Introduction

Use case diagram is one of the primary tools used in the Unified Modeling Language (UML) for capturing requirements and functionalities by using actors, use cases, and relationships. Use case diagrams assists to model the tasks, functions, services that a system needs to perform, by that stakeholders of the system can acquire a better understanding about scope and behavior of the system. Mainly it represents high-level functionalities of the system.

Components of a Use Case Diagram

Actors:

Actors are the roles that interact with the particular system. The users can be divided into two parts as primary actors and secondary actors. Actors are drawn using stick figures or simple shapes outside the system boundary. Each actor corresponds to a set of behaviors that they perform within the system boundary.

Use Cases:

Use cases are used to represent a particular functionality provided by the system. They describe the behavior of the system from the perspective according to the actors of the system.
Use cases are represented by using ellipses within the system boundary.

Relationships:

Actors and use cases are connected by relationships. It illustrates how they interact with each component of the system. There are primary relationships and other type of relationships.
The primary relationship is the association between an actor and a use case. It indicates that the actor participates in that use case. Other relationships include include, extend, generalization, and dependency relationships.

System Boundary:

The system boundary defines the scope of the system being modeled.
It helps to identify the system's scope from its external actors and environments. Use cases are consists within this system boundary.

Characteristics and Guidelines:

Simplicity:

Use case diagrams should not be more complex. There should be not be included too much details or technical implementation specifics.

Clarity:

Use clear labels and tags for actors and use cases to ensure understanding of the stakeholders. Use of meaningful names that accurately reflect the system's functionalities helps to increase the clarity of the diagram also.

Consistency:

Ensure consistency with other UML diagrams and project documentation by using standard notations.

Completeness:

The diagram should cover all significant interactions between actors and the system.

Validation:

After preparing the diagram it is better to validate the diagram with stakeholders to ensure that it accurately captures their requirements and expectations. Otherwise need to do modification according to their requirements.