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Chapter: Software Design : Structured System Analysis and Design

Overview of Diagrams: Collaboration-Sequence, Class

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.

Overview of Diagrams: Collaboration-Sequence, Class



ü   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.

ü   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.


ü   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


ü   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.


ü   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.


ü   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


ü    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


Generalization Between Use Cases


ü    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.




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


Generalization between use cases


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.



Extend Relationship


Ø  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.


Ø  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.

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


Ø  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.


Ø  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





ü    Registration use case is meaningful on its own, and it could be extended with optional Get Help On Registration use case

ü    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.

ü    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.


ü    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>]

ü    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.

ü    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


Include Relationship


ü    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.


ü    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.


ü    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.


ü    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


ü    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.



The primary purpose of this phase is to complete the most essential parts of the project that are high risk and plan the construction phase. This is the part of the project where technical risk is fully evaluated and/or eliminated by building the highest risk parts of the project. During this phase personnel requirements should be more accurately determined along with estimated man hours to complete the project.


The complete cost and time frame of the project is more firmly determined. During this phase how the system will work must be considered. Use cases will help identify risks. Steps to take during this phase:


1.     Complete project plan with construction iterations planned with requirements for each iteration.


2.     80% of use cases are completed. Significant use cases are described in detail.


3.     The project domain model is defined. (Don't get bogged down)


4.     Rank use cases by priority and risk. Do the highest priority and highest risk use cases first. Items that may be high risk:


-Overall system architecture especially when dealing with communication between subsystems.


-Team structure.


-Anything not done before or used before such as a new programming language, or using the


unified/iterative process for the first time.


5.     Begin design and development of the riskiest and highest priority use cases. There will be an iteration for each high risk and priority use case.


6.     Plan the iterations for the construction phase. This involves choosing the length of the iterations


and deciding which use cases or parts of use cases will be implemented during each iteration. Develop the higher priority and risk use cases during the first iterations in the construction phase.


ü    As was done on a preliminary level in the previous phase, the value (priority) of use cases and their respective risks must be more fully assessed in this phase. This may be done be either assigning an number to each use case for both value and risk. or categorize them by high, medium, or low value and risk. Time required for each use case should be estimated to the man week. Do the highest priority and highest risk use cases first.

ü    Requirements to be completed for this phase include:


1.     Description of the software architecture. Therefore most use cases should be done, activity diagrams, state charts, system sequence diagrams, and the domain model should be mostly complete.


2.     A prototype that overcomes the greatest project technical risk and has minimal high priority functionality.


3.     Complete project plan.


4.     Development plan.

ü   There may be an elaboration phase for each high risk use case.


ü   Considering the various diagrams and charts to be created, when they are created, and the best order to create them in, there seems to be a variety of opinions. This is because in the real world there may be more than one correct solution and there are no hard and fast rules that work everytime. In a way, this flexibility is a strength of UML.


ü   Some documentation indicates that most use cases should be done before creating a domain model and others indicate that the domain model can be built on a use case by use case basis. A good compromise is to spend a short time on a brief domain model during the elaboration phase, then enhance the domain model as each use case is developed during the elaboration and construction phase iterations.

ü   Some documentation indicates that activity diagrams and class diagrams should be complete


before the domain model is done. It is possible to create some of the diagrams and charts (artifacts) in parallel with each otherCompletion of 80% of use case diagrams.


1.     Completion of 80% of high level use case diagrams.


2.     Completion of expanded use case diagrams for major use cases only.


3.     System sequence diagrams for major use cases.


4.        Domain model (Don't get bogged down here with details). Just get a good idea of concepts involved. Use use cases to create the domain model. Any use case that strongly impacts the domain model should be considered and concepts from that use case should be incorporated in the domain model. The initial domain model may be drawn without lines and attributes to avoid too much detail and determine important use cases. The domain model may be refined later as the project analysis continues. If the system is large, domain models should be done on a per use case basis.


5.        Optionally create a glossary of terms for concepts to improve team communication.




ü After this point the design part of the project begins (although more analysis is done for each use case) and the following will be done in each iteration of the elaboration and construction phases.


1.     Operation contracts based on domain model and use cases.


2.     Collaboration diagrams.


3.     Class diagrams.


4.     Map class and collaboration diagrams to code.


5.     Update the domain model but do not force it to the class diagrams. Considerations during this project should be the following:

ü   Consider possible significant changes (down the road) to the system during analysis.


ü   Regarding system functional ability what do you expect to be able to change?


ü   Elaboration phase plans the necessary activities and required resources and specifies the features and designing the architecture.


Things to do: With the input of the use case model generated from the previous phase, we transform it into a design model via an analysis model. In brief, both an analysis model and a design model are structures made up of classifiers and a set of use-case realizations that describe how this structure realizes the use cases. Classifiers are, in general, "class-like" things.


ü   The analysis model is a detailed specification of the requirements and works as a first cut at a design model, although it is a model of its own. It is used by developers to understand precisely the use cases as described in the requirements. The analysis model is different from the design model in that it is a conceptual model rather than a blueprint of the implementation.

ü    Class Diagrams


ü   Sequence Diagrams


ü   Collaboration Diagrams


Exit Criteria:


ü   A detailed software development plan, containing:

1.     An updated risk assessment,


2.       A management plan,


3.       A staffing plan,



4.     A phase plan showing the number and contents of the iteration


5.     An iterative plan, detailing the next iteration


6.     The development environment and other tools required


7.     A test plan

ü    A baseline vision, in the form of a set of evalution criteria for the final product


ü    Objective, measurable evalution criteria for assessing the results of the initial iterations of the construction phase


ü    A domain analysis model (80% complete), sufficient to be able to call the corresponding architecture 'complete'.


ü    A software architecture description (stating constraints and limitations)


ü    An executable architectural baseline.


ü    During the Elaboration phase the project team is expected to capture a healthy majority of the system requirements. However, the primary goals of Elaboration are to address known risk factors and to establish and validate the system architecture. Common processes undertaken in this phase include the creation of use case diagrams, conceptual diagrams (class diagrams with only basic notation) and package diagrams (architectural diagrams).


ü    The architecture is validated primarily through the implementation of an Executable Architecture Baseline. This is a partial implementation of the system which includes the core, most architecturally significant, components. It is built in a series of small, timeboxed iterations. By the end of the Elaboration phase the system architecture must have stabilized and the executable architecture baseline must demonstrate that the architecture will support the key system functionality and exhibit the right behavior in terms of performance, scalability and cost.


ü    The final Elaboration phase deliverable is a plan (including cost and schedule estimates) for the Construction phase. At this point the plan should be accurate and credible, since it should be based on the Elaboration phase experience and since significant risk factors should have been addressed during the Elaboration phase.

ü    The Lifecycle Architecture Milestone marks the end of the Elaboration phase.


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Software Design : Structured System Analysis and Design : Overview of Diagrams: Collaboration-Sequence, Class |

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