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Data Flow Modelling
Introduction to DFDs
What is a Data Flow Model ?
What is a Data Flow Diagram ?
A means of documenting the flow of
data through a system.
It is a graphical technique which helps: Provide the analyst in the development with an
understanding of the system.
Convey the analyst’s ideas to others.
Introduction to DFD’s
•A DFD is a diagrammatic representation of the
information flows within a system showing:
How the information enters and leaves the system;
What changes the information;
Where information is stored.
•Data Flow Diagrams are an important technique for
systems analysis and design.
What does a DFD give us ? Boundary definition
Checking the completeness of analysis
Basis of system design
Data Flow Diagram (DFD)
• Maps the route of data around a system
Data Source and Sink
Data Store
Flow Line
Process is easy to understand and communicate does not show procedures and control as a
flowchart does
D1 OrdersD1 Orders
1 Accounts
Check Customer Details and Create Order
1 Accounts
Check Customer Details and Create Order
CustomerCustomer
Data Flow Modeling
Customer andOrder Details
Valid Order Details
Modelling the way that data flows within a system
Looks at data in terms of Processes (which transfer
data from one form to another)
Data Stores (which are places where data is held)
External Entities (objects from which data comes, or objects which receive data)
Data Flows (routes by which the data can flow)
External Entities
An External Entity is whatever or whoever donates information to the system or receives information from it.
An external entity may be a user of the system, an external organization, a computer system, or any other source or recipient of information.
A Process
A process transforms or manipulates data within the system. Processes are represented by rectangles on a Data Flow Diagram.
Each process box contains the name of the process, an identifier, and possibly a location.
Process Boxes
The process name describes the process performed on the data received by the process.
Process identifiers are numerical. The location of the process is used to
denote either - The place where the process is performed Or the member of staff responsible for
performing the process.
Process Box Components
1 Manager
Process NewCustomerName
Location
Identifier
Data Stores
A data store is where information is held within the
system.
A data store is represented on a DFD by an open-
ended box.
D1 Student Details
Data store Components
D1 Student DetailsIdentifier
Description of Data Store
Data Flows
A package of information flowing between objects on the Data Flow Diagram.
Represented by a line and an arrow to denote the direction of the information flow.
Labeled with the name or details of the information represented by the data flow.
Example - Data Flows
Customer
A
Process New Customer
1 Manager
Customer Name
Customer Details
Customer Name
D1
Basic rules for a DFD
Data does not flow directly between processes
Data does not flow directly between data stores
Data cannot be transferred directly from store to sink or source to store
External entity Data store
D1 Student Details
Customer
A
Data store Data store
D1 Student Details
D3 Course Details
Drawing Data Flow Diagrams Identify
main inputs & outputs processes data flows
Draw Data Flow Diagram associate data stores internal data flows
Example
Process New Student
1 Academic
Student
AStudent Details
D1 Student Details
Student Details
Checking Data Flow Diagrams ?
Every process must have at least one data flow coming from it and two data flow flowing into it.
Every external entity and data-store must have at least one data flow either coming from it, or flowing into it.
For every data flow either its source is a process or its sink is a process.
A physical DFD represents HOW things are happening. It contains the problems of the current system, and is people or machine dependent. It tends to contain redundant data stores/ processing. It tends to mention names of departments, people, forms, devices used and where data is stored
A logical DFD is extracted from a physical DFD. It is a logical representation of the system, which indicates WHAT the system accomplishes. It is implementation-independent: it focuses on the flow of data between processes without regard to the specific devices, storage location or people in the system.
Types of DFDs
Create the context diagram (Level 0), which shows the external interfaces with the outside world, i.e. the global view of a system.
How to get a DFD
Dispatch note
WarehouseCustomer Invoice
Cheque
Order form Picking notes
Sales & Accountancy
System
0
DFD Level 1
How to get a DFD
Dispatch note
WarehouseCustomer
Invoice
Cheque
Order form Picking notesProcess Order
1 Sales Dept.
Prepare Invoice2 Accounts Dept.
Ensure all data flows are given a name
“A DFD cannot be considered to be completed unless all data flows and functions have been given a meaningful name”
Add Data Stores where needed
How to get a DFD
Example - Supermarket DFD A supermarket uses point of sale bar code
scanners at their checkout tills. The bar code on the product is scanned and is then checked to see if it is valid. If it is a valid bar code then the stock database is updated and an entry for that product is printed on the till receipt. If it is an invalid bar code then an alarm will sound on the checkout till and the operator will need to deal with the problem.
Draw the processes on the Supermarket Data Flow Diagram below.