In UML Sequence diagrams, is it possible to model optional external inputs

Question

In UML Sequence Diagrams you have the combined fragment type Alt to branch based on different values for parameters. But let's say that in the middle of your sequence you are waiting for one of two different messages from two different external actors and you shall branch the code depending on which one arrives, what would be the best way to model this? And to make the question a little more challenging, let's throw in the possibility that neither message comes (triggering a timeout).

Without a better solution, I would divide the sequence diagram into multiple sequence diagrams, each new one starting with the one of the two possible messages. Or possibly just go over to state machines. But is their a not too convoluted way that would allow me to show these different cases within one sequence diagram?

Answer 1

In addition to the answer I referred in the comment, I made a little sample with a duration constraint for the timeout.

If you have a lot of conditional logic to show Activity Diagrams are an alternative. They do not have object responsibilities or a time axis, but because of this they can freely use two dimensions to show flow control.

Answer 2

I would simply go for the two SDs which you can name accordingly. One should always keep in mind that a SD shall highlight a certain aspect of a complex chain of actions in a system. Trying to put more and more information in a single SD will mess it up and hinder more than it helps.

It is also possible to use diagram fragments which allows navigation through zooming into the two fragments.

The timing diagram will not really help here. You would still need a large alt-fragment to show the sequences depending on which message arrived first.