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Chapter 5 – System ModelingSummaryChapter 5 System modeling1

Topics covered Context modelsInteraction modelsStructural modelsBehavioral modelsModel-driven engineeringChapter 5 System modeling2

System modeling System modeling is the process of developingabstract models of a system, with each modelpresenting a different view or perspective of thatsystem. System modeling has now come to meanrepresenting a system using some kind of graphicalnotation, which is now almost always based onnotations in the Unified Modeling Language (UML). System modelling helps the analyst to understandthe functionality of the system and models are usedto communicate with customers.Chapter 5 System modeling3

Existing and planned system models Models of the existing system are used during requirementsengineering. They help clarify what the existing system does andcan 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 requirementsengineering to help explain the proposed requirements to othersystem stakeholders. Engineers use these models to discuss designproposals and to document the system for implementation. In a model-driven engineering process, it is possible to generate acomplete or partial system implementation from the system model.Chapter 5 System modeling4

System perspectives An external perspective, where you model the context orenvironment of the system. An interaction perspective, where you model theinteractions between a system and its environment, orbetween the components of a system. A structural perspective, where you model theorganization of a system or the structure of the data thatis processed by the system. A behavioral perspective, where you model the dynamicbehavior of the system and how it responds to events.Chapter 5 System modeling5

UML diagram types Activity diagrams, which show the activities involved in aprocess or in data processing . Use case diagrams, which show the interactions betweena system and its environment. Sequence diagrams, which show interactions betweenactors and the system and between system components. Class diagrams, which show the object classes in thesystem and the associations between these classes. State diagrams, which show how the system reacts tointernal and external events.Chapter 5 System modeling6

Use of graphical models As a means of facilitating discussion about an existingor proposed system– Incomplete and incorrect models are OK as their role is tosupport discussion. As a way of documenting an existing system– Models should be an accurate representation of thesystem but need not be complete. As a detailed system description that can be used togenerate a system implementation– Models have to be both correct and complete.Chapter 5 System modeling7

Context models Context models are used to illustrate the operationalcontext of a system - they show what lies outside thesystem boundaries. Social and organisational concerns may affect thedecision on where to position system boundaries. Architectural models show the system and itsrelationship with other systems.Chapter 5 System modeling8

System boundaries System boundaries are established to define what isinside and what is outside the system.– They show other systems that are used or depend on thesystem being developed. The position of the system boundary has a profoundeffect on the system requirements. Defining a system boundary is a political judgment– There may be pressures to develop system boundaries thatincrease / decrease the influence or workload of differentparts of an organization.Chapter 5 System modeling9

The context of the MHC-PMSChapter 5 System modeling10

Process perspective Context models simply show the other systems in theenvironment, not how the system being developed isused in that environment. Process models reveal how the system beingdeveloped is used in broader business processes. UML activity diagrams may be used to definebusiness process models.Chapter 5 System modeling11

Process model of involuntary detentionChapter 5 System modeling12

Interaction models Modeling user interaction is important as it helps toidentify user requirements. Modeling system-to-system interaction highlights thecommunication problems that may arise. Modeling component interaction helps usunderstand if a proposed system structure is likely todeliver the required system performance anddependability. Use case diagrams and sequence diagrams may beused for interaction modeling.Chapter 5 System modeling13

Use case modeling Use cases were developed originally to supportrequirements elicitation and now incorporated intothe UML. Each use case represents a discrete task that involvesexternal interaction with a system. Actors in a use case may be people or other systems. Represented diagramatically to provide an overviewof the use case and in a more detailed textual form.Chapter 5 System modeling14

Transfer-data use case A use case in the MHC-PMSChapter 5 System modeling15

Tabular description of the ‘Transfer data’ usecaseMHC-PMS: Transfer dataActorsMedical receptionist, patient records system (PRS)DescriptionDataA receptionist may transfer data from the MHC-PMS to ageneral patient record database that is maintained by ahealth authority. The information transferred may eitherbe updated personal information (address, phonenumber, etc.) or a summary of the patient’s diagnosisand treatment.Patient’s personal information, treatment summaryStimulusUser command issued by medical receptionistResponseConfirmation that PRS has been updatedCommentsThe receptionist must have appropriate securitypermissions to access the patient information and thePRS.Chapter 5 System modeling16

Use cases in the MHC-PMS involving the role‘Medical Receptionist’Chapter 5 System modeling17

Sequence diagrams Sequence diagrams are part of the UML and are used tomodel the interactions between the actors and theobjects within a system. A sequence diagram shows the sequence of interactionsthat take place during a particular use case or use caseinstance. The objects and actors involved are listed along the topof the diagram, with a dotted line drawn vertically fromthese. Interactions between objects are indicated by annotatedarrows.Chapter 5 System modeling18

Sequence diagram for View patient informationChapter 5 System modeling19

Sequence diagram for Transfer DataChapter 5 System modeling20

Structural models Structural models of software display theorganization of a system in terms of the componentsthat make up that system and their relationships. Structural models may be static models, which showthe structure of the system design, or dynamicmodels, which show the organization of the systemwhen it is executing. You create structural models of a system when youare discussing and designing the system architecture.Chapter 5 System modeling21

Class diagrams Class diagrams are used when developing an object-orientedsystem model to show the classes in a system and theassociations between these classes. An object class can be thought of as a general definition ofone kind of system object. An association is a link between classes that indicates thatthere is some relationship between these classes. When you are developing models during the early stages ofthe software engineering process, objects representsomething in the real world, such as a patient, a prescription,doctor, etc.Chapter 5 System modeling22

UML classes and associationChapter 5 System modeling23

Classes and associations in the MHC-PMSChapter 5 System modeling24

The Consultation classChapter 5 System modeling25

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’scontext, interactions, structure and behavior. Context models show how a system that is being modeled is positioned inan environment with other systems and processes. Use case diagrams and sequence diagrams are used to describe theinteractions between users and systems in the system being designed. Usecases describe interactions between a system and external actors;sequence diagrams add more information to these by showinginteractions 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 asystem and their associations.Chapter 5 System modeling26

Generalization Generalization is an everyday technique that we useto manage complexity. Rather than learn the detailed characteristics ofevery entity that we experience, we place theseentities in more general classes (animals, cars,houses, etc.) and learn the characteristics of theseclasses. This allows us to infer that different members ofthese classes have some common characteristics e.g.squirrels and rats are rodents.Chapter 5 System modeling27

Generalization In modeling systems, it is often useful to examine the classes in asystem to see if there is scope for generalization. If changes areproposed, then you do not have to look at all classes in the system tosee if they are affected by the change. In object-oriented languages, such as Java, generalization isimplemented using the class inheritance mechanisms built into thelanguage. In a generalization, the attributes and operations associated withhigher-level classes are also associated with the lower-level classes. The lower-level classes are subclasses inherit the attributes andoperations from their superclasses. These lower-level classes then addmore specific attributes and operations.Chapter 5 System modeling28

A generalization hierarchyChapter 5 System modeling29

A generalization hierarchy with added detailChapter 5 System modeling30

Object class aggregation models An aggregation model shows how classes that arecollections are composed of other classes. Aggregation models are similar to the part-ofrelationship in semantic data models.Chapter 5 System modeling31

The aggregation associationChapter 5 System modeling32

Behavioral models Behavioral models are models of the dynamic behavior of asystem as it is executing. They show what happens or what issupposed to happen when a system responds to a stimulusfrom its environment. You can think of these stimuli as being of two types:– Data Some data arrives that has to be processed by thesystem.– Events Some event happens that triggers systemprocessing. Events may have associated data, although thisis not always the case.Chapter 5 System modeling33

Data-driven modeling Many business systems are data-processing systemsthat are primarily driven by data. They are controlledby the data input to the system, with relatively littleexternal event processing. Data-driven models show the sequence of actionsinvolved in processing input data and generating anassociated output. They are particularly useful during the analysis ofrequirements as they can be used to show end-toend processing in a system.Chapter 5 System modeling34

An activity model of the insulin pump’soperationChapter 5 System modeling35

Order processingChapter 5 System modeling36

Event-driven modeling Real-time systems are often event-driven, withminimal data processing. For example, a landlinephone switching system responds to events such as‘receiver off hook’ by generating a dial tone. Event-driven modeling shows how a system respondsto external and internal events. It is based on the assumption that a system has afinite number of states and that events (stimuli) maycause a transition from one state to another.Chapter 5 System modeling37