Which one is the external system actors?

Question

I have Use case like this.

This is an Emergency Monitoring Use Case and actors, which one is the external system actors? I just learn Use Case Diagram, and I think the answer is Monitoring Operator, But I confused with the another actor, especially Monitoring sensor and Remote System? Is Another actor an external system actor too? But the option just : a. Remote System b. Monitoring Operator c. Monitoring Sensor d. Generate Alarm Can you give a link, so I can read it, too prove my answer is true / wrong.

Answer 1

An actor is always external to the system. An external system actor is a non-human actor.

In your diagram, the Monitoring operator seems to be a security agent looking at the screen and reacting to what's happening.

The Remote service seems to be a non-human actor that interact with some use case, probably to automatically feed data into the system:

• The diagram has a syntactic error: the arrow from Remote System and Monitoring sensor to Remote service needs to be a generalization arrow (normally a big white triangle. A use case does not allow to associate actors between them). This means that both are special forms of Remote service.

• All these three remote actors are therefore candidate for an external system:

  • However, Remote service seems meant here more as a placeholder for its specializations. You could therefore as well omit it.
  • If the sensor interacts with a system as an actor, it should be complex enough to be autonomous. This means that it's not a passive sensor but could be conisdered as a small active system. I would name it as well.
  • And of course, the Remote system is without a doubt an external system actor.