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System Design: Identification of processes, input, output in a data flow diagram
Many basic business operations/processes/activities, e.g.
withdrawal, deposit of money at an ATM,
sales payment using Octopus,
booking and payment of airline tickets and hotel rooms via the internet,
shopping on the internet, and payment of goods via internet e.g.Taobao.com, Amazon.com
can be automated by machines and/or computers as in business information systems.
In order to build and develop such automatic systems, these systems need to be designed properly by IT people (who know IT and computer knowledge, but not much about businesses) and business people (who know business operations in sales, finance, inventory, but not much in IT and computer).
Data Flow diagram (DFD) provides a graphical representation of how information/data moves between processes around a system, the way data/information is changed by processes and stored into databases. DFDs assist design and development of a system.
Objectives:
to graphically document boundaries (main functions) of a system;
to provide hierarchical breakdown of the system into smaller steps (or processes);
to show movement of information between a system and its environment (external entities) which interact (or exchange data and information) with the system;
to document information flows within the system;
to aid communication between users of system and designers of system.
Components of a Data Flow Diagram, (SSADM notation)
We are now going to look inside the box and diagrammatically represent the processing of the inputs to the system to get the outputs from the system. We are going to use the following notation (this is SSADM notation – the most popular system development, it’s used by the Government):
a) Information Flow :
Data Flow Symbol
A data flow can be thought of as a pipeline through which packets of data of known composition flow. Data flows must be an input or output of a Process Box. Physical flows are sometimes represented by a dotted line.
b) Process (performed by computers/machines/hands e.g. count cash money):
Process Symbol
Each process transforms incoming data flows into outgoing data flows. Each process can / should be documented with a brief function description, a brief outline of the processing activity which is taking place.
c) Source or Destination of Data, (External entity) :
External Entity Symbol
External entities (people, place or organisations) are sources or destinations of data that are lying outside the context of the system. Source/Destination must be external to system, and must be a source or destination of input or output to/from the system.
d) Internal Data Store, (Database or File in hard disk) :
Stock Database or File
Flight Booking File / DB
Data Store Symbol
Data stores can hold permanent, temporary, historical or extract data. Files receive inputs and outputs only from Processes, NOT from Externals or other Files.
An example DFD using all Components
After a service job is finished, a service engineer returns a service job description (either written on paper or typed on an electronic device via computer or phone) to the system for recording process and purpose, and this recording process checks any error in input and will store the service job description into the database.
Fragment of DFD Using all Components
Hints for Drawing Data Flow Diagrams
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 the process;
ignore timing, except for logical procedures;
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.
Duplicate external entities are usually represented by:
Duplicate External Entity Symbol
An example DFD in real life
Below is the data flow diagram for Electrical World. This shows the processes in the system.
These processes reflect the basic operations in Electrical World, the data flow arrows into and out of each process show the necessary required data (or information) for each operation, and the resulting data (or information) from each operation.
Data Flow Diagram for processes inside Electrical World
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