Steps And Terms Used In Drawing A Use Case Diagram
Use case
A use case is always depicted in a horizontal ellipse and represents a certain sequence of actions that renders an actor with a measurable value.
Actors
Actors in a UML diagram is always depicted with the help of a stick figure and are important role players within a system. Some examples of actors include: an individual, an organization or a complete external system itself.
Association
Association is a way of depicting the relationship between an actor and a use case in a UML diagram and is represented with a solid line connecting the two and in some cases with an arrowhead. The presence of an association shows that an actor is associated with an interaction of a use case. The importance of arrowhead is in its property to show the primary actor of a particular use case or to show the direction in which initial invocation of a particular relationship is taking place. But the use of arrowhead sometimes creates ambiguity with respect to the data flow.
System Boundary Box
To represent a system’s scope a rectangle is drawn around the concerned use case and is termed as the system boundary box. The things contained in that system boundary are considered to be functional for the scope and the things outside of it are not. The use of this system boundary box is not very common to see; yet its use is significant in identification of the use cases that are going to be delivered fro every significant release of the systems. Use of the system boundary can be understood with the help of figure2.
Package
The entities of the UML diagrams that are used in grouping of the model elements are called packages and are used as the UML constructs. Packages are always represented as file folders and are used in any sort of UML diagrams like, case or class diagrams. Many a times a package is used to constrain a larger diagram into a smaller one so that it could be printed easily on a single page. Figure3 illustrates the importance of a package through reorganizing figure1.
Here, figure1 shows the overall process of the enrollment of students in different courses with the help of registrars. Students are assigned the marks, for their assignments, by the professors and the authorization of handing out of the report cards are done by the registrars. You can see here that there are some use cases that involves more than one a single actor. You can also see the use of arrowheads in this figure, but always remember that associations of a particular use case cannot have more than one arrowhead at a time. In this figure the solid lines representing associations are drawn between “student” and “enroll in seminar”. So it is clear that in this case of use case the initial invocation is carried out by the student rather than by registrar despite its presence in the same use case. But an association only indicates the involvement of a particular actor with a use case without clearing anything about the flow of information.
The flow of information is taking place in both the directions between an actor and a use case, e.g., it is imperative for the students to show their choice of enrollment for a particular seminar and on the other hand the system needs to inform the students of their effective enrollment. This information can’t be depicted with the help of use cases instead the flow of information are shown with the help of UML activity diagrams. The solid line, indicating the associations, doesn’t contain any arrowhead in case of association between “enroll in seminar” and “registrar”. So it shows the uncertainty of direction of the interaction. So in this case any of the two instances can take place: either the registrar itself provides help to the student without any request from the student or the student can approach registrar for a favor. But it is sure that an actor will not be there without any involvement with at least a single use case. Actors are represented on the external edges of a particular use case diagram.
Figure 1. Diagram of system use case:
Figure 2. Indication of release with the help of system boundary box:
Figure 3. Application of packages
How to create a use case diagram
The creation of a use case diagram should begin with the identification of number of possible actors and the effort should always be targeted towards finding as many actors as possible. To find the actors one should try to guess the interactions taking place in the system and this will eventually bring out the initial number of use cases. After the determination of the numbers one should connect the use cases and the actors with each other with the help of solid lines. Te association between them could be the result of the receipt of any information by an actor due to a use case or the supplying of any information by an actor that creates a use case. Try to avoid the use of arrowheads in the diagram lets people take them as an indication of the direction of information flow rather than the indication towards an initial invocation. As soon as, it becomes clear that a particular similarity exist between the use cases or the between the actors, modeling of proper relationship should begin immediately.
There are a number of different styles of modeling a use case diagram. The modeling style used to illustrate the model in the given example is called actors first style. Another style that can be used, involves the identification of use cases related to a specific actor and then evolving the model. So it is up to you that what style you are going to apply in your creation of use case diagram. Only thing that you should be strict about is to be very flexible while utilizing the AM’s practice of Model.
There is basically three relations that exist among use cases and the relations are: inheritance extends & includes. Extend relations could be taken as an interruption to the hardware process due to the uncertainty of invocation of the extending use case. Besides, the relationships are just like a procedure call. The application of the inheritance relationship is utilized just like it is used in UML class diagrams. The inheritance relationship is used to model the specialization of the actors and the use cases.
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