Mypresentation
- 1. Information Systems Development Methods Jackie Croft Room N5.107 croftj@wmin.ac.uk ISDM11 Lecture 3 Structured Methods 1
- 2. Information Systems Development Methods Structured Methods ISDM11 Lecture 3 Structured Methods 2
- 3. Pre-methodology Problems? ISDM11 Lecture 3 Structured Methods 3
- 4. The Systems Development Life Cycle - Waterfall Strategy Planning Feasibility Analysis Design Implementation Maintenance ISDM11 Lecture 3 Structured Methods 4
- 5. Strategy Planning Formal mechanism for deciding which areas of the business require new or enhanced computer systems Involves assessing the relative priorities of different areas, with a view to initiating one or more development projects. ISDM11 Lecture 3 Structured Methods 5
- 6. Feasibility Study Establish the feasibility of potential systems ideas from strategy planning Look at economic, technical and operational feasibility (see Kendall and Kendall) ISDM11 Lecture 3 Structured Methods 6
- 7. Systems Analysis Establish the requirements of users, and hence of the business Concentrate on what it should deliver, rather than how it should deliver it ISDM11 Lecture 3 Structured Methods 7
- 8. System Design Translate the user requirements gathered during systems analysis into a computer system design Detail exactly how the requirements will be satisfied ISDM11 Lecture 3 Structured Methods 8
- 9. Implementation The system design provides a blueprint for building, testing and introducing the new system Programs are constructed and hardware is installed Provide training for users and assistance in cutting over to the new system ISDM11 Lecture 3 Structured Methods 9
- 10. Maintenance Production or operational phase The period when the system is up and running in support of the business The system needs to be kept up to date in responding to changing requirements and system errors Uses 70% of the total development effort required over the life of a system ISDM11 Lecture 3 Structured Methods 10
- 11. Structured Methods Structured methods consist of: 1 A default structure of steps and tasks which the project team should consider following 2 A set of techniques to be applied in each step that provide (largely diagrammatic) structured definitions of user requirements and system components. ISDM11 Lecture 3 Structured Methods 11
- 12. Why are Structured Methods Desirable? Diagrams!!! Improve communications - diagrams Reduce ambiguity - diagrams ISDM11 Lecture 3 Structured Methods 12
- 13. Advantages of Structured Methods Structured methods use the following core concepts: Abstraction Diagrammatic modelling techniques User involvement ISDM11 Lecture 3 Structured Methods 13
- 14. Abstraction Simplifies the area under study Concentrates on certain aspects while disregarding others Look at the physical and logical (conceptual) levels separately Can then consider the organisational aspects ISDM11 Lecture 3 Structured Methods 14
- 15. Physical and Logical Levels of Abstraction (Tudor and Tudor) Existing Physical Required Physical Required Logical Existing Logical User/business Requirements ISDM11 Lecture 3 Structured Methods 15
- 16. Physical, Logical and Organisational levels of Abstraction (Tudor and Tudor) Required Physical (Operational System) Existing Physical System Required Organisational Required Logical Existing Logical (Conceptual, Essential) (Conceptual, Essential) User/business Requirements User/business Requirements ISDM11 Lecture 3 Structured Methods 16
- 17. Three-Schema Architecture Conceptual Model – required logical data and processes External Design – required system DFDs, functions, menus and dialogues Internal Design – physical database and physical process design All contribute to the construction ISDM11 Lecture 3 Structured Methods 17
- 18. Diagrammatic Modelling Techniques Models are produced for each level of abstraction Can produce diagrams to model processes, data and events ISDM11 Lecture 3 Structured Methods 18
- 19. Diagrams Can model: Data Processes Events System boundary Physical and logical ISDM11 Lecture 3 Structured Methods 19
- 20. The Development of Structured Methods Structured programming Structured design Structured analysis – the process view Structured analysis – the data view Structured analysis – the event view Blended methods ISDM11 Lecture 3 Structured Methods 20
- 21. Structured Programming Dijkstra (1965) suggested that the greatest single problem was the ‘GOTO’ statement Bohm and Jacopini (1966) proved that any program could be written using three basic constructs: sequence, selection and iteration ISDM11 Lecture 3 Structured Methods 21
- 22. Structured Design Advantages: ‘divide and conquer’ approach using a modular approach, several developers can work on a project information hiding cohesion coupling ISDM11 Lecture 3 Structured Methods 22
- 23. Structured Analysis Emphasis shifted to an earlier part of the SDLC Uses DFDs, ERDs, Data Dictionary See: DeMarco (1978) ISDM11 Lecture 3 Structured Methods 23
- 24. Analysis Considered More emphasis on the systems analysis and design phases of the SDLC Roots in process flow diagrams used in industrial engineering More emphasis on systems for the organisation as a whole ISDM11 Lecture 3 Structured Methods 24
- 25. Process Driven Methods Problem: Processes are quite volatile and liable to change ISDM11 Lecture 3 Structured Methods 25
- 26. Modelling Processes Early structured methods (DeMarco, Youdon) concentrated on modelling the processes and the data flowing between them DFDs originated with engineering flow diagrams ISDM11 Lecture 3 Structured Methods 26
- 27. Processes – Context Diagram a Supplier b Purchaser Payment Delivery Note Purchase Order Rejected P.O. Copy #2 Delivery Invoice Details Matched P.O. Copy #2 e d SRW Matched Invoice Depot Despatch Note Accounts Customer System Stock Report Matched C.O. Customer Order Copy #1 P.O.Quantities b Customer Order c Sales and Purchaser Marketing ISDM11 Lecture 3 Structured Methods 27
- 28. DFD a b c Sales and Purchaser Supplier Marketing Purchase Order Rejected P.O. Copy #2 P.O.Quantities Delivery Details Customer Order 1 P.O.Clerk Place Product Product Details M3 and Monitor Info Orders 5 Despatch Clk f Customer 2 x C.O. Copies Despatch Report 2 x Rejected M4 Allocate Despatch P.O.Copies Orders Despatch Supervisor Rejected P.O. Copy #1 2 x P.O. Copies Stock To Current b Be Used Stock Levels T1 Rejected (M) P.O.'s Supplier 2 x C.O. Copies M2 Stock Matched Matched Despatch Rpt 2 x Rejected Despatch Rpt Purchase M1 Delivery Note P.O.Copies Orders d 2 Goods In 5 Despatch Clk Customer 2 x P.O. Copies Despatch Note Check Purchase Complete M1 Delivery Orders Customer Order Matched C.O. c Copy #1 Sales and Invoice Copy Marketing Matched P.O. Copy #1 Matched P.O. Matched C.O. Copy #2 Copy #2 T2 Matched P.O. Copy #1 P.O. Copy #1 (M) P.O.'s a 7 P.O.Clerk e Product Matched P.O. M3 Matched Invoice Info Match Purchaser Copy #1 Supplier Accounts Invoice Product Info 3 Stock Dept Store New Stock M2 Stock New Stock Stock Report Invoice Stock Info Adjustment 4 Stock Clerk f b Maintain Despatch Despatch Stock Rpt Copy Supplier Supervisor Information ISDM11 Lecture 3 Structured Methods 28
- 29. BAM with Potential System Boundary Receive Customer Order Arrange Despatch Details Outside Forward Despatch Contents Assemble Goods for Despatch ISDM11 Lecture 3 Structured Methods 29
- 30. Main Methods: Process-Driven: Largely superseded by data-driven and mixed methodologies as databases assumed a more prominent role. Look at work by DeMarco, Gane and Sarson, Yourdon and Constantine ISDM11 Lecture 3 Structured Methods 30
- 31. Data Driven Approaches Early methods were process-oriented (computers were able to carry out functions long before they were able to implement a database) The seventies and eighties saw work on relational databases (Codd, Chen etc) (See Codd, 1970 onwards; Chen, 1976; Jackson, 1975, 1983; Martin and Finkelstein, 1981; Orr, 1977; Warnier, 1976 etc) ISDM11 Lecture 3 Structured Methods 31
- 32. Data Driven Approaches The eighties saw the rise of methods based on data modelling Information Engineering ISDM11 Lecture 3 Structured Methods 32
- 33. Modelling Data The structure of the data is important and often more stable than the processing E-R Modelling, (data modelling) techniques were introduced ISDM11 Lecture 3 Structured Methods 33
- 34. Data Transfer Delivery Line made of delivery of delivered by part of delivered by transfer of Transfer Line Stock reduced by transfer from ISDM11 Lecture 3 Structured Methods 34
- 35. Summary of Process-Driven and Data-Driven Perspectives Most structured methods are either process-driven or data-driven although some have aspects of both and a few also incorporate a time dimension Structured approach is better suited to business data processing than real time systems development ISDM11 Lecture 3 Structured Methods 35
- 36. The Event View So far the behaviour or state perspective has been ignored Introduction of the State Transition Diagram to allow for a behaviour or state perspective ISDM11 Lecture 3 Structured Methods 36
- 37. Integrative Approaches Sometimes known as blended SSADM and JSD take a combined view of data, process and events SSADM uses Entity Life Histories ISDM11 Lecture 3 Structured Methods 37
- 38. Event Modelling Event modelling (and the effect of the event on the data) became increasingly important State-oriented approach (after and before states) Command-oriented approach (procedural description of the command Interaction-oriented approach (rules and constraints controlling the interaction between events, processing and data ISDM11 Lecture 3 Structured Methods 38
- 39. Event Purchase Order Line assumed case alternative case Q3 Q1 Q4 Q2 Delivery Confirmation (first) Q4 Q2 Q3 9 1,3/4 Delivery Ad Hoc Purchase Confirmation events Order Raised (subsequent) R1 R4 R2 R3 1 2 3 4 5 6 7 8 9 Q4 Q2 Q1 Delivery -/1 4,5/5 Purchase Order Supplier Ceased Out of Time Confirmation Cancellation Trading Purchase Order Purchase Order (last) Proposal Confirmation 1,2,3/6 1-5/7 1-5/8 1,3,4,5 1 2 3 4 5 6 7 8 -/2 2/3 ISDM11 Lecture 3 Structured Methods 39
- 40. Another view of event Delivery Number Delivery Date Supplier Number Depot Number Purchase Order Number Supplier Depot Delivery Start Time Delivery End Time Product Number Quantity Due Delivery Set of Delivery Line Purchase Order Delivery Line Line Purchase Order Line Purchase Order Line Purchase Order Line (subsequent) (first) (last) ISDM11 Lecture 3 Structured Methods 40
- 41. Factors Modelled by Structured Techniques An event in the outside world triggers a process. This causes and effect on data in a given state, and may cause the data to be transformed to a different state. ISDM11 Lecture 3 Structured Methods 41
- 42. Techniques Wide range of techniques – some linked to a specific method, eg SSADM Many ways of drawing some of the diagrams – LDS, ERD etc Some techniques involve description rather than diagrams ISDM11 Lecture 3 Structured Methods 42
- 43. More Techniques Think about what you are trying to model Requirements Processes Data Events Which diagram is clearer or more specific Is a particular notation mandated by your chosen method ISDM11 Lecture 3 Structured Methods 43
- 44. User Involvement Different structured methods differ in the amount of user involvement Sometimes the user involvement is not explicit ISDM11 Lecture 3 Structured Methods 44
- 45. How Do Methods Differ? Life-cycle coverage Underlying philosophy User role ‘Structuredness’ Size of system aimed at Techniques within the method CASE tool support ISDM11 Lecture 3 Structured Methods 45
- 46. Poor Quality Still!!!!! Information system does not address the right problem Wider social or psychological problems are missed Information needs not recognised, ignorance of what may be possible System developed for the wrong reasons - technological push, or political push ISDM11 Lecture 3 Structured Methods 46
- 47. Read: Kendall and Kendall, Systems Analysis and Design, latest edition Tudor and Tudor; Systems Analysis and Design A Comparison of Structured Methods, Palgrave, 1997 Dijkstra, E. W. (March 1968). "Letters to the editor: go to statement considered harmful". Communications of the ACM 11 (3): 147–148 ISDM11 Lecture 3 Structured Methods 47
- 48. Read……… Bohm and Jacopini (1966), "Flow Diagrams, Turing Machines, and Languages with Only Two Formation Rules," Communications of the ACM 9:5, p. 266, May 1966. DeMarco, Tom. Structured Analysis and System Specification. Yourdon, Gane and Sarson, Yourdon and Constantine etc Codd, 1970 onwards work on relational databases ISDM11 Lecture 3 Structured Methods 48
- 49. Read…….. Chen (ER Model) Michael Jackson, Jackson Structured Programming, Jackson Structured Development Martin, James and Clive Finkelstein. Nov 1981. "Information Engineering", Technical Report, two volumes, Lancs, UK : Savant Institute, Carnforth. ISDM11 Lecture 3 Structured Methods 49
- 50. Read….. Kenneth T. Orr, Structured Systems Development, Prentice Hall PTR ; Weaver, Lambrou and Walkley. Practical Business Systems Development Using SSADM, Prentice Hall, 3rd ed, 2002 ISDM11 Lecture 3 Structured Methods 50