Data Flow Diagrams

Context Diagram Order Company Customer Rejected Order Despatch Note + Goods

A Complete DFD 1 Sales Order Check Credit Rating Customer Rejected Order D1 Credit Rating Dispatch Note Accounts 3 Maintain Credit Ratings 2 Warehouse Dispatch Goods

Data Flow Diagrams (DFDs) show: The processes within the system The datastores (files) supporting its operation The information flows within it The system boundary Interactions with external entities

A Data Flow Customer DFD Symbols An External Entity Sales 3 A Process Check Credit Rating D1 Credit Rating A Data Store

The Process Symbol Unique Number Where it’s done 3 Sales Input Data Check Credit Rating Output Data What’s done

The data store symbol D1 Credit Rating Writing/Updating Accounts 3 Maintain Credit Rating Reading 1 Sales M = Manual data Store D = Computer File Check Credit Rating

The Data Flow Symbol Use Meaningful names Customer Balance Self Evident Data Flows need not be named D1 Credit Rating Sales 3 Check Credit Rating

Physical Resource Flow Customer Returned Library Book

System Boundary The External Entity Symbol Customer A Source and/or sink of data outside the system boundry

Levelling – to avoid complex DFDs, processes can be grouped into higher level processes 2 Dispatch Goods Process 2.1 Validated Order 1 Dispatch Note 2

A Complete DFD 1 Sales Order Check Credit Rating Customer Rejected Order D1 Credit Rating Dispatch Note Accounts 3 Maintain Credit Ratings 2 Warehouse Dispatch Goods

Repeated Symbols – to avoid crossing lines, data stores and external entities can be repeated: Customer Customer A Duplicated External Entity D1 Credit Rating Duplicated Data Store

DFD Levels Elementary Functional Description Level 2 E F D Level 1 Get cust dets If order>cred refer Else raise order ….. E F D Level 1

A wines and spirits wholesaler stocks a range of approximately 400 different lines and delivers by van on regular weekly runs to customers who are mainly off-licenses and clubs. The majority of orders are received by telephone and any one customer may place several orders during the week which must be accumulated and delivered in one consignment. When a customer telephones he expects to be advised on the availability of an item requested and, if not in stock, the alternatives that are available. At present, sales order forms are filled in by the telephone salesmen and sent to the warehouse where they are used for the despatch of the items. Hand-written despatch notes are used as the basis of the invoices which are typed and sent by post sometime later. Prepare a dataflow diagram (DFD) which represents the above system.

Developing Dataflow Diagrams 1. Identify external entities 2. Draw context diagram 3. Who/Where does what 4. Identify datastores 5. Draw diagram

External Entities Customer Context Diagram order advice customer company des. note + goods invoice

What Where/who Receive orders Sales Accumulate orders Sales Advise availability Sales Complete Sales Order Form Sales Despatch Items Warehouse Write despatch notes Warehouse Type Invoices Warehouse Send Invoices Warehouse Datastore Stock file

Draw DFD customer Stock file customer Order Advice 1 Sales M1 Accumulate orders Advise availability Complete forms Sales Order Forms Warehouse 2 Produce despatch notes Produce Invoices Despatch note + goods Invoice customer

Extensions Maintenance of Stock File Customer file – address, credit etc Payments Accounts dept – invoices, payments etc Accumulate orders – temporary file