones are false statements, and why the last one – which was called simply nonsense – is actually true.