Synchronous and Asynchronous Calls

If you're exceptionally alert, you'll have noticed that the arrowheads in the last couple of diagrams are different from the arrowheads earlier on. That minor difference is quite important in UML 2. In UML 2, filled arrowheads show a synchronous message, while stick arrowheads show an asynchronous message.

If a caller sends a synchronous message, it must wait until the message is done, such as invoking a subroutine. If a caller sends an asynchronous message, it can continue processing and doesn't have to wait for a response. You see asynchronous calls in multithreaded applications and in message-oriented middleware. Asynchrony gives better responsiveness and reduces the temporal coupling but is harder to debug.

The arrowhead difference is very subtle; indeed, rather too subtle. It's also a backward-incompatible change introduced in UML 1.4, before then an asynchronous message was shown with the half-stick arrowhead, as in Figure 4.5.

I think that this arrowhead distinction is too subtle. If you want to highlight asynchronous messages, I would recommend using the obsolete half-stick arrowhead, which draws the eye much better to an important distinction. If you're reading a sequence diagram, beware of making assumptions about synchrony from the arrowheads unless you're sure that the author is intentionally making the distinction.