Short note: External agent in use case

An external agent (actor) in a use case model is anything outside the system that interacts with it: a person, another system, a device, or time. It is not part of the system, but it either starts a use case or takes part in it.

Notation

Stick figure with the actor's name, linked by a solid line to the use cases it takes part in. External systems may also be shown as a box with <<actor>>.

              +-------------------+
   O          |                   |
  /|\---------|-( Withdraw Cash ) |
  / \         |                   |
 Customer     +-------------------+

Types of actors

• Primary actor: has a goal and starts the use case (Customer, in "Withdraw Cash").
• Supporting (secondary) actor: provides a service to the system (Bank server, Payment gateway, Printer).
• Offstage actor: interested in the result but not directly involved (Tax authority).
• Actors can also be human, external system, hardware device, or timer.

Points

• Actors represent a role, not a particular person: one person may play many roles and one role may be played by many people.
• Actors are found by asking who uses the system, who maintains it, which other systems exchange data with it, and what events happen on time.
• Actors can be related by generalization (e.g. Student and Teacher are specializations of User).
• They define the boundary of the system: what is inside the system and what is outside.