Learn the basics of UML use case diagrams and try your hand at drawing one. Additionally, see what a UML use case specification is.
In the previous two articles, you saw a brief review of the nine UML diagrams and what kind of tools can be used to model UML diagrams. Now that you have the clear , you'll start to learn about the the nine UML diagrams. In this article, you'll explore the Use case diagram. We'll present the basics of use case diagrams and show you how to draw a use case diagram. Additionally, you will see what a use case specification is. Finally, you'll see how to use case diagrams for the case study application—the Courseware Management System—used in the previous articles.
Basics of Use Case Diagrams
Before starting off today's article, iti is worth revisiting the definition of use a case diagram, as described in the first article.
The Use case diagram is used to identify the primary elements and processes that form the system. The primary elements are termed as "actors" and the processes are called "use cases." The Use case diagram shows which actors interact with each use case.
The above statement pretty much sums up what a use case diagram is primarily made up of—actors and use cases.
A use case diagram captures the functional aspects of a system. More specifically, it captures the business processes carried out in the system. As you discuss the functionality and processes of the system, you discover significant characteristics of the system that you model in the use case diagram. Due to the simplicity of use case diagrams, and more importantly, because they are shorn of all technical jargon, use case diagrams are a great storyboard tool for user meetings. Use case diagrams have another important use. Use case diagrams define the requirements of the system being modeled and hence are used to write test scripts for the modeled system.
So who should normally be involved in the creation of use cases? Normally, domain experts and business analysts should be involved in writing use cases for a given system. Use cases are created when the requirements of a system need to be captured. Because, at this point no design or development activities are involved, technical experts should not be a part of the team responsible for creating use cases. Their expertise comes in use later in the software lifecycle.
Elements of a UML Use Case Diagram
A use case diagram is quite simple in nature and depicts two types of elements: one representing the business processes and the other representing the business roles. Take a closer look at what elements constitute a use case diagram:
- Actors: An actor portrays any entity (or entities) that performs certain roles in a given system. The different roles the actor represents are the actual business roles of users in a given system. An actor in a use case diagram interacts with a use case. For example, for modeling a banking application, a customer entity represents an actor in the application. Similarly, the person who provides service at the counter is also an actor. But it is up to you to consider what actors make an impact on the functionality that you want to model. If an entity does not affect a certain piece of functionality that you are modeling, it makes no sense to represent it as an actor. An actor is shown as a stick figure in a use case diagram depicted "outside" the system boundary, as shown in Figure 3.1.
Figure 3.1: an actor in a UML use case diagram
To identify an actor, search in the problem statement for business terms that portray roles in the system. For example, in the statement "patients visit the doctor in the clinic for medical tests," "doctor" and "patients" are the business roles and can be easily identified as actors in the system.
- Use case: A use case in a use case diagram is a visual representation of a distinct business functionality in a system. The key term here is "distinct business functionality." To choose a business process as a likely candidate for modeling as a use case, you need to ensure that the business process is discrete in nature. As the first step in identifying use cases, you should list the discrete business functions in your problem statement. Each of these business functions can be classified as a potential use case. Remember that identifying use cases is a discovery rather than a creation. As business functionality becomes clearer, the underlying use cases become more easily evident. A use case is shown as an ellipse in a use case diagram (see Figure 3.2).
Figure 3.2: use cases in a use case diagram
Figure 3.2 shows two uses cases: "Make appointment" and "Perform medical tests" in the use case diagram of a clinic system. As another example, consider that a business process such as "manage patient records" can in turn have sub-processes like "manage patient's personal information" and "manage patient's medical information." Discovering such implicit use cases is possible only with a thorough understanding of all the business processes of the system through discussions with potential users of the system and relevant domain knowledge.
- System boundary: A system boundary defines the scope of what a system will be. A system cannot have infinite functionality. So, it follows that use cases also need to have definitive limits defined. A system boundary of a use case diagram defines the limits of the system. The system boundary is shown as a rectangle spanning all the use cases in the system.
Figure 3.3: a use case diagram depicting the system boundary of a clinic application
Figure 3.3 shows the system boundary of the clinic application. The use cases of this system are enclosed in a rectangle. Note that the actors in the system are outside the system boundary.
The system boundary is potentially the entire system as defined in the problem statement. But this is not always the case. For large and complex systems, each of the modules may be the system boundary. For example, for an ERP system for an organization, each of the modules such as personnel, payroll, accounting, and so forth, can form the system boundary for use cases specific to each of these business functions. The entire system can span all of these modules depicting the overall system boundary.
Relationships in UML Use Cases
UML use cases share different kinds of relationships. A relationship between two use cases is basically a dependency between the two use cases. Defining a relationship between two use cases is the decision of the modeler of the use case diagram. This reuse of an existing use case using different types of relationships reduces the overall effort required in defining use cases in a system. A similar reuse established using relationships, will be apparent in the other UML diagrams as well. Use case relationships can be one of the following:
- Include: When a use case is depicted as using the functionality of another use case in a diagram, this relationship between the use cases is named as an include relationship. Literally speaking, in an include relationship, a use case includes the functionality described in the another use case as a part of its business process flow. An include relationship is depicted with a directed arrow having a dotted shaft. The tip of the arrowhead points to the parent use case and the child use case is connected at the base of the arrow. The stereotype "<<include>>" identifies the relationship as an include relationship.
Figure 3.4: an example of an include relationship
For example, in Figure 3.4, you can see that the functionality defined by the "Validate patient records" use case is contained within the "Make appointment" use case. Hence, whenever the "Make appointment" use case executes, the business steps defined in the "Validate patient records" use case are also executed.
- Extend: In an extend relationship between two use cases, the child use case adds to the existing functionality and characteristics of the parent use case. An extend relationship is depicted with a directed arrow having a dotted shaft, similar to the include relationship. The tip of the arrowhead points to the parent use case and the child use case is connected at the base of the arrow. The stereotype "<<extend>>" identifies the relationship as an extend relationship, as shown in Figure 3.5.
Figure 3.5: an example of an extend relationship
Figure 3.5 shows an example of an extend relationship between the "Perform medical tests" (parent) and "Perform Pathological Tests" (child) use cases. The "Perform Pathological Tests" use case enhances the functionality of the "Perform medical tests" use case. Essentially, the "Perform Pathological Tests" use case is a specialized version of the generic "Perform medical tests" use case.
- Generalizations: A generalization relationship is also a parent-child relationship between use cases. The child use case in the generalization relationship has the underlying business process meaning, but is an enhancement of the parent use case. In a use case diagram, generalization is shown as a directed arrow with a triangle arrowhead (see Figure 3.6). The child use case is connected at the base of the arrow. The tip of the arrow is connected to the parent use case.
Figure 3.6: an example of a generalization relationship
On the face of it, both generalizations and extends appear to be more or less similar. But there is a subtle difference between a generalization relationship and an extend relationship. When you establish a generalization relationship between use cases, this implies that the parent use case can be replaced by the child use case without breaking the business flow. On the other hand, an extend relationship between use cases implies that the child use case enhances the functionality of the parent use case into a specialized functionality. The parent use case in an extend relationship cannot be replaced by the child use case.
An example should help you understand things better. From the diagram of a generalization relationship (refer to Figure 3.6), you can see that "Store patient records (paper file)" (parent) use case is depicted as a generalized version of the "Store patient records (computerized file)" (child) use case. Defining a generalization relationship between the two implies that you can replace any occurrence of the "Store patient records (paper file)" use case in the business flow of your system with the "Store patient records (computerized file)" use case without impacting any business flow. This would mean that in future you might choose to store patient records in a computerized file instead of as paper documents without impacting other business actions.
Now, if you had defined this as an extend relationship between the two use cases, this would imply that the "Store patient records (computerized file)" use case is a specialized version of the "Store patient records (paper file)" use case. Hence, you would not be able to seamlessly replace the occurrence of the "Store patient records (paper file)" use case with the "Store patient records (computerized file)" use case.
Creating the UML Use Case Diagram
For drawing UML use case diagrams, you need to use any tool that supports use case diagrams. You will be using the Poseidon Community Edition tool for drawing the use case diagram, as shown in Figure 3.7. You can use any tool that you are comfortable with. A use case modeling tool provides a palette of options to draw actors and use cases and to define relationships between the use cases.
Click here for a larger image.
Figure 3.7: a screen shot of the Poseidon tool
Take a look at the screen shot of the Poseidon tool. You can see the different options it provides to draw the use case diagram elements. In addition to drawing the use case diagram elements such as actors and use cases, you also can define relationships between use cases. Apart from this, the tool also provides capability to document the different elements that we draw. This documentation can be viewed as a consolidated report for future reference.
|An additional feature that you can check in your modeling tool is support for generating test scripts from the use case diagram. A comprehensive use case diagram provides a good foundation for basing test cases for the system that you model.
Writing a UML Use Case Specification
A use case diagram, as we have seen, is a visual depiction of the different scenarios of interaction between an actor and a use case. The usefulness of use case diagrams is more as a tool of communication between the requirements capture team and the user group. The next step after finalizing of use case diagrams is to document the business functionality into clear-cut and detailed use case specifications. Because use cases are used as an input to the other project phases such as design, development, and testing, we need to ensure that the visual depiction of the business requirements is translated into clear and well-defined requirements in the form of use case specifications. Elaborate use case specifications are used as an input for design and development and for writing test cases (unit, system, and regression tests, as the case may be).
A use case specification document should enable us to easily document the business flow. Information that you document in a use case specification includes what actors are involved, the steps that the use case performs, business rules, and so forth. A use case specification document should cover the following areas:
- Actors: List the actors that interact and participate in this use case.
- Pre-conditions: Pre-conditions that need to be satisfied for the use case to perform.
- Post-conditions: Define the different states in which you expect the system to be in, after the use case executes.
- Basic Flow: List the basic events that will occur when this use case is executed. Include all the primary activities that the use case will perform. Be fairly descriptive when defining the actions performed by the actor and the response of the use case to those actions. This description of actions and responses are your functional requirements. These will form the basis for writing the test case scenarios for the system.
- Alternative flows: Any subsidiary events that can occur in the use case should be listed separately. Each such event should be completed in itself to be listed as an alternative flow. A use case can have as many alternative flows as required. But remember, if there are too many alternative flows, you need to revisit your use case design to make it simpler and, if required, break the use case into smaller discrete units.
- Special Requirements: Business rules for the basic and alternative flows should be listed as special requirements in the use case narration. These business rules will also be used for writing test cases. Both success and failure scenarios should be described here.
- Use case relationships: For complex systems, it is recommended that you document the relationships between use cases. If this use case extends from other use cases or includes the functionality of other use cases, these relationships should be listed here. Listing the relationships between use cases also provides a mechanism for traceability.
|You can download a template for a use case specification document and customize it for your use here.
Dos and Don'ts with UML Use Cases
UML use cases should not be used to capture all the details of a system. The granularity to which you define use cases in a diagram should be enough to keep the use case diagram uncluttered and readable, yet, be complete without missing significant aspects of the required functionality. You will encounter such decision points of the level of granularity that you need to define when you build any of the UML diagrams.
An important rule that gets forgotten during use creation is the creeping in of design issues. Use cases are meant to capture "what" the system is, not "how" the system will be designed or built. Use cases should be free of any design characteristics. If you end up defining design characteristics in a use case, you need to go back to the drawing board and start again.
UML Case study—Courseware Management System
Use case modeling, as you have learnt today, involves analyzing the problem statement to determine the business processes of the system. We will now design the use case model for the Courseware Management System case study.
Note: In case you need to revisit the problem statement of the Courseware Management System described in Article 2, click here.
Analyze the problem statement to identify the potential actors and use cases of the system. First, let us list the potential actors. A quick look at the problem statement shows up the following terms and entities specific to the system:
- Courses and Topics that make up a course
- Tutors who teach courses
- Course administrators who mange the assignment of the courses to tutors
- Calendar or Course Schedule is generated as a result of the
- Students who refer to the Course schedule or Calendar to decide which courses they wish to take up for study
Identifying Actors of the Courseware Management System
Out of the preceding list, one thing is clear. There are certain terms and entities in the list that identify that they perform certain roles or business processes. We will discuss what these business processes are after we complete our analysis for identifying actors. For now, focus on identifying the actors in the system. From the preceding list, you can see that there are some entities that perform an action and some that form the target for the action. The entities that perform action will be the actors for the Courseware Management System. In the above list, the actors that we can identify are:
- Course administrators
But, because students are not the potential active participants for this system, we will drop them from the list of actors. Similarly, tutors are not active participants from our system's perspective, and hence, we will exclude tutors from our list if roles. Yet, we will still record them in our use case model since we do not wish to lose this business information. Our final list of primary actors has now come down to only one:
Identifying Use Cases of the Courseware Management System
Next, let us identify the potential business processes in the Courseware Management System. The primary business flows in the system are:
- Manage courses
- Manage course assignments
As we analyze the problem statement further, we can determine some discrete processes within these primary business flows. To manage courses, the actor needs to have the ability to view existing courses, manage the course information for a course, such as duration and so forth, and also manage the addition or removal of topics for a course. So, within the "Manage courses" use case, we can identify the following sub processes:
- View courses
- Manage topics for a course
- Manage course information
And similarly, the "Manage course assignment" use case can be refined into smaller discrete processes such as viewing the course calendar, viewing tutors, managing the tutor information of tutors working for the organization, and of course, assigning courses to tutors. Now, the use cases that we have identified within the "Manage course assignment" use case are:
- View course calendar
- View tutors
- Manage tutor information
- Assign courses to tutors
Our final list of use cases for the courseware management system will now be:
- View courses
- Manage topics for a course
- Manage course information
- View course calendar
- View tutors
- Manage tutor information
- Assign courses to tutors
|If you were analyzing a sentence in English, the subject in the sentence can be identified as a potential actor and the verb part of the sentence can be a potential use case. Remember, this may or may not apply to the problem at hand, but is a good starting point for use case modeling.
Use Case Diagram
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Figure 3.8: the UML use case diagram for the Courseware Management System
You have completed identifying potential use cases and actors. Take a look at the use case diagram for the Courseware Management System in Figure 3.7. The use case diagram of the Courseware Management System includes all the actors and use cases that we identified during our analysis of the problem statement.
Use case diagrams were the starting point of our journey in exploring each of the UML diagrams. Business functionality can be quickly represented in a simple and lucid fashion by using use case diagrams. Once the groundwork for depicting use cases is completed, the next step, as we learnt today, is writing detailed use case scenarios that will be used as the base functional requirements for the system. Our exercise in defining the use case diagram for the Courseware Management System case study was useful and enabled us to get a hands-on experience in applying what we learnt today.
In the coming article, you will study the next UML diagram—the Class diagram.
About the Authors
Mandar S. Chitnis, Lakshmi Ananthamurthy, and Pravin S. Tiwari are the co-founders of Novusware, Inc. They have co-authored the book Teach Yourself BEA WebLogic Server 7.0 in 21 Days (SAMS publishing Oct 2002) based on the recently launched WebLogic Server 7.0 by BEA Systems inc.
For any questions or queries regarding the article contents, please contact firstname.lastname@example.org.
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