1 Introduction to Data Flow Modelling The data flow approach to requirements determination in building a system for business use. This type of computer systems is commonly called Transaction Processing System, which is the earliest use (1970 ’ s to 1990 ’ s) of computer technologies in businesses. Examples are: Bank ATM (automatic teller machines) Bank passbook update and printing Point of sale cashier in supermarket Payroll Airline ticket reservation Accounting packages Buy/sell stocks, gold, foreign currencies etc. Octopus

2 Transaction Processing Systems (Operational level) A transaction is any business-related exchange e.g. payments to employees, sales to customers, payments to suppliers. Transaction processing system (TPS) - an organized collection of people, procedures, software, databases, and hardware devices used to record completed business transactions. TPS is the earliest use of computers to reduce labour costs by automating routine, repetitive, labour-intensive business procedures.

3 Software specification The process of establishing what services are required and the constraints on the system’s operation and development Requirements engineering process involves the following activities: –Feasibility study (e.g. contract) –Requirements elicitation and analysis –Requirements specification –Requirements validation

4 The requirements engineering process

5 Introduction Data Flow Modeling or diagram (DFDs) represents the flow of information around a system, the way it is changed, processed and stored and the ‘ sources ’ (sender) and ‘ sinks ’ (receiver) of information outside the computer system. DFDs represent a situation from the viewpoint of the data (input or output); DFDs - a technique to assist analysis of processes in the computer system.

6 Data-flow models Show the processing steps as data flows through a computer system Simple and intuitive notation that customers (non-technical, non-IT business people) can understand Show end-to-end processing of data

7 Data-flow diagrams may be used to show processing at different levels of abstraction from fairly abstract to fairly detailed may also be used for architectural description showing data interchange between the sub-systems making up the system

8 Data Flow Diagrams They show the overall data flow through a system and they do NOT show control order time errors It is primarily a systems analysis tool used to draw the basic procedural components and the data that pass among them

9 Objectives of DFDs to graphically document boundaries of a system; to provide hierarchical breakdown of the system; to show movement of information between a system and its environment; to document information flows within the system; to aid communication between users (or customers) and developers.

10 How to Draw a DFD

11 Context Diagram A Context Diagram simply shows the system as a box, things external to the system as circles and the information flows into and out of the system. It is usually regarded as a Level 0 DFD. The context diagram can be a presentational aid for us to discuss the interfaces to and from the system without our audience getting concerned with the processes within the system.

12 Components of a DFD (1) Information Flow: l Data flows must be an input or output of a Process Box. l Physical flows are sometimes represented by a double or dotted line.

13 Components of a DFD (2) Process:

14 Components of a DFD (3) Source/Destination of Data, (External): l External entities are sources or sinks of data (people or organisations) that are lying outside the context of the system. l Source/Destination must be external to system, and must be a source or destination of input or output to/from the system. l Externals don't speak to each other.

15 Components of a DFD (4) Internal Data Store, (File): l Data stores can hold permanent, temporary, historical or extract data. l Files receive inputs and outputs only from Processes, NOT from Externals or other Files. l Identifier may be D or M.

16 Example Fragment of DFD using all components

17 Hints for Drawing DFDs (1) For a diagram to be useful it must be at an appropriate level of detail: –avoid detail initially; –identify external entities - they provide the boundary; –identify main processes, then concentrate on data flows; –ensure enough data flows go into a process to perform transformations;

18 Hints for Drawing DFDs (2) duplicate external entities and data store to improve clarity of diagram; use meaningful names; do not duplicate data flows; be prepared to modify and re-draw; prepare in conjunction with users.

19 Hints for Drawing DFDs (3) Duplicate external entities are usually represented by: Duplicate data stores are usually represented by:

20 Some remarks (1) From top to bottom: Context Diagram is a zero level data flow diagram (0-DFD) Next level is a first level data flow diagram (1- DFD) and builds on the Context Diagram by giving more detail

21 Some remarks (2) Naming Conventions All processes must use a VERB and NOUN All Data Flows must only use a NOUN All files must be named: Invoice File (notice no underscore between the words-this is not a data flow)

22 Some remarks (3) Data Flow Diagram Conventions Each context diagram must fit on one page The process name in the context diagram should be the name of the system Use unique names with each set of symbols Do not cross lines Use abbreviated identifications Use a unique reference number for each process symbol