A UML use case diagram summarizes the interactions between a system and its users. A use case diagram can be made for a general scenario or multiple scenarios that encapsulate the system’s functioning.
Three kinds of entities are displayed in the use case diagram: the system, the actors, and the use cases.
1. System: The product in question that is being developed. This is represented by a box that encompasses the use cases.
2. Actors: The users or systems that interact with the main system. An actor can be primary or secondary. Primary actors are those that initiate use cases and interact with the system. They are usually placed on the left side of the system. On the other hand, secondary actors are used by the system but they do not interact with the system on their own. They are displayed on the right side of the system. Actors are represented by figures.
3. Use cases: The functions performed by the actors as they interact with the system. Use cases are represented by ellipses.
In the use case diagram of an airline booking system, the system is the airline booking, represented by the box and containing many different use cases. The primary actor is the customer, and the secondary actor is the admin. The customer initiates use cases such as booking, browsing, and canceling flights, while the admin initiates use cases such as updating flight records, while also overseeing the customer’s use cases. The admin is considered a secondary actor as she is used for the completion of the use cases initiated by the customer.
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