Two actors invoking same functionality in sequence diagram. How to represent?

Question

I have a equipment which I am representing with a class and there are two actors a remote and local operator who can put the equipment on or off. Both actors will use the functionality of the equipment. But How do I now represent them using sequence diagram, since if I draw an event from both local and remote its going to show at the equipment the one happened after the other but in reality two actors are using the same function and can invoke it any time. So how do I represent the two actors in the below sequence diagram.

P.S. The RAN40L is the equipment and CMS is remote operator and Simulator Operator is the local operator.

Answer 1

As it happens I have extensive experience from the defence industry, including naval CMS, so I am familiar with the domain.

The crucial question is, as always with UML, what you want to show in the diagram, which of course ties in with what you are showing in other diagrams. No diagram is ever read in isolation and you will never capture the entire radar functionality in a single sequence diagram.

Remember that a sequence diagram is intended to show things happening in a strict sequence. It is possible to show some rudimentary concurrency using the appropriate fragment, but if you want to show that the two actors do exactly the same thing, that the sequence is in fact one and the same in both cases, then the sequence diagram is the wrong place to show that.

Assuming that this sequence is intended as an elaboration of a use case, then the solution is to replace the two actors with a single actor, eg "Radar Controller". This actor can then be specialized into CMS and Simulator, which makes sense if the radar is unaware of, or unconcerned with, who is interacting with it in some (use) cases but not in others.

If the radar never makes the distinction, there shouldn't be two actors at all. The actors must make sense to the system they're interacting with, otherwise there's something wrong with your actor model.

So one solution is to structure the use cases as below.

Answer 2

http://sdedit.sourceforge.net/images/webserver.png This is a good example where two actors are used. It is default to put one actor to the opposite the other (this is not done in the example).

Answer 3

Actor is considered to be just another object in the sequence diagram. You can plase arbitrarily many actors and use them just like any other object, no restrictions in this sense.

There are some stylistic guidelines though, most of all regarding Actors positioning on the diagram. It is a common practice to show the actores on the border of the diagram, keeping internal system objects inside. Moreover, human actors are typically shown on the left side, while system actors are kept on the right. Actors should not be "mixed" with system objects. Here is a simple example:

Answer 4

Everything in behavioral diagram is executed after behavior defined by diagram started. If actors interacts individually, and their interaction are not moxed in single execution, you must draw diagram for each case. I would say you need two diagrams, each for one actor.