Who is the actor in a Use Case Diagram

Question

I'm making "UML Use Case" for an application that will have actors:

• user (a normal user),
• google (to make OpenID registration).

What will I use: OpenID or Google? What is the real actor here? The actor which will make the interaction with the user to be represented in the Use case diagram?

Answer 1

Both are valid choices. If you want to express that Google acts as a company you can add it as an actor. If you want to express that the user interacts with a general "OpenID Service" you can put it there as well. Or you can combine it and name the actor "Google OpenID Service" to make it more specific.

Answer 2

An actor is a person, organization or system that interacts with your system. OpenID is none of these. Google is. So you should model Google as the actor.