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Chapter 8 - System Models

Chapter 8 - System Models. Instructor : Haya Sammaneh. System Models is:. Abstract descriptions of systems whose requirements are being analyzed. Objectives. To explain why the context of a system should be modelled as part of the RE process

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Chapter 8 - System Models

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  1. Chapter 8 - System Models Instructor : Haya Sammaneh

  2. System Models is: • Abstract descriptions of systems whose requirements are being analyzed

  3. Objectives • To explain why the context of a system should be modelled as part of the RE process • To describe behavioural modelling, data modelling and object modelling • To introduce some of the notations used in the Unified Modeling Language (UML) • To show how CASE workbenches support system modelling

  4. Topics covered • Context models • Behavioural models • Data models • Object models • CASE workbenches

  5. System modeling • System modeling helps the analyst to understand the functionality of the system, and models are used to communicate with customers (blueprint to a kitchen or 3D model to see where every element of the kitchen exists and how it relates to others) • Different models present the system from different perspectives • External perspective showing the system’s context or environment • Behavioral perspective showing thebehavior of the system • Structural perspective showing the system or data architecture

  6. Examples on Model types (which may produced during the analysis process) • Data processing model showing how the data is processed at different stages (DFD). • Composition(aggregation) model showing howentities are composed of other entities (E-R). • Architectural model showing principal sub-systems

  7. Model types (which may produced during the analysis process) , cont…. • Classification model(object class/inheritance diagram) showing how entities have common characteristics • Stimulus-response model (state transition diagram) showing the system’s reaction to events

  8. 8.1 Context models • Context models are used to illustrate the boundaries of a system • In some cases, the boundary between a system and its environment is relatively clear. • Architectural models show the a system and its relationship with other systems

  9. The context of an ATM system

  10. Process models • Process models show the overall process and the processes that are supported by the system • Data flow models may be used to show the processes and the flow of information from one process to another • The following slide illustrate a process model for procuring equipment in an organization. • This involves specifying the equipment required, finding and choosing a supplier, ordering the equipment,taking delivery, and testing the equipment.(dotted lines for the system boundary).

  11. Process (activity) model

  12. 8.2 Behavioral models • Behavioral models are used to describe the overall behavior of a system • Two types of behavioral model are shown here: • Data Flow models (DFD) that show how data is processed as it moves through the system • State machine models (STD) that show the systems response to events • Both of these models are required for a description of the system’s behavior

  13. Data-processing models (DFD) • Data flow diagrams are used to model the system’s data processing • By nature it is part of many analysis methods • Simple notation that customers can understand • Show end-to-end processing of data • Four symbols only for DFD, Source/Sink (Square), Data flow(arrow), process (oval), and Data store (rectangle)

  14. Order processing DFD

  15. 8.2.1 Data flow diagrams • DFDs model the system from a functional perspective • Tracking and documenting how the data associated with a process is helpful to develop an overall understanding of the system • Data flow diagrams may also be used in showing the data exchange between a system and other systems in its environment • DFD do not try to represent conditional processing or loops in it, it is simply show the flow of data

  16. Insulin pump DFD

  17. 8.2.2 State machine models • These models are used to model the behavior of the system in response to external and internal events • They show the system’s responses 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 (essential) part of the UML

  18. State machine Model of Microwave oven ,initial state then transition to another state when receive the command

  19. Microwave oven state description

  20. Microwave oven

  21. The problem with the state machine approach is that the number of possible states increases rapidly. So some structuring of these state models is necessary. One way to do this is by using the notion of a super state (looks like a single state) which encapsulate a number of separate states.

  22. Super State-charts • Allow the decomposition of a model into sub-models (see next slide) • A brief description of the actions is included following the ‘do’ in each state • Can be complemented by tables describing the states

  23. Microwave oven operation

  24. 8.3 Semantic data models(E-R Models) • Used to describe the logical structure of data processed by the system • Entity-relation-attribute model sets out the entities in the system, the relationships between these entities and the entity attributes • Widely used in database design.

  25. LibrarySemantic data (a.k.a. E-R) model

  26. Data dictionaries • Data dictionaries are lists of all of the names used in the system models. Descriptions of the entities,relationships attributes, and services are also included • Advantages of data dictionary • Support name management and avoid duplication • Store oforganizational information that canlink analysis, design, implementation and evolution. (all In one place) • Many CASE workbenches support data dictionaries

  27. Data dictionary entries

  28. 8.4 Object models • Object models describe the system in terms of object classes, if they are developed during requirements analysis, they would simplify the transition to object oriented design and programming. • An object class is an abstraction over a set of objects with common attributes and the services (operations) provided by each object • Various object models may be produced • Inheritance models- how object are related to each other • Aggregation models- how object form other object • Interaction models- how object interact with each other

  29. Object models • Natural ways of reflecting the real-worldentities manipulated by the system, such entities as cars,aircraft,books… etc • More abstract entities are more difficult to model using this approach, such library, medical record systems. • Object class identification is recognized as a difficult process requiring a deep understanding of the application domain

  30. Object models , cont… • Object classes reflecting domain entities are reusable across systems

  31. Advice: One of the first important thing to think about when building an OO system is how to classify the objects that are to be manipulated by the system.

  32. 8.4.1 Inheritance models • Organize the domain object classes into ahierarchy, it is not an easy task,so analyst has to understandindetail the domain in which the system is to be installed. • Classes at the top of the hierarchy reflect the common features of all classes • Object classes inherit their attributes and services from one or more super-classes, these may then be specialized as necessary

  33. Object models and the UML • The UML is a standard representation devised by the developers of widely used object-oriented analysis and design methods. • It has become an effective standard for object-oriented modelling. • Notation • Object classes are rectangles with the name at the top,attributes in the middle section and operations in the bottom section; • Relationships between object classes (known as associations) are shown as lines linking objects; • Inheritance is referred to as generalisation and is shown ‘upwards’ rather than ‘downwards’ in a hierarchy.

  34. State Diagrams State Diagrams State Diagrams State Diagrams State Diagrams State Diagrams Class Diagrams Object Diagrams Component Diagrams Component Diagrams Component Diagrams Deployment Diagrams Use Case Diagrams Use Case Diagrams Scenario Diagrams Scenario Diagrams Use Case Diagrams Use Case Diagrams Scenario Diagrams Scenario Diagrams Sequence Diagrams Use Case Diagrams Statechart Diagrams Collaboration Diagrams Models Activity Diagrams The Unified Modelling Language,UML • Has nine different notations

  35. Library class hierarchy

  36. User class hierarchy

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