Chapter 5: System Modeling

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

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

Model of the new/planned 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

External system perspective

where you model the context or environment of the system.

Interaction system perspective

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

structural system perspective

where you model the organization of a system or the structure of the data that is processed by the system

behavioral system perspective

where you model the dynamic behavior of the system and how it responds to events.

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.

Use of a graphical model

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

used to illustrate the operational

context of a system - they show what lies outside the

system boundaries.

 Architectural models show the system and its

relationship with other systems.

 Social and organizational concerns may affect the

decision on where to position 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.

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.

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 modeling.

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.

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.

Structural models

• They display the organization of a system in terms of the components that make up that system and their relationships – like class diagrams

• They 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.

Class diagram

are used when developing an object- oriented system model to show the classes in a system and the associations between these classes.

 An object class can be thought of as a general definition of one kind of system object.

 An association is a link between classes that indicates that there is some relationship between these classes.

 When you are developing models during the early stages of the software engineering process, objects represent something in the real world, such as a patient, a prescription, doctor, etc.

Generalization

an everyday technique that we use to manage complexity.

 Rather than learn the detailed characteristics of every entity that we experience, we place these entities in more general classes (animals, cars, houses, etc.) and learn the characteristics of these classes.

 This allows us to infer that different members of these classes have some common characteristics e.g. squirrels and rats are rodents.

Object Class Aggregation Models

 An aggregation model shows how classes that are

collections are composed of other classes.

 Aggregation models are similar to the part-of relationship

in semantic data models.

Behavioral Models

are models of the dynamic behavior of a system as it is executing. They show what happens or what is supposed to happen when a system responds to a stimulus from its environment.

 You can think of these stimuli as being of two types:

 Data Some data arrives that has to be processed by the system.

 Events Some event happens that triggers system processing.

Events may have associated data, although this is not always the case.

Data driven Models

 Many business systems are data-processing systems that are primarily driven by data. They are controlled by the data input to the system, with relatively little external event processing.

 They show the sequence of actions involved in processing input data and generating an associated output.

 They are particularly useful during the analysis of requirements as they can be used to show end-to-end processing in a system.

Event driven modeling

 Real-time systems are often event-driven, with minimal data processing. For example, a landline phone switching system responds to events such as ‘receiver off hook’ by generating a dial tone.

 This shows how a system responds to external and internal events.

 It is based on the assumption that a system has a finite number of states and that events (stimuli) may cause a transition from one state to another.

State machine models

 These model the behavior of the system in response to external and internal events.

 They show the system’s responses to stimuli so are often used for modeling real-time systems.

 State machine models show system states as nodes and events as arcs between these nodes. When an event occurs, the system moves from one state to another.

 State charts are an integral part of the UML and are used to represent state machine models.

Model Driven Engineering

an approach to software development where models rather than programs are the principal outputs of the development process.

 The programs that execute on a hardware/software platform are then generated automatically from the models.

 Proponents of MDE argue that this raises the level of abstraction in software engineering so that engineers no longer have to be concerned with programming language details or the specifics of execution platforms.

Usage of model-driven engineering

Model-driven engineering is still at an early stage of development, and it is unclear whether or not it will have a significant effect on software engineering practice.

Pros

 Allows systems to be considered at higher levels of abstraction

 Generating code automatically means that it is cheaper to adapt systems to new platforms.

Cons

 Models for abstraction and not necessarily right for implementation.

 Savings from generating code may be outweighed by the costs of developing translators for new platforms.

Model Driven Architecture

the precursor of more general model-driven engineering

 MDA is a model-focused approach to software design and implementation that uses a subset of UML models to describe a system.

 Models at different levels of abstraction are created. From a high-level, platform independent model, it is possible, in principle, to generate a working program without manual intervention.

A computation independent model (CIM)

These model the important domain abstractions used in a system. CIMs are sometimes called domain models.

A platform independent model (PIM)

These model the operation of the system without reference to its implementation. The PIM is usually described using UML models that show the static system structure and how it responds to external and internal events.

Platform specific models (PSM)

These are transformations of the platform-independent model with a separate PSM for each application platform. In principle, there may be layers of PSM, with each layer adding some platform-specific detail.

Agile methods and Model Driven Architecture

 The developers of MDA claim that it is intended to support an iterative approach to development and so can be used within agile methods.

 The notion of extensive up-front modeling contradicts the fundamental ideas in the agile manifesto and I suspect that few agile developers feel comfortable with model-driven engineering.

 If transformations can be completely automated and a complete program generated from a PIM, then, in principle, MDA could be used in an agile development process as no separate coding would be required.