Write short notes on use case diagram with suitable diagram and their utility in system design.

Use case diagrams :

1. Interaction with External Users:
   • Use cases illustrate how a system interacts with external users or actors.
2. Functionality Representation:
   • Each use case represents a specific piece of functionality that the system offers to its users.
3. Informal Requirements:
   • Use cases are effective for capturing informal requirements, providing a high-level view of system functionality.
4. Actor Inclusion:
   • Use cases consist of actors, which are objects or sets of objects directly communicating with the system but not part of it.
5. Quantization of Interactions:
   • Interactions of actors with the system are quantized into use cases, simplifying the understanding of system functionality.
6. Coherent Functionality:
   • Use cases represent coherent pieces of functionality that the system can provide by interacting with actors.
7. Example:
   • For a vending machine, use cases include “Buy a beverage,” “Perform scheduled maintenance,” “Make repairs,” and “Load items.”

Utility of Use Case Diagrams in System Design:

1. Functionality Identification:
   • Use cases help identify the functionality of a system, organizing it based on user perspectives.
2. Transaction Completeness:
   • Use cases describe complete transactions, reducing the likelihood of omitting necessary steps in system operations.
3. Focus on System Purpose:
   • The main purpose of a system is often revealed through its use cases, with requirements lists providing additional implementation constraints.