Comprehensive Guide to Elaborating a Use Case Scenario

Introduction

A use case scenario is a detailed description of how a system should behave as it responds to a request from an external actor (user or another system). It provides a step-by-step narrative of the interactions between the actors and the system, helping to clarify requirements and ensure that the system meets the needs of its users.

This guide will explain how to elaborate a use case scenario, the relationship between use cases, use case scenarios, and sequence diagrams, using the attached sequence diagram as an example.

1. Understanding Use Cases

A use case is a description of how a user will perform tasks with your system. It outlines the interactions between the user (actor) and the system to achieve a specific goal. Use cases are typically represented in a use case diagram, which shows the actors and the use cases they are involved in.

2. Use Case Scenarios

A use case scenario is a specific instance of a use case. It provides a detailed, step-by-step description of the interactions between the actors and the system for a particular use case. Scenarios help to clarify the requirements and ensure that the system behaves as expected in different situations.

3. Sequence Diagrams

A sequence diagram is a visual representation of the interactions between objects in a particular scenario of a use case. It shows the sequence of messages exchanged between the objects and the order in which they occur. Sequence diagrams are useful for understanding the dynamic behavior of the system and for identifying potential issues in the interaction between objects.

4. Relationship Between Use Cases, Use Case Scenarios, and Sequence Diagrams

• Use Case: Defines the high-level interaction between actors and the system.
• Use Case Scenario: Provides a detailed, step-by-step description of a specific instance of a use case.
• Sequence Diagram: Visually represents the interactions between objects in a use case scenario, showing the sequence of messages exchanged.

5. Elaborating a Use Case Scenario

To elaborate a use case scenario, follow these steps:

1. Identify the Use Case: Start by identifying the use case you want to elaborate. For example, in the attached sequence diagram, the use case is “Buy Tickets.”

2. Define the Actors: Identify the actors involved in the use case. In the example, the actors are the Ticket Vending Machine, Clerk, Credit Card Service, Box Office, and Supervisor.

3. Describe the Main Success Scenario: Write a step-by-step description of the main success scenario, which is the most common and successful path through the use case. In the example, the main success scenario involves the following steps:

   • The Ticket Vending Machine requests the count and performance from the Ticket Selling System.
   • The Ticket Selling System shows the availability (seat-list) to the Ticket Vending Machine.
   • The Ticket Vending Machine selects seats and demands payment (cost) from the user.
   • The user inserts the card (card number) into the Ticket Vending Machine.
   • The Ticket Vending Machine charges the card number and cost to the Credit Card Service.
   • The Credit Card Service authorizes the charge.
   • The Ticket Vending Machine prints the tickets (performance, seats) and ejects the card.

4. Identify Alternative Scenarios: Describe any alternative scenarios that may occur, such as error conditions or exceptional cases. For example, if the credit card is declined, the system should handle this gracefully and inform the user.

5. Create a Sequence Diagram: Use a sequence diagram to visually represent the interactions between the objects in the use case scenario. The attached sequence diagram shows the interactions between the Ticket Vending Machine, Ticket Selling System, Credit Card Service, Box Office, and Supervisor.

6. Example Use Case Scenario

Using the attached sequence diagram as an example, here is a detailed use case scenario for “Buy Tickets”:

1. Request Count and Performance: The Ticket Vending Machine sends a request to the Ticket Selling System to get the count and performance of available tickets.

2. Show Availability: The Ticket Selling System responds with a list of available seats (seat-list) for the requested performance.

3. Select Seats: The Ticket Vending Machine allows the user to select the desired seats from the available list.

4. Demand Payment: The Ticket Vending Machine calculates the total cost and demands payment from the user.

5. Insert Card: The user inserts their credit card into the Ticket Vending Machine.

6. Charge Card: The Ticket Vending Machine sends the card number and cost to the Credit Card Service for authorization.

7. Authorize Charge: The Credit Card Service authorizes the charge and sends a confirmation back to the Ticket Vending Machine.

8. Print Tickets: The Ticket Vending Machine prints the tickets with the performance and seat details.

9. Eject Card: The Ticket Vending Machine ejects the user’s credit card.

Conclusion

Elaborating a use case scenario involves identifying the use case, defining the actors, describing the main success scenario, identifying alternative scenarios, and creating a sequence diagram. By following these steps, you can ensure that your system meets the needs of its users and behaves as expected in different situations. The attached sequence diagram provides a visual representation of the interactions between objects in the “Buy Tickets” use case scenario, helping to clarify the requirements and ensure the system’s success.