The Use Case Model

The Use Case Model

       The Use Case Model describes the proposed functionality of the new system. A Use Case represents a discrete unit of interaction between a user (human or machine) and the system. A Use Case is a single unit of meaningful work; for example login to system, register with system and create order are all Use Cases. Each Use Case has a description which describes the functionality that will be built in the proposed system. A Use Case may 'include' another Use Case's functionality or 'extend' another Use Case with its own behaviour.

       Use Cases are typically related to 'actors'. An actor is a human or machine entity that interacts with the system to perform meaningful work

A Use Case description will generally include:

1.       General comments and notes describing the use case.

2.       Requirements

3.       Constraints

4.       Scenarios

5.       Scenario diagram

Actors

       An Actor is a user of the system. This includes both human users and other computer systems. An Actor uses a Use Case to perform some piece of work which is of value to the business. The set of Use Cases an actor has access to defines their overall role in the system and the scope of their action.

Constraints, Requirements and Scenarios

The formal specification of a Use Case includes:

1.       Requirements. These are the formal functional requirements that a Use Case must provide to the end user. They correspond to the functional specifications found in structured methodologies. A requirement is a contract that the Use Case will perform some action or provide some value to the system.

2.       Constraints. These are the formal rules and limitations that a Use Case operates under, and includes pre- post- and invariant conditions. A pre-condition specifies what must have already occurred or be in place before the Use Case may start. A post-condition documents what will be true once the Use Case is complete. An invariant specifies what will be true throughout the time the Use Case operates.

3.       Scenarios. Scenarios are formal descriptions of the flow of events that occurs during a Use Case instance. These are usually described in text and correspond to a textual representation of the Sequence Diagram.

Includes and Extends relationships between Use Cases

       One Use Case may include the functionality of another as part of its normal processing. Generally, it is assumed that the included Use Case will be called every time the basic path is run. An example may be to list a set of customer orders to choose from before modifying a selected order - in this case the <list orders> Use Case may be included every time the <modify order> Use Case is run.

       A Use Case may be included by one or more Use Cases, so it helps to reduce duplication of functionality by factoring out common behaviour into Use Cases that are re-used many times.

       One Use Case may extend the behaviour of another - typically when exceptional circumstances are encountered. For example, if before modifying a particular type of customer order, a user must get approval from some higher authority, then the <get approval> Use Case may optionally extend the regular <modify order> Use Case.

Sequence Diagrams

       UML provides a graphical means of depicting object interactions over time in Sequence Diagrams. These typically show a user or actor, and the objects and components they interact with in the execution of a use case. One sequence diagram typically represents a single Use Case 'scenario' or flow of events.

       Sequence diagrams are an excellent way to document usage scenarios and to both capture required objects early in analysis and to verify object usage later in design. Sequence diagrams show the flow of messages from one object to another, and as such correspond to the methods and events supported by a class/object.

       The diagram illustrated below shows an example of a sequence diagram, with the user or actor on the left initiating a flow of events and messages that correspond to the Use Case scenario. The messages that pass between objects will become class operations in the final model.

Implementation Diagram

       A Use Case is a formal description of functionality the system will have when constructed. An implementation diagram is typically associated with a Use Case to document what design elements (eg. components and classes) will implement the Use Case functionality in the new system. This provides a high level of traceability for the system designer, the customer and the team that will actually build the system. The list of Use Cases that a component or class is linked to documents the minimum functionality that must be implemented by the component

Relationships Between Use Cases

Use cases could be organized using following relationships:

1.       Generalization

2.       Association

3.       Extend

4.       Include

1. Generalization Between Use Cases

       Generalization between use cases is similar to generalization between classes – child use case inherits properties and behavior of the parent use case and may override the behavior of the parent.

Notation:

       Generalization is rendered as a solid directed line with a large open arrowhead (same as generalization between classes).

Generalization between use cases

2. Association Between Use Cases

       Use cases can only be involved in binary Associations. Two use cases specifying the same subject cannot be associated since each of them individually describes a complete usage of the system.

3. Extend Relationship

       Extend is a directed relationship from an extending use case to an extended use case that specifies how and when the behavior defined in usually supplementary (optional) extending use case can be inserted into the behavior defined in the use case to be extended

       Note: Extended use case is meaningful on its own, independently of the extending use case, while the extending use case typically defines behavior that is not necessarily meaningful by itself.

       The extension takes place at one or more extension points defined in the extended use case.

       The extend relationship is owned by the extending use case. The same extending use case can extend more than one use case, and extending use case may itself be extended.

       Extend relationship between use cases is shown by a dashed arrow with an open arrowhead from the extending use case to the extended (base) use case. The arrow is labeled with the keyword «extend».

       Registration use case is meaningful on its own, and it could be extended with optional

Get Help On Registration use case

       The condition of the extend relationship as well as the references to the extension points are optionally shown in a Note attached to the corresponding extend relationship.

       Registration use case is conditionally extended by Get Help On Registration use case in extension point Registration Help

Extension Point

       An extension point is a feature of a use case which identifies (references) a point in the behavior of the use case where that behavior can be extended by some other (extending) use case, as specified by an extend relationship.

       Extension points may be shown in a compartment of the use case oval symbol under the heading extension points. Each extension point must have a name, unique within a use case. Extension points are shown as text string according to the syntax: <extension point> ::= <name> [: <explanation>]

       The optional description is given usually as informal text, but can also be given in other forms, such as the name of a state in a state machine, an activity in an activity diagram, a precondition, or a postcondition.

       Registration use case with extension points Registration Help and User Agreement

       Extension points may be shown in a compartment of the use case rectangle with ellipse icon under the heading extension points.

Extension points of the Registration use case shown using the rectangle notation

4. Include Relationship

       An include relationship is a directed relationship between two use cases, implying that the behavior of the required (not optional) included use case is inserted into the behavior of the including (base) use case. Including use case depends on the addition of the included use case.

       The include relationship is intended to be used when there are common parts of the behavior of two or more use cases. This common part is extracted into a separate use case to be included by all the base use cases having this part in common.

       Execution of the included use case is analogous to a subroutine call or macro command in programming. All of the behavior of the included use case is executed at a single location in the including use case before execution of the including use case is resumed.

       As the primary use of the include relationship is to reuse common parts, including use cases are usually not complete by themselves but dependent on the included use cases.

       Include relationship between use cases is shown by a dashed arrow with an open arrowhead from the including (base) use case to the included (common part) use case. The arrow is labeled with the keyword «include».

Four Phases of Unified Process

       The Unified Process consists of cycles that may repeat over the long-term life of a system. A cycle consists of four phases: Inception, Elaboration, Construction and Transition. Each cycle is concluded with a release, there are also releases within a cycle. Let's briefly review the four phases in a cycle:

Inception Phase - During the inception phase the core idea is developed into a product vision. In this phase, we review and confirm our understanding of the core business drivers. We want to understand the business case for why the project should be attempted. The inception phase establishes the product feasibility and delimits the project scope.

Elaboration Phase - During the elaboration phase the majority of the Use Cases are specified in detail and the system architecture is designed. This phase focuses on the "Do-Ability" of the project. We identify significant risks and prepare a schedule, staff and cost profile for the entire project.

Construction Phase - During the construction phase the product is moved from the architectural baseline to a system complete enough to transition to the user community. The architectural baseline grows to become the completed system as the design is refined into code.

Transition Phase - In the transition phase the goal is to ensure that the requirements have been met to the satisfaction of the stakeholders. This phase is often initiated with a beta release of the application. Other activities include site preparation, manual completion, and defect identification and correction. The transition phase ends with a postmortem devoted to learning and recording lessons for future cycles.