Unit 7 University of Sunderland COMM1B Information Systems Analysis Process Modelling: Introduction to Data Flow Modelling Information System Analysis COMM1B

Unit 7 University of Sunderland COMM1B Information Systems Analysis Is the existing system important? How do we describe an existing system? Problems with narrative Data Flow Diagrams Outline

Unit 7 University of Sunderland COMM1B Information Systems Analysis 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” How important is the existing system?

Unit 7 University of Sunderland COMM1B Information Systems Analysis 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 Current system model

Unit 7 University of Sunderland COMM1B Information Systems Analysis 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 How do we describe an existing system?

Unit 7 University of Sunderland COMM1B Information Systems Analysis Accounts Customer Accounts Supplier * Enter Orders Sales1 * Package goods & despatch 2 Control stock Warehouse3 Delivery note Order acknowledgement Order Weekly credit limit list Goods received note Internal order form Despatch instruction set Re-order card Goods received note Copy delivery note Example: Data Flow Diagram Level 1 Order Processing System Despatch

Unit 7 University of Sunderland COMM1B Information Systems Analysis 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

Unit 7 University of Sunderland COMM1B Information Systems Analysis 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

Unit 7 University of Sunderland COMM1B Information Systems Analysis Data Flow Diagram Symbols A data flow A process A data store

Unit 7 University of Sunderland COMM1B Information Systems Analysis Data Flow Diagrams an external entity a physical resource flow (rarely used, never in logical DFDs)

Unit 7 University of Sunderland COMM1B Information Systems Analysis Data Flow Diagram Symbols A data flow A process Location ID Order Details Labelling: A data store M1 Name

Unit 7 University of Sunderland COMM1B Information Systems Analysis Data Flow Diagram Symbols A process Description (include a ‘doing’ verb) an external entity Name b

Unit 7 University of Sunderland COMM1B Information Systems Analysis 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

Unit 7 University of Sunderland COMM1B Information Systems Analysis Accounts Customer Accounts Supplier Sales Despatch Delivery note Order acknowledgement Order Weekly credit limit list Goods received note Internal order form Despatch instruction set Re-order card Goods received note Copy delivery note Document Flow Order Processing System Warehouse

Unit 7 University of Sunderland COMM1B Information Systems Analysis Accounts Customer Accounts Supplier Sales Despatch Delivery note Order acknowledgement Order Weekly credit limit list Goods received note Internal order form Despatch instruction set Re-order card Goods received note Copy delivery note Document Flow Order Processing System Boundary Warehouse

Unit 7 University of Sunderland COMM1B Information Systems Analysis Context Diagram (level 0) Order Processing System Accounts Customer Supplier Order Delivery note Order acknowledgement Weekly credit limit list Copy delivery note Re-order card Goods received note Goods received note Order Processing System

Unit 7 University of Sunderland COMM1B Information Systems Analysis Level 1 Data Flow Diagram Order Processing System Accounts Customer Accounts Supplier * Enter Orders Sales1 * Package goods & despatch 2 Control stock Warehouse3 Delivery note Order acknowledgement Order Weekly credit limit list Goods received note Internal order form Despatch instruction set Re-order card Goods received note Copy delivery note Despatch

Unit 7 University of Sunderland COMM1B Information Systems Analysis Level 2 DFD: Control Stock

Unit 7 University of Sunderland COMM1B Information Systems Analysis Level 3 DFD: Monitor Stock Levels

Unit 7 University of Sunderland COMM1B Information Systems Analysis 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

Unit 7 University of Sunderland COMM1B Information Systems Analysis Overall process Agency Level 0 Components of a Data Flow Model

Unit 7 University of Sunderland COMM1B Information Systems Analysis Overall process Agency Level 0 Level 1 Process Components of a Data Flow Model

Unit 7 University of Sunderland COMM1B Information Systems Analysis Overall process Agency Level 0 Level 1 Level 2 Process sub Process sub Process sub Process Components of a Data Flow Model

Unit 7 University of Sunderland COMM1B Information Systems Analysis Overall process Agency Level 0 Level 1 Level 2 Level 3 Process sub Process sub Process sub Process sub-sub Process sub-sub Process sub-sub Process Components of a Data Flow Model *

Unit 7 University of Sunderland COMM1B Information Systems Analysis Overall process Elementary Process Description Decision Tree Decision Table Structured English Agency Level 0 Level 1 Level 2 Level 3 Level 4 Process sub Process sub Process sub Process sub-sub Process sub-sub Process sub-sub Process Components of a Data Flow Model *

Unit 7 University of Sunderland COMM1B Information Systems Analysis 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

Unit 7 University of Sunderland COMM1B Information Systems Analysis 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

Unit 7 University of Sunderland COMM1B Information Systems Analysis 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

Unit 7 University of Sunderland COMM1B Information Systems Analysis 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

Unit 7 University of Sunderland COMM1B Information Systems Analysis 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