Ch t 8 Chapter 8. System Models

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1 Ch t 8 Chapter 8. System Models

2 Objectives To explain why the context t of a system should be modelled d as a part of requirements engineering process To describe behavioural modelling, data modelling and object modelling To show how CASE workbenches support system modelling 138

3 System Modelling Helps analysts to understand d the functionality of the system. System models are used to communicate with customers. Different models present the system from different perspectives External perspective : showing the system s context or environment Behavioural perspective : showing the behaviour of the system Structural perspective :showing the system or data architecture System model types Data processing model: showing how the data is processed at different stages Composition model: showing how entities are composed of other entities Architectural model: showing principal sub-systems Classification model: showing how entities have common characteristics Stimulus/response model: showing the system s reaction to events Many ones 139

4 System Context Model System Context (models) are used to illustrate the operational context of a system Showing what lies outside the system boundaries Showing the system and its relationship with other systems Social and organizational concerns may affect the decision of system boundaries. System Context Model for ATM Branch Accounting System Security System Auto-Teller System Account Database Branch Counter System Maintenance System Usage Database 140

5 Process Model Process models show the overall process supported by the system. Specify equipment required Equipment Spec. Validate specification Checked Spec. Get cost estimate Delivery Note Accept delivery of equipment Delivery Note Check delivery items Equipment Spec. Supplier list Spec. + Supplier + estimate Order Notification Installation Instructions Supplier Database Find supplier Choose Supplier Place equipment order Install equipment Equipment procurement process Order Details and Blank Order Checked and Form Signed Order Form Installation Acceptance Accept delivered equipment Equipment Details Equipment Database 141

6 Behavioural Model Behavioural models are used dto describe the overall llbehaviour of the system. Data processing models : showing how data is processed as it moves through the system State machine models : showing how the system responses to events Two models show different perspectives. Both of them are required to describe the system s behaviour. 142

7 Data Processing Model Data flow diagrams(dfds) are used to model the system s data processing. Show the processing steps as data flows through a system Use simple and intuitive notation that customers can understand Show end-to-end processing of data Order processing DFD Checked and singed Order + Order details + Blank order form Complete order form Equipment Spec. Completed order form Validate order Signed order form Record order Od Order details Signed order form Signed order form Send to supplier Adjust available budget Order amount + Account details Order notification Order file Budget file 143

8 State Machine Model State machine models model the behaviour of the system in response to external and internal events. Show the system s responses to stimuli Often used for modelling real-time systems Show system states as nodes and events as arcs between these nodes Full pow er Full power do: set power = 600 Waiting do: display time Half power Half power Full power Half power do: set power = 300 Timer Timer Set time do: get number exit: set time Door open Door closed Door closed Number Enabled do: display 'Ready' Start Operation do: operate oven Door open Cancel Waiting do: display time State machine mode for Microwave model Disabled do: display 'Waiting' 144

9 Semantic Data Model Semantic data models are used to describe the logical l structure of data processed by the system. Entity-relation-attribute model : setting out the entities in the system, relationships between these entities, and the entity attributes t Widely used in rational database design Library semantic model delivers Article title authors pdf file fee 1 n Order order number total payment date tax status n places 1 Buyer name address billing info m 1 published-in fee-payable-to Copyright Agency name has-links address 1 in n 1 1 Source title publisher issue date pages 1 in 1 Country copyright form tax rate 145

10 Object Model Object models describe the system in terms of object classes and their associations. An object class is an abstraction over a set of objects with common attributes and dthe services (operations). Object classes are reusable across systems. Various object models Inheritance model Aggregation model Interaction model 146

11 Inheritance Model Inheritance models organize domain object classes into a hierarchy. h Classes at the top of the hierarchy reflect common features of all classes. Object classes inherit their attributes and services from one or more super- classes. User class hierarchy Library user Name Address Phone Registration # Register () De-register () Reader Affiliation Borrower Items on loan Max. loans Staff Department Department phone Student Major subject Home address 147

12 Multiple Inheritance Multiple l inheritance i allows object classes to inherit i from several superclasses. May lead to semantic conflicts where attributes/services with the same name in different super-classes have different semantics Make class hierarchy reorganisation more complex Book Author Edition Publication date ISBN Voice recording Speaker Duration Recording date Talking book # Tape 148

13 Object Aggregation Model Aggregation models show how classes are composed of other classes. Similar to the part-of relationship in semantic data models Study pack Course title Number Year Instructor Assignment Credits OHP slides Slides Lectur e notes Text Videotape Tape ids. Exercises #Problems Description Solutions Text Diagrams 149

14 Object Behaviour Model Object behavioural models show the interactions i between objects To produce some particular system behaviour specified as in use-cases Called interaction model Sequence diagrams (or collaboration diagrams) in the UML Ecat: Catalog :Library Item Lib1: NetServer :Library User Lookup Display Issue Issue licence Accept licence Compress Deliver 150

15 Structured Method Structured methods incorporate system modelling as an inherent part of the method. Structured methods define a set of models a process for deriving these models rules and guidelines that should apply to the models CASE tools to support system modelling CASE Workbench: A coherent set of tools that is designed to support related software process activities such as analysis, design or testing. Analysis and design workbenches support system modelling during both requirements engineering and system design. May support a specific design method May support to create several different types of system model 151

16 Analysis and Design Workbench: An example Data Dictionary Structural Diagramming Tools Report Generation Facilities i Code Generator Central Information Repository Query Language Facilities Form Creation Tools Design, Analysis and Checking tools Import/Export Facilities 152

17 Summary A model is an abstract t system view. Complementary types of model provide different system information. Context models show the position of a system in its environment with other systems and processes. Data flow models are used to model the data processing in a system. State machine models model the system s behaviour in response to internal or external events. Semantic data models describe the logical structure of data which is imported to or exported by the systems. Object models describe logical system entities, their classification and aggregation. Sequence models show the interactions between actors and the system objects that they use. Structured methods provide a framework for developing system models. 153

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System models Abstract descriptions of systems whose requirements are being analysed. System modelling. Structured methods

System models Abstract descriptions of systems whose requirements are being analysed Ian Sommerville 995/2000 (Modified by Spiros Mancoridis 999) Software Engineering, 6th edition. Chapter 7 Slide System