3SL Reference - Use Case Diagram (uc)

Use Case Diagram (uc)

A Use Case identifies the ways of using a proposed system to achieve a particular goal or mission for a particular stakeholder. Taken together the set of all the Use Cases gives the project all of the useful ways to use the system, and illustrates the value that it will provide.

A Use Case Diagram (uc) consists of a set of Actors and Use Cases and the relationships between them. Each Use Case description is first captured in text, and identifies the actors, pre-conditions, assumptions, post-conditions, and primary activities needed to accomplish the Use Case goal/mission.

For each Use Case one or more operational scenarios should be developed using Activity Diagrams so that all the operational activities and information exchanges can be understood for each possible scenario (e.g. normal conditions, off-normal conditions, different situations). The information derived from these scenarios help identify stakeholder functional requirements, external systems that must be interacted with and a high-level view of the information to be exchanged with the external systems (interfaces).

Symbols

An example uc is:

The symbols available in ucs are:

Symbol	Name	Description
	Comment	Makes a note anywhere in the diagram. Are always surrounded by * characters. Note: If you do not want Cradle to automatically add an * go to the Graphics Settings section of Project Setup and turn off the Automatically add asterisks to diagram's comment symbols option.
	Picture	Allows you to choose the location of a GIF or JPEG image to be displayed as a diagram symbol. An image can also be embedded in the other diagram symbols.
	Note Callout	Multi-purpose symbol which attaches to other symbols in order to show certain attributes of the model element. It can show frames, categories, tags, related items, or simply a piece of standalone text.
	Tag	A stereotype can be tagged with additional information/data needed to fully specify the element such as a time, event, constraint, etc. Note that there are currently no tags defined for the symbols used on a Use Case Diagram.
	Actor	The users and other external participants in an interaction with a system are described by «actor» elements. An Actor represents the role of a human, an organisation, or any external system that participates in the use of a subject system. They are also called Stakeholders. An Actor on a Use Case Diagram is shown as a stick figure with the actor’s name underneath the symbol. Associations between Actors and Use Cases are shown using standard association notation without arrowheads. The default multiplicity of the associations, if not shown, is 0..1. Actors can be classified using a Generalisation relationship. A Generalisation relationship is a line with a hollow triangle pointing to the more general end. In the following example Driver and Passenger are more specialised elements of the Vehicle Occupant element.
	Subject System	The entity for which Use Cases are defined is called the Subject System or the Subject System of Interest. The symbol is associated with a «block». The Use Cases that will be nested within the Subject System symbol will be cross referenced to the subject «block» with the «has usecase» relationship. The linked elements can be seen in the sidebar as shown below.
	Use Case	A Use Case describes how stakeholders use the system to achieve their goals. A Use Case is shown as an oval with the Use Case name inside it. The Use Cases can be grouped within the Subject System block symbol. Use Cases can be related to one another by the following kinds of relationships (path symbols on the diagram): generalisation, inclusion and extension. The «include» relationship allows a base Use Case to include the capabilities of an included Use Case as part of its capability. A dashed line with an open arrowhead, and annotated by an «include» keyword, is used to establish the «include» relationship. The arrowhead points to the Use Case to be included. A dashed line with an open arrowhead, and annotated by an «extend» keyword, is used to establish the «extend» relationship. The arrowhead points to the Use Case to be extended. Generalise/specialise relationship has a hollow triangle pointing to the more general Use Case. Indicates that the child Use Case inherits functionality from the parent Use Case. A text-based Use Case description is associated with the Use Case symbol to provide additional information to support the Use Case definition. A typical Use Case description may include the following: Pre conditions - the conditions that must hold for the Use Case to begin Post conditions - the conditions that must hold once the Use Case has completed Primary flow - the most frequent sequence of activities of the Use Case Alternate and/or exception flows - the sequence of activities that are less frequent or off normal Elaborating Use Cases with behaviours - if more details is available with the Use Case description is desired, the Use Case can be modelled with interactions, activities, or state the Use Case element is cross reference linked to a Behaviour Diagram (act, sd or stm) using the «scenario» relationship. The Use Case relationships can be seen in the sidebar.
	Association	Actors are related to Use Cases by reference association paths. Associations between Actors and Use Cases are shown using standard association notation, a line between two nodes: The multiplicity at the Actor end describes the number of Actors involved, and the multiplicity at the Use Case end describes the number of instances in which the Actor or Actors can be involved.
	Dependency	The Dependency paths that can be drawn on a Use Case Diagram are «include» and «extend». See the Use Case symbol description for a definition of these relationships
	Generalisation	A Generalisation path represents a classification hierarchy of Actors or Use Cases. In a classification hierarchy each classifier is described as being more general or more specialised than another. See the Use Case symbol and the Actor symbol descriptions for a definition of this relationship.

The symbols along with their default stereotypes, referenced elements, stereotype frames and cross reference link attributes are:

Symbol	Name	Default Stereotype	Referenced Element	Stereotype Frames or Cross Reference Link Attributes
	Comment	None	None	None
	Picture	None	None	None
	Note Callout	None	None	None
	Tag	None	None	None
	Actor	«actor»	None	None
	Subject System	«block»	None	None
	Use Case	«useCase»	None	None
	Association	None	None	None
	Dependency	None	None	None
	Generalisation	None	None	None