Explanation

To fully understand the system's purpose, you must know who the system is for, that is: Who will use the system? The answer to this question is: the Actors.

An Actor is a role that a person or external system plays when interacting with the system.  Instances of an Actor can be an individual or an external system, however each Actor provides a unique and important perspective on the system that is shared by every instance of the Actor.

This difference between an actor and an instance of an actor is illustrated below.  Figure 1 shows a case in which Ivar and Mark are operators of a recycling machine. When they are using the machine in this capacity, each is represented by an instance of the actor called Operator that expects certain functionality of the system (Print Daily Reports in this example).

Figure 1: Example Actor with multiple instances 

Conversely, the same user can act as several actors (that is, the same person can take on different roles). In Figure 2, Charlie uses the Depot-Handling System primarily as Depot Manager, but sometimes he also uses the Depot-Handling System as an ordinary Depot Staff member. Each of these actors expects different functionality of the system.

Figure 2: Example of user playing different roles

Actors help you to identify external interfaces and to determine the scope the system (what is in the system, vs. what is outside the system boundary).  Each Actor has associated use cases which describe what that particular actor expects of the system.  It will be very difficult, if not impossible, to assess the completeness of the set of Use Cases without the context provided by the associated Actors. Furthermore, missing an actor may result in missing important stakeholder perspectives, resulting in a solution that does not meet all stakeholder needs.

Hence, identifying the Actors for the system should be done early in the lifecycle.  Actors are captured, including their names, brief descriptions, and relationships to use cases, in the Artifact: Use-Case Model.

Properties of Actors

Name

Each actor should have a name that clearly describes the role played by the user.

Brief Description

Each actor should have a brief description that clearly describes:

• What or who the actor represents.
• Why the actor is needed.
• What interests the actor has in the system
• Major characteristics of the actor.

The major characteristics of the actor are important as they may influence how the system is developed, for example characteristics of the user interface such as accessibility or globalization.  Examples of important characteristics include:

• The actors level of domain knowledge
• The actors level of computer experience
• The actors abilities and disabilities
• The actors native language