---
title: "Short note: External agent in use case"
subject: "Object Oriented Analysis and Design"
chapter: 2
key: "semester_6/object_oriented_analysis_and_design:short-note-external-agent-use-case"
source: https://www.bctengineer.com/q/semester_6/object_oriented_analysis_and_design/short-note-external-agent-use-case
site: BCT Engineering
---

# Short note: External agent in use case

Object Oriented Analysis and Design · Chapter 2: Object Oriented Analysis

- Asked in: 2070 Magh, 4 marks
- Video: [All About Use Case Diagrams - What is a Use Case Diagram, Use Case Diagram Tutorial, and More](https://www.youtube.com/watch?v=Omp4RbHbB0s) — SmartDraw
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/object_oriented_analysis_and_design/short-note-external-agent-use-case
- Whole chapter: https://www.bctengineer.com/md/semester_6/object_oriented_analysis_and_design/chapter-2

## Question

Write a short note on External agent in Use Case.

## Short answer

An external agent (actor) is anything outside the system that interacts with it, such as a person, system, device, or time. It initiates or participates in use cases and is depicted as a stick figure. Actors represent roles, not specific individuals, and include primary, supporting, and offstage types.

## Answer

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>>`.

```text
              +-------------------+
   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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is an external agent (actor) in a use case model?
   A. A class inside the system that stores data
   B. Anything outside the system that interacts with it
   C. A use case that is always optional
   D. A database used by the system
   Answer: B. An actor is a person, system, device or time outside the system that starts or takes part in a use case. (Key point: Definition of external agent)

2. In 'Withdraw Cash', which actor is the primary actor and which is supporting?
   A. Bank server is primary, Customer is supporting
   B. Both are primary
   C. Customer is primary, Bank server is supporting
   D. Tax authority is primary
   Answer: C. The primary actor has the goal and starts the use case; a supporting actor provides a service to the system. (Key point: Primary vs supporting actor)

3. What does it mean that an actor represents a role?
   A. One actor equals exactly one particular person
   B. Actors must be human
   C. Actors are inside the system boundary
   D. One person may play many roles and one role may be played by many people
   Answer: D. Actors are roles, not individuals, so the mapping between people and actors is many-to-many. (Key point: Actors are roles)

4. How do actors help define the system?
   A. They fix the boundary between what is inside and what is outside the system
   B. They list the classes of the system
   C. They give the database design
   D. They decide the programming language
   Answer: A. Since actors are outside the system, identifying them marks the system boundary. (Key point: System boundary)

## Related in Object Oriented Analysis and Design

- [External actors and library system sequence diagram](https://www.bctengineer.com/question.md?key=semester_6%2Fobject_oriented_analysis_and_design%3Aexternal-actors-library-system-sequence-diagram) — Object Oriented Analysis and Design, chapter 2