Process Modelling Introduction to Data Flow Modelling Information

Process Modelling: Introduction to Data Flow Modelling Information System Analysis COMM 1 B University of Sunderland COMM 1 B Information Systems Analysis Unit 7

Outline • • Is the existing system important? How do we describe an existing system? Problems with narrative Data Flow Diagrams University of Sunderland COMM 1 B Information Systems Analysis Unit 7

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” University of Sunderland COMM 1 B Information Systems Analysis Unit 7

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 University of Sunderland COMM 1 B Information Systems Analysis Unit 7

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 University of Sunderland COMM 1 B Information Systems Analysis Unit 7

Example: Data Flow Diagram Level 1 Customer 2 Delivery note Order Package goods & despatch Order acknowledgement 1 Despatch * Sales Enter Orders Weekly credit limit list Internal order form Accounts Goods received note 3 Goods received note Supplier University of Sunderland Copy delivery note Despatch instruction set * Warehouse Accounts Control stock Re-order card Order Processing System COMM 1 B Information Systems Analysis Unit 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 University of Sunderland COMM 1 B Information Systems Analysis Unit 7

Data Flow Diagrams • A major generic technique – Powerful and useful – Not just in computer based systems development • Notation – There 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 University of Sunderland COMM 1 B Information Systems Analysis Unit 7

Data Flow Diagram Symbols A data flow A process A data store University of Sunderland COMM 1 B Information Systems Analysis Unit 7

Data Flow Diagrams an external entity a physical resource flow (rarely used, never in logical DFDs) University of Sunderland COMM 1 B Information Systems Analysis Unit 7

Data Flow Diagram Symbols Labelling: Order Details ID A data flow Location A process M 1 Name University of Sunderland A data store COMM 1 B Information Systems Analysis Unit 7

Data Flow Diagram Symbols Description (include a ‘doing’ verb) A process b Name University of Sunderland an external entity COMM 1 B Information Systems Analysis Unit 7

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 University of Sunderland COMM 1 B Information Systems Analysis Unit 7

Document Flow Customer Delivery note Despatch Order acknowledgement Sales Weekly credit limit list Copy delivery note Despatch instruction set Internal order form Goods received note Accounts Goods received note Warehouse Accounts Re-order card Supplier University of Sunderland Order Processing System COMM 1 B Information Systems Analysis Unit 7

Document Flow Customer Delivery note Despatch Order acknowledgement Sales Weekly credit limit list Despatch instruction set Internal order form Goods received note Accounts Goods received note Copy delivery note Warehouse Accounts Re-order card Supplier University of Sunderland Order Processing System Boundary Unit 7 COMM 1 B Information Systems Analysis

Context Diagram (level 0) Accounts Customer Copy delivery note Delivery note Weekly credit limit list Order Goods received note Re-order card Order acknowledgement Order Processing System Goods received note Order Processing System University of Sunderland Supplier COMM 1 B Information Systems Analysis Unit 7

Level 1 Data Flow Diagram Customer 2 Delivery note Order Package goods & despatch Order acknowledgement 1 Despatch * Sales Enter Orders Weekly credit limit list Internal order form Accounts Goods received note 3 Goods received note Supplier University of Sunderland Copy delivery note Despatch instruction set * Warehouse Accounts Control stock Re-order card Order Processing System COMM 1 B Information Systems Analysis Unit 7

Level 2 DFD: Control Stock 2 Sales 1 Despatch Package goods & despatch Order Entry * Despatch instruction set Internal order form Accounts Re-order card 3 * Control stock 3. 1 Stock Clerk Allocate stock to order 3. 2 Typist Type dispatch instruction sheet Internal order form * * read & update stock M 3/1 3. 3 W'h men Stock stock level Monitor stock levels * Re-order card Goods received note University of Sunderland Accounts stock additions 3. 4 Goods Rec Check goods receipt note to stock recei- Goods received note Supplier COMM 1 B Information Systems Analysis Unit 7

Level 3 DFD: Monitor Stock Levels University of Sunderland COMM 1 B Information Systems Analysis Unit 7

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 University of Sunderland COMM 1 B Information Systems Analysis Unit 7

Components of a Data Flow Model Level 0 Agency Overall process Agency University of Sunderland COMM 1 B Information Systems Analysis Unit 7

Components of a Data Flow Model Level 0 Level 1 Agency 1 Process Overall process 2 Process 3 Agency University of Sunderland Process COMM 1 B Information Systems Analysis Unit 7

Components of a Data Flow Model Level 0 Level 1 Level 2 2 Agency 1 2. 1 Process Overall process 2 Process 3 Agency University of Sunderland Process sub Process 2. 2 sub Process 2. 3 sub Process COMM 1 B Information Systems Analysis Unit 7

Components of a Data Flow Model Level 0 Level 1 Level 2 2 Agency 1 2. 1 Process Overall process 2 Process 3 Agency University of Sunderland Process Level 3 2. 2. 1 sub Process sub-sub Process 2. 2. 2 sub-sub Process 2. 3 sub Process 2. 2. 3 sub-sub Process COMM 1 B Information Systems Analysis * Unit 7

Components of a Data Flow Model Level 0 Level 1 Level 2 2 Agency 1 2. 1 Process Overall process 2 Process 3 Agency University of Sunderland Process Level 3 Level 4 2. 2. 1 sub Process sub-sub Process 2. 2. 2 sub-sub Process 2. 3 sub Process 2. 2. 3 sub-sub Process COMM 1 B Information Systems Analysis Elementary Process Description * Decision Tree Decision Table Structured English Unit 7

Data Flow Diagrams 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 University of Sunderland COMM 1 B Information Systems Analysis Unit 7

Data Flow Diagrams 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 University of Sunderland COMM 1 B Information Systems Analysis Unit 7

Data Flow Diagrams 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 University of Sunderland COMM 1 B Information Systems Analysis Unit 7

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 University of Sunderland COMM 1 B Information Systems Analysis Unit 7

Further Reading • Lejk and Deeks – An Introduction to Systems Analysis Techniques – Chapter 4 Introduction to Data Flow Diagrams • M Goodland C Slater – SSADM version 4: a Practical Approach – Mc. Graw Hill 1995 • Weaver, Lambrou and Walkley – Practical SSADM version 4+, 2 nd Edition • Chapter 2 – Pitman Publishing 1998 University of Sunderland COMM 1 B Information Systems Analysis Unit 7