Actors in the use case digram

Question

I have a question regarding the use case diagram, I started by identifying the actors but I came across some subsystems which make me confuse whether to take them as actors or not.I know that an actor can be human, system and anything that has a goal in the system but I just want to clarify this point to target the right actors in my diagram. Thank you

Answer 1

Anything that can change the system state can be treated as an Actor.

Answer 2

@xmojmr is right. http://en.wikipedia.org/wiki/Actor_%28UML%29

The thing is; do you want to model functionality that other systems use of your system? Or only functionality your system provides to the user? You can model whatever you want in an Use-case (or any UML diagram for that matter). Will other people benefit if you include other systems? Maybe it is better to make diagrams per set of functionality (webshopping, maintaining supply etc) or per type of user (customer, sub-system, external system etc). You decide, as long as people understand and it's not against the rules of UML. :-)