Process Modelling: Introduction to Data Flow Modelling

1. Process Modelling:Introduction to Data Flow Modelling Information System AnalysisCOMM1B

2. Outline • Is the existing system important? • How do we describe an existing system? • Problems with narrative • Data Flow Diagrams

3. How important is the existing system? It can been argued: “If we analyse the existing system we will eventually end up with a computerised version of the existing system with all of its problems and weaknesses”

4. Current system model • Why bother? • Occasionally we do not • Promotes understanding • Provides an accurate and complete view of the existing system • Can reveal weaknesses and identify strengths • Can help identifies information requirements for new system • Examination of the whole picture can enable discussion between developers and users

5. How do we describe an existing system? Narrative description • Can be ambiguous • Can be long-winded • Can be incomplete Data Flow Modelling (DFM) • Diagrammatic • Readable • Even by those without knowledge of the technique

6. Example: Data Flow Diagram Level 1 Customer 2 Despatch Delivery note Package Order Order goods acknowledgement & despatch * 1 Sales Enter Orders Copy delivery Despatch * note instruction set Weekly credit limit list Internal order form Accounts Goods received note 3 Warehouse Accounts Goods received Control stock note Re-order card Supplier Order Processing System

7. Data Flow Diagrams • Provide a complete model of the information system showing: • View of system focusing on its data • Where it arrives from • external sources • What the system does to it • processing • Where it is kept • storage • Who receives it • What it is used for

8. Data Flow Diagrams • A major generic technique • Powerful and useful • Not just in computer based systems development • Notation • There are a number of variations in symbols used (esp. methods used in USA) • Yourdon • Gane and Sarson • We use SSADM notation • All share exactly the same construction

9. Data Flow Diagram Symbols A data flow A process A data store

10. Data Flow Diagrams an external entity a physical resource flow (rarely used, never in logical DFDs)

11. Data Flow Diagram Symbols Labelling: Order Details A data flow Location ID A process Name M1 A data store

12. Data Flow Diagram Symbols Description (include a ‘doing’ verb) A process b an external entity Name

13. Example • Following example shows a Data Flow Model of an order processing system • An initial document flow diagram can be used as a starting point • Helps define the boundaries of the system and therefore the agencies which are external

14. Document Flow Customer Delivery note Despatch Order Order acknowledgement Sales Copy delivery Weekly credit limit list Despatch note instruction set Internal order form Goods received Accounts note Warehouse Accounts Goods received note Re-order card Supplier Order Processing System

15. Document Flow Customer Delivery note Despatch Order Order acknowledgement Sales Copy delivery Weekly credit limit list Despatch note instruction set Internal order form Goods received Accounts note Warehouse Accounts Goods received note Re-order card Supplier Order Processing System Boundary

16. Context Diagram (level 0) Accounts Customer Goods received note Copy delivery note Delivery note Re-order card Weekly credit limit list Order Order acknowledgement Order Processing System Goods received note Order Processing System Supplier

17. Customer 2 Despatch Delivery note Package Order Order goods acknowledgement & despatch * 1 Sales Enter Orders Copy delivery Despatch * note instruction set Weekly credit limit list Internal order form Accounts Goods received note 3 Warehouse Accounts Goods received Control stock note Re-order card Supplier Level 1Data Flow Diagram Order Processing System

18. Despatch 2 Sales 1 Package goods Order & despatch Entry * * Despatch Internal order instruction set form Accounts 3 Control stock Re-order card 3.1 3.2 Stock Clerk Typist Internal order Type dispatch instruction sheet form Allocate stock to order * * read & update stock M3/1 Stock stock additions 3.3 W'h men Goods Rec 3.4 stock level Check goods Monitor receipt note stock to stock recei- levels * Re-order card Goods received note Goods received note Accounts Accounts Supplier Level 2 DFD:Control Stock

19. Level 3 DFD:Monitor Stock Levels

20. Components of a Data Flow Model • Hierarchic structure • At highest level shows an overview of the system • Level 1 is the most important • Gradually refined into further detail • Level 2, 3 etc • Until system processing is described in the utmost detail: • Elementary process descriptions

21. Components of a Data Flow Model Level 0 Agency Overall process Agency

22. Components of a Data Flow Model Level 0 Level 1 Agency 1 Process 2 Overall process Process 3 Agency Process

23. Components of a Data Flow Model Level 0 Level 1 Level 2 2 Agency 1 2.1 sub Process Process 2 2.2 Overall process sub Process Process 3 2.3 sub Process Agency Process

24. Components of a Data Flow Model Level 0 Level 1 Level 2 Level 3 2 2.2 Agency 1 2.1 2.2.1 sub Process sub-sub Process Process 2 2.2 2.2.2 Overall process sub Process sub-sub Process Process * 3 2.3 2.2.3 sub Process sub-sub Process Agency Process

25. Components of a Data Flow Model Level 0 Level 1 Level 2 Level 3 Level 4 2 2.2 2.2.2 Elementary Process Description Decision Tree Decision Table Structured English Agency 1 2.1 2.2.1 sub Process sub-sub Process Process 2 2.2 2.2.2 Overall process sub Process sub-sub Process Process * 3 2.3 2.2.3 sub Process sub-sub Process Agency Process

26. Basic Rules Data flow lines must NOT cross each other Data MUST pass through a process not between external entities nor between data stores nor external entity to data store (or vice-versa) Process descriptions MUST contain a verb and describe what is happening to DATA They must be concise ie. not a list of sub-processes Data Flow Diagrams

27. Basic Rules Data stores used by only one process are internal to that process and are not shown in that level diagram A guideline is max. of six processes in one diagram at a particular level You will see examples of eight or more level 1 processes - this is not good practice too much detail difficult to read Key features - clarity, simplicity, completeness Data Flow Diagrams

28. Basic Rules DFDs are not Systems Flowcharts (an older technique used to describe processing) and do NOT show: the passage of time or sequence of processing operations Although it is common practice to label and draw processes in approximate order for clarity and sense therefore order is sometimes implicit, but not explicit Data Flow Diagrams

29. Data Flow Diagrams: Summary Powerful modelling tool used at: • Analysis phase • Physical representation of current system • Transformed into Logical view of current system • by removing physical circumstances • Design phase • Logical view of required system • by adding requirements. • Physical representation of required system • CASE tools are often used in their construction

30. Further Reading • Lejk and Deeks • An Introduction to Systems Analysis Techniques • Chapter 4 Introduction to Data Flow Diagrams • M Goodland and C Slater • SSADM version 4: a Practical Approach • McGraw Hill 1995 • Weaver, Lambrou and Walkley • Practical SSADM version 4+, 2nd Edition • Chapter2 • Pitman Publishing 1998