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IMS1001 – Information Systems 1 CSE1204 - Information Systems 1

IMS1001 – Information Systems 1 CSE1204 - Information Systems 1. Process Modelling Using Data Flow Diagrams - Building and Levelling Them; Process Modelling Using Function Decomposition. Levelling Data Flow Diagrams. any "real" system is too large to represent as a single data flow diagram

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IMS1001 – Information Systems 1 CSE1204 - Information Systems 1

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  1. IMS1001 – Information Systems 1CSE1204 - Information Systems 1 Process Modelling Using Data Flow Diagrams - Building and Levelling Them; Process Modelling Using Function Decomposition

  2. Levelling Data Flow Diagrams • any "real" system is too large to represent as a single data flow diagram • the solution is to decompose the system into a hierarchy of levels of processing • the process model of the system then consists of a set of levelled data flow diagrams • levelling of DFDs improves their readability and usefulness as a communication tool

  3. Levelling of DFDs Levelling creates a hierarchical decomposition of the processing within the system • Context diagram • Level 0 diagram • Level 1 diagrams • Level 2 diagrams . . . • Level n diagrams

  4. Context Diagram • the highest level data flow diagram is the context diagram • the context diagram shows the interaction of the system with its environment in terms of data flows • the context diagram defines the boundary of the system (the scope of the system) • only the data flows which leave the system and the data flows which come from outside the system are shown

  5. Context Diagram • the entire system is represented as a single process • all external agents (sources and sinks) are shown • no data stores are shown: they are inside the boundary of the system

  6. Example Context Diagram Sales Order System sales order Suppliers purchase order goods returned supplier delivery Inventory System goods unavailable notice shipping slip checked supplier invoice Accounts Department Warehouse

  7. Level Zero Diagram • the level zero data flow diagram is the diagram at the level immediately below the context diagram • it "expands" the single process on the context diagram to show the major, high-level processes (or functions) within the system

  8. Level Zero Diagram • all external agents (sources and sinks) are included because the level zero diagram, like the context diagram, represents the entire system • the number of each process ends in .0 which corresponds to the level of the diagram: e.g. 1.0, 2.0, 3.0 etc. often just 1, 2, 3 etc. are used

  9. Example Level Zero Diagram Order Entry System Suppliers purchase order sales order 2.0 Monitor Stock levels 3.0 1.0 supplier delivery Fill Sales Orders Accept Deliveries out of stock notice Stock file shipping slip Accounts Department checked supplier invoice Warehouse

  10. Levelling DFDs Context diagram 1 2 3 1.1 Level zero diagram 1.2 Diagram 1 (at Level 1)

  11. Level 1 diagrams • a set of data flow diagrams is created at Level 1 • there is one Level 1 diagram for each of the processes at Level zero • each diagram decomposes a Level zero process into several processes

  12. Level 1 and lower level diagrams • each Level 1 diagram and diagrams at lower levels (e.g. Level 2, Level 3) show only a part of the processing • Level 1 processes are numbered 1.1, 1.2, 1.3, and 2.1, 2.2, 2.3 etc • that part is shown in more detail than on the Level zero diagram • no external agents are shown on a Level 1 or lower diagrams, as the entire system is not being represented

  13. CUSTOMERS SUPPLIERS Example Level zero diagram p.o. number Customer details Product details p.o. invoice product product delivered qty product qty product holding INVENTORY order qty 1 product holding Record and Invoice Sales product order qty 2 delivery qty Maintain Product Inventory SALES ORDERS customer invoice product order qty

  14. Example Level 1 diagram 1.2 INVENTORY Customer details Check Stock Available 1.1 product holding Check Customer Credit product qty INVOICES product customer credit status available qty SALES ORDERS Product details 1.4 1.3 Create Sales Invoice customer invoice product qty Record Sales Demand

  15. Levelling of DFDs Context Level 0 1 2 3 4 1.3 4.2 Level 1 1.1 1.2 2.1 2.2 3.1 3.2 4.1 4.3 Level 2 1.1.1 1.1.2 3.2.1 3.2.2

  16. Guidelines for Levelling DFDs External communication: • external agents represent entities in the environment of our information system • external agents are outside the scope of our information system • we do NOT model interactions between external agents • we do NOT allow external agents to interact directly with data stores

  17. Guidelines for Levelling DFDs • numbering: when a process is decomposed, its diagram is given the same number as that process • balancing of levelled DFDs: all data flows entering and leaving a process must appear on the corresponding diagram which decomposes that process • external agents: are only included on the two diagrams which represent the entire system, i.e. the context and level zero diagrams

  18. Guidelines for Levelling DFDs the access to data stores across levels of diagrams must be consistent: • the direction of accesses must match and all accesses on higher level diagrams must appear on corresponding lower level diagrams • a data store is first shown on the highest level diagram where it is accessed by more than one process • it can then appear on all lower level diagrams where it is accessed

  19. Guidelines for Levelling DFDs the access to data stores across levels of diagrams must be consistent: 1 1.2 1.1 1.3 2

  20. Guidelines for Levelling DFDs How many levels should be in a set of DFDs? • each diagram usually has between 3 and 7 processes • level the diagrams until bottom level or primitive processes are reached: primitive processeshave only 1 or 2 inputs and outputs, and cannot be further decomposed as a data flow diagram

  21. Guidelines for Levelling DFDs • partition processes to minimise the data flows between them • partition processes to form cohesive, related groups of activities • not all parts of the system may need to be decomposed to the same level

  22. An example - Context Diagram Line Manager Applicant Address Position Spec Applicant name Position Acknow- ledgement Employment System Successful Applicant Payroll System Decision Position

  23. An example - Level zero diagram Applicant Applicant name Line Manager Position Address Ackn’d Appn Position Spec 1 Accept Applic- ation Decision 2 Acknow- ledgement Evaluate Applicants Applicant Position Unsuccessfuls file Evaluation results Payroll System Successful Applicant

  24. Level 1- Diagram 1 Address Applicant name Position Acknow- ledgement Verified Application 1.1 1.2 Verify for Complete- ness Acknow- ledge Application Acknowledged Application

  25. Level 1 - Diagram 2 Ackn’d Appn Position Spec Qualified applicant 2.1 unqualified applicant Screen Applicants Decision 2.2 2.3 Reject Unsuitables Schedule Evaluation Decision Successful Applicant Unsuccessfuls file Evaluation results

  26. Function Decomposition: a Process Modelling Technique • function decomposition is the decomposing of a system into its component functions and processes as a way of managing complexity • function decomposition creates a top-down view: it shows a hierarchy of levels of increasingly detailed processes within a system • a function decomposition model (or diagram) is used to represent the hierarchical decomposition and structure of the processes of a system • the decomposition of functions corresponds to levelling of processes in DFDs

  27. Function Decomposition Diagrams • a function is a high-level set of related activities that are ongoing: a function is a broad, generic activity • a process is a lower level activity that is repeatedly carried out • functions consist of groups of related processes • the depth and scope of function decomposition diagrams depend on the size and complexity of the system represented

  28. Function Decomposition Diagrams • function decomposition diagrams showing only higher levels can be built early in systems analysis • more detailed process decomposition can be carried out as the system is studied in more detail • function decomposition diagrams show the structure of functions and processes within the system • this structure may not correspond to the structure of the organisation shown in an organisation chart See Whitten et al (2001) p 5, p 338 for examples

  29. Function Decomposition Diagrams • each function and process should have a unique name which indicates what it does ABC Sales System 1. Sell Products 2. Manage Inventory 3. Control Finance a top level function decomposition diagram

  30. Example Function Decomposition Diagram ABC Sales System 1. Sell Products 2. Manage Inventory 3. Control Finance 2.1 Deliver Product 2.2 Accept Delivery 2.3 Check Stock levels functions and processes are further decomposed

  31. Function Decomposition the decomposition of functions and processes corresponds to the levelling of DFDs 3.0 1.0 2.0 1.1 1.2 1.2.1 1.2.2

  32. Function Decomposition • function decomposition diagrams are an alternative representation of the hierarchy of functions and processes within a system • they may be built using either a top down or a bottom up approach • they provide a useful overview of the processing within a system

  33. References • WHITTEN, J.L., BENTLEY, L.D. and DITTMAN, K.C. (2001) 5th ed., Systems Analysis and Design Methods, Irwin/McGraw-HilI, New York, NY. Chapters 5, 8 • HOFFER, J.A., GEORGE, J.F. and VALACICH (1999) 2nd ed., Modern Systems Analysis and Design, Benjamin/Cummings, Massachusetts. Chapter 8

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