1. DATA FLOW DIAGRAM PART 2

2. Creating Data Flow Diagrams
Creating DFDs is a highly iterative process of gradual refinement.
General steps:
1. Create a preliminary Context Diagram
2. Identify Use Cases, i.e. the ways in which users most commonly use the system
3. Create DFD fragments for each use case
4. Create a Level 0 diagram from fragments
5. Decompose to Level 1,2,…
6. Go to step 1 and revise as necessary
7. Validate DFDs with users.

3. Context Diagram Top-level view of IS
A data flow diagram (DFD) of the scope of an organizational system that shows the system boundaries, external entities that interact with the system and the major information flows between the entities and the system
Example: Order system that a company uses to enter orders and apply payments against a customer’s balance

4. DFD Rules—Context Diagram
One process, numbered 0.
Sources and sinks (external entities) as squares
Main data flows depicted
No internal data stores are shown
They are inside the system
External data stores are shown as external entities
How do you tell the difference between an internal and external data store?

5. Context Diagram of Order System

6. Level-0 DFD
Shows the system’s major processes, data flows, and data stores at a high level of abstraction
When the Context Diagram is expanded into DFD level-0, all the connections that flow into and out of process 0 needs to be retained.

7. Context Diagram of Order System

8. Level-0 DFD of Order System

9. Lower-Level Diagrams
Functional Decomposition
Balancing

10. Decomposition of DFDs Functional decomposition Level-N Diagrams
An iterative process of breaking a system description down into finer and finer detail
Uses a series of increasingly detailed DFDs to describe an IS
Act of going from one single system to many component processes
Repetitive procedure
Lowest level is called a primitive DFD
Level-N Diagrams
A DFD that is the result of n nested decompositions of a series of subprocesses from a process on a level-0 diagram

11. Balancing DFDs
When decomposing a DFD, you must conserve inputs to and outputs from a process at the next level of decomposition
Ensures that the input and output data flows of the parent DFD are maintained on the child DFD
This is called balancing
Example: Hoosier Burgers
In Figure 1, notice that there is one input to the system, the customer order
Three outputs:
Customer receipt
Food order
Management reports

12. Figure 1 Context diagram of Hoosier Burger’s Food ordering system

13. Balancing DFDs Example (Continued)
Notice Figure 2. We have the same inputs and outputs
No new inputs or outputs have been introduced
We can say that the context diagram and level-0 DFD are balanced

14. Figure 2 Level-0 DFD of Hoosier Burger’s food ordering system

15. Balancing DFDs: An unbalanced example
Figure 3:
In context diagram, we have one input to the system, A and one output, B
Level-0 diagram has one additional data flow, C
These DFDs are not balanced

16. Figure 3: An unbalanced set of data flow diagramsB
SINK
SOURCE
(a) Context diagram
1.0 A
SOURCE 1 C
SOURCE 2
2.0 B
SINK
(b) Level-0 diagram

17. Balancing DFDs
We can split a data flow into separate data flows on a lower level diagram (see Figure 4)
Balancing leads to four additional advanced rules

18. Example of data flow splitting
Payment & coupon
Composite data flow
x.1
Payment
Disaggregated data flow
x.2
Coupon

19. Advanced Rule for DFD
A composite DF on 1 level can be split into component DF as the next level, but no new data can be added n all data in the composite must be accounted for in 1 or more subfollows
The inputs to a process must b sufficient to produce the outputs from the process
At the lowest level of DFD, new data flows may be added to represent data that r transmitted under exceptional condition, these data flows typically represent error message
To avoid having data flow lines cross each other, u may repeat data strores or sources/sinks on a DFD. Use an additional symbol, like a double line on the middle vertical line of data strore symbol, or a diagonal line in a corner of a sink/source square to indicate a repeated symbol

20. Strategies for Developing DFDs
Top-down strategy
Create the high-level diagrams (Context Diagram), then low-level diagrams (Level-0 diagram), and so on
Bottom-up strategy
Create the low-level diagrams, then higher-level diagrams

21. Exercise:
Precision Tools sells a line of high-quality woodworking tools. When customers place orders on the company’s Web site, the system checks to see if the items are in stock, issues a status message to the customer, and generates a shipping order to the warehouse, which fills the order. When the order is shipped, the customer is billed. The system also produces various reports, such as inventory reports for Accounting.
Draw a context diagram for the order system
Draw DFD diagram 0 for the order system

22. Identify Entities,Process,Data Stores & Data Flow
Customer
Warehouse
Accounting
Processes
1.0 Check Status
2.0 Issue Status Messages
3.0 Generate Shipping Order
4.0 Manage Accounts Receivable
5.0 Produce Reports
Data Stores
D1 Pending Orders
D2 Accounts Receivable
Data Flows
Order
In-Stock Request
Order Data
Status Data
Status Message
Shipping Order
Invoice
Shipping Confirmation
Payment
Accounting Data
Accounts Receivable Data
Inventory Reports
1.0
2.0
3.0
4.0
5.0

23. Context Diagram of Order System

24. Level-0 of Order System

25. HOMEWORK (group)
Based on the use cases, draw your project data flow in:
Context diagram
Level 0-DFD
If neccesary, decompose the DFD to the next level DFDs
Put them on your SRS’s document as the attachment