Software Construction & Development lecture #4: System Modelling, Slides of Software Development Methodologies

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System Modelling

Software Construction & Development

Today’s Agenda

• Context models
• Interaction models
• Structural models
• Behavioral models

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.

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.

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.

Context models

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.

The context of the Mentcare system

Process model of involuntary detention

Interaction models

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.

Transfer-data use case

• A use case in the Mentcare system

Use cases in the Mentcare system involving

the role ‘Medical Receptionist’

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.