Use Case Model

The use case model for any system consists of a set of “use cases”. Intuitively, use cases represent the different ways in which a system can be used by the users. A simple way to find all the use cases of a system is to ask the question: “What the users can do using the system?” Thus for the Library Information System (LIS), the use cases could be:

• issue-book

• query-book

• return-book

• create-member

• add-book, etc

Use cases correspond to the high-level functional requirements. The use cases partition the system behavior into transactions, such that each transaction performs some useful action from the user’s point of view. To complete each transaction may involve either a single message or multiple message exchanges between the user and the system to complete.

Purpose of use cases

The purpose of a use case is to define a piece of coherent behavior without revealing the internal structure of the system. The use cases do not mention any specific algorithm to be used or the internal data representation, internal structure of the software, etc. A use case typically represents a sequence of interactions between the user and the system. These interactions consist of one mainline sequence. The mainline sequence represents the normal interaction between a user and the system. The mainline sequence is the most occurring sequence of interaction. For example, the mainline sequence of the withdraw cash use case supported by a bank ATM drawn, complete the transaction, and get the amount. Several variations to the main line sequence may also exist. Typically, a variation from the mainline sequence occurs when some specific conditions hold. For the bank ATM example, variations or alternate scenarios may occur, if the password is invalid or the amount to be withdrawn exceeds the amount balance. The variations are also called alternative paths. A use case can be viewed as a set of related scenarios tied together by a common goal. The mainline sequence and each of the variations are called scenarios or instances of the use case. Each scenario is a single path of user events and system activity through the use case.

Representation of use cases

Use cases can be represented by drawing a use case diagram and writing an accompanying text elaborating the drawing. In the use case diagram, each use case is represented by an ellipse with the name of the use case written inside the ellipse. All the ellipses (i.e. use cases) of a system are enclosed within a rectangle which represents the system boundary. The name of the system being modeled (such as Library Information System) appears inside the rectangle.

 The different users of the system are represented by using the stick person icon. Each stick person icon is normally referred to as an actor. An actor is a role played by a user with respect to the system use. It is possible that the same user may play the role of multiple actors. Each actor can participate in one or more use cases. The line connecting the actor and the use case is called the communication relationship. It indicates that the actor makes use of the functionality provided by the use case. Both the human users and the external systems can be represented by stick person icons. When a stick person icon represents an external system, it is annotated by the stereotype <>.

Example :

 The use case model for the Tic-tac-toe problem is shown in fig. 7.2. This software has only one use case “play move”. Note that the use case “get-user move” is not used here. The name “get-user-move” would be inappropriate because the use cases should be named from the users’ perspective.

Use case model for tic-tac-toe game

Text Description

 Each ellipse on the use case diagram should be accompanied by a text description. The text description should define the details of the interaction between the user and the computer and other aspects of the use case. It should include all the behavior associated with the use case in terms of the mainline sequence, different variations to the normal behavior, the system responses associated with the use case, the exceptional conditions that may occur in the behavior, etc. The behavior description is often written in a conversational style describing the interactions between the actor and the system. The text description may be informal, but some structuring is recommended. The following are some of the information which may be included in a use case text description in addition to the mainline sequence, and the alternative scenarios.

Contact persons:

 This section lists the personnel of the client organization with whom the use case was discussed, date and time of the meeting, etc.

Actors:

 In addition to identifying the actors, some information about actors using this use case which may help the implementation of the use case may be recorded.

Pre-condition:

 The preconditions would describe the state of the system before the use case execution starts.

Post-condition:

 This captures the state of the system after the use case has successfully completed.

Non-functional requirements:

 This could contain the important constraints for the design and implementation, such as platform and environment conditions, qualitative statements, response time requirements, etc.

Exceptions, error situations:

 This contains only the domain-related errors such as lack of user’s access rights, invalid entry in the input fields, etc. Obviously, errors that are not domain related, such as software errors, need not be discussed here.

Sample dialogs:

 These serve as examples illustrating the use case.

Specific user interface requirements:

 These contain specific requirements for the user interface of the use case. For example, it may contain forms to be used, screen shots, interaction style, etc.

Document references:

 This part contains references to specific domain related documents which may be useful to understand the system operation.

Example

The use case model for the Supermarket Prize Scheme described. As discussed earlier, the use cases correspond to the high-level functional requirements. From the problem description and the context, we can identify three use cases: “register-customer”, “register-sales”, and “select-winners”. As a sample, the text description for the use case “register-customer” is shown.

Use case model for Supermarket Prize Scheme

Text description

U1:register-customer: Using this use case, the customer can register himself by providing the necessary details.

The description for other use cases is written in a similar fashion.