Chapter 5 – System Modeling

Lecture 1

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Topics covered

• Context models• Interaction models• Structural models• Behavioral models• Model-driven engineering

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System modeling

• System modeling is the process of developing abstract models of a system, with each model presenting a different view or perspective of that system.

• System modeling has now come to mean representing a system using some kind of graphical notation, which is now almost always based on notations in the Unified Modeling Language (UML).

• System modelling helps the analyst to understand the functionality of the system and models are used to communicate with customers.

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Existing and planned system models• Models of the existing system are used during requirements

engineering. They help clarify what the existing system does and can be used as a basis for discussing its strengths and weaknesses. These then lead to requirements for the new system.

• Models of the new system are used during requirements engineering to help explain the proposed requirements to other system stakeholders. Engineers use these models to discuss design proposals and to document the system for implementation.

• In a model-driven engineering process, it is possible to generate a complete or partial system implementation from the system model.

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System perspectives

• An external perspective, where you model the context or environment of the system.

• An interaction perspective, where you model the interactions between a system and its environment, or between the components of a system.

• A structural perspective, where you model the organization of a system or the structure of the data that is processed by the system.

• A behavioral perspective, where you model the dynamic behavior of the system and how it responds to events.

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UML diagram types

• Activity diagrams, which show the activities involved in a process or in data processing .

• Use case diagrams, which show the interactions between a system and its environment.

• Sequence diagrams, which show interactions between actors and the system and between system components.

• Class diagrams, which show the object classes in the system and the associations between these classes.

• State diagrams, which show how the system reacts to internal and external events.

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Use of graphical models• As a means of facilitating discussion about an existing

or proposed system• Incomplete and incorrect models are OK as their role is to

support discussion.

• As a way of documenting an existing system• Models should be an accurate representation of the system but

need not be complete.

• As a detailed system description that can be used to generate a system implementation• Models have to be both correct and complete.

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Context models

• Context models are used to illustrate the operational context of a system - they show what lies outside the system boundaries.

• Social and organisational concerns may affect the decision on where to position system boundaries.

• Architectural models show the system and its relationship with other systems.

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System boundaries

• System boundaries are established to define what is inside and what is outside the system.• They show other systems that are used or depend on the

system being developed.

• The position of the system boundary has a profound effect on the system requirements.

• Defining a system boundary is a political judgment• There may be pressures to develop system boundaries that

increase / decrease the influence or workload of different parts of an organization.

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The context of the MHC-PMS

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Process perspective

• Context models simply show the other systems in the environment, not how the system being developed is used in that environment.

• Process models reveal how the system being developed is used in broader business processes.

• UML activity diagrams may be used to define business process models.

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Activity diagrams • In UML, an activity diagram is used to display the sequence

of activities.

• Activity diagrams show the workflow from a start point to the finish point detailing the many decision paths that exist in the progression of events contained in the activity.

• Activity diagrams are useful for business modeling where they are used for detailing the processes involved in business activities.

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Activity diagram Components• Activities: An activity is shown as a

round-cornered rectangle enclosing all the actions, control flows and other elements that make up the activity.

• Actions: An action represents a single step within an activity. Actions are denoted by round-cornered rectangles.

• Control Flow: A control flow shows the flow of control from one action to the next. Its notation is a line with an arrowhead.

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Activity diagram Components• Action Constraints: constraints

can be attached to an action. The following diagram shows an action with local pre- and post-conditions.

• Initial Node: an initial or start node is depicted by a large black spot

• Final Node: the activity final node is depicted as a circle with a dot inside.

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Activity diagram Components

• Objects: objects or data can pass

• Object Flows: is shown as a connector with an arrowhead denoting the direction the object is being passed.

• Data store: is shown as an object with the «datastore» keyword.

• System: is shown as an object with the «system» keyword.

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system

<<system>>

System nam

Activity diagram Components• Decision and Merge Nodes: have

the same notation: a diamond shape.

• Fork and Join Nodes: forks and joins have the same notation: either a horizontal or vertical bar (the orientation is dependent on whether the control flow is running left to right or top to bottom). They indicate the start and end of concurrent threads of control.

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Example

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Process model of involuntary detention

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Interaction models

• Modeling user interaction is important as it helps to identify user requirements.

• Modeling system-to-system interaction highlights the communication problems that may arise.

• Modeling component interaction helps us understand if a proposed system structure is likely to deliver the required system performance and dependability.

• Use case diagrams and sequence diagrams may be used for interaction modelling.

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Use case modeling

• Use cases were developed originally to support requirements elicitation and now incorporated into the UML.

• Each use case represents a discrete task that involves external interaction with a system.

• Actors in a use case may be people or other systems.

• Represented diagrammatically to provide an overview of the use case and in a more detailed textual form.

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Use case diagram components

• Actor: is a user of the system. Actors are not limited to humans. They maybe system or application. It is depicted using a stick figure.

• Use Case: is functionality provided by the system, typically described as verb+object (eg. Register Car, Delete User). Use Cases are depicted with an ellipse.

• Associations: are used to link Actors with Use Cases, and indicate that an Actor participates in the Use Case . They are depicted by a line connecting the Actor and the Use Case.

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Relationships in Use Cases

• Include: a use case includes the functionality described in another use case as a part of its business process flow.

• 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.

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The functionality defined by the "Validate patient records" use case is contained within the "Make appointment" use case.

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.

Relationships in Use Cases

• Generalizations: A generalization relationship is 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. The child use case is connected at the base of the arrow. The tip of the arrow is connected to the parent use case.

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Transfer-data use case

• A use case in the MHC-PMS

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Tabular description of the ‘Transfer data’ use-case

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MHC-PMS: Transfer data

Actors Medical receptionist, patient records system (PRS)

Description A receptionist may transfer data from the MHC-PMS to a general patient record database that is maintained by a health authority. The information transferred may either be updated personal information (address, phone number, etc.) or a summary of the patient’s diagnosis and treatment.

Data Patient’s personal information, treatment summary

Stimulus User command issued by medical receptionist

Response Confirmation that PRS has been updated

Comments The receptionist must have appropriate security permissions to access the patient information and the PRS.

Use cases in the MHC-PMS involving the role ‘Medical Receptionist’

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Sequence diagrams

• Sequence diagrams are part of the UML and are used to model the interactions between the actors and the objects within a system.

• A sequence diagram shows the sequence of interactions that take place during a particular use case or use case instance.

• The objects and actors involved are listed along the top of the diagram, with a dotted line drawn vertically from these.

• Interactions between objects are indicated by annotated arrows.

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Sequence diagrams components

• Lifeline: is a named element which represents an individual participant in the interaction.

• A lifeline is shown using a symbol that consists of a rectangle forming its "head" followed by a vertical line (which may be dashed) that represents the lifetime of the participant.

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Sequence diagrams components• Message: is a named element that defines one specific kind of

communication between lifelines of an interaction. The message specifies not only the kind of communication, but also the sender and the receiver.

• Message could be one of: • synchronous call• asynchronous call• asynchronous signal• create• delete• reply

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Sequence diagrams components

• Synchronous Call: typically represents operation call - send message and suspend execution while waiting for response. Synchronous call messages are shown with filled arrow head.

• Asynchronous call: send message and proceed immediately without waiting for return value. Asynchronous messages have an open arrow head.

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Sequence diagrams components

• Asynchronous signal: message corresponds to asynchronous send signal action.

• Create message: is sent to a lifeline to create itself. It is shown as a dashed line with open arrowhead (looks the same as reply message), and pointing to the created lifeline's head.

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Sequence diagrams components

• Delete message: is sent to terminate another lifeline. The lifeline usually ends with a cross in the form of an X at the bottom denoting destruction occurrence.

• Reply: message to an operation call is shown as a dashed line with open arrow head (looks similar to creation message). C

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Sequence diagrams components• Conditional interaction: a

message can include a guard, which signifies that the message is only sent if a certain condition is met. The guard is simply that condition between brackets.

• If you want to show that several messages are conditionally sent under the same guard, you'll have to use an 'opt' combined fragment. The combined fragment is shown as a large rectangle with an 'opt' operator plus a guard, and contains all the conditional messages under that guard.

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Sequence diagrams components

• Conditional interaction: If you want to show several alternative interactions, use an 'alt' combined fragment. The combined fragment contains an operand for each alternative. Each alternative has a guard and contains the interaction that occurs when the condition for that guard is met.

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Sequence diagram for View patient information

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Sequence diagram for Transfer Data

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Structural models

• Structural models of software display the organization of a system in terms of the components that make up that system and their relationships.

• Structural models may be static models, which show the structure of the system design, or dynamic models, which show the organization of the system when it is executing.

• You create structural models of a system when you are

discussing and designing the system architecture.

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Key points• A model is an abstract view of a system that ignores system details.

Complementary system models can be developed to show the system’s context, interactions, structure and behaviour.

• Context models show how a system that is being modeled is positioned in an environment with other systems and processes.

• Use case diagrams and sequence diagrams are used to describe the interactions between users and systems in the system being designed. Use cases describe interactions between a system and external actors; sequence diagrams add more information to these by showing interactions between system objects.

• Structural models show the organization and architecture of a system. Class diagrams are used to define the static structure of classes in a system and their associations.

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First exam

• The first progress test will be held on Next Monday• Get ready!

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