1. DFD Process & Examples

2. Learning Objective: Success Criteria: Learn process of a DFD
Know the components of a DFD
Understand the rules and conventions of creating DFD’s
Create 3 DFD’s
Success Criteria:
Notes about DFD’s in books
3 DFD’s created in books

3. Purpose of a DFD
A diagram is a much easier way to view the processes that take place in an information system than by reading a written description.
A data flow diagram (DFD) is a process-modelling tool that depicts the flow of data through a system. It depicts all the input, the processing and the output and storage of data.

4. Components of a DFD
Data Source or External entity – a source or destination of data flow that is outside the area of study.
Data Store – repository of data; ‘D’ indicates a permanent computer file; ‘M’ indicates a manual file, ‘T’ indicates a transient store, deleted after processing.
Process – transforms incoming data flow into outgoing data flow.
Data flow – a connector shows relationships between the representative shapes.
Entity
Datastore M
Level
Process

5. The Steps in Drawing a DFD
DFDs are drawn in a number of steps (at least two), each step is referred to as a level. The steps involve top-down design and functional decomposition. That is we start at the top with an overall picture and then add detail as we work down.
Example: A teacher gives his IT class a test on databases. The result for each student is out of a possible 40, which is first calculated as a percentage, but is then converted to an A to E grade. This grade is recorded in the teacher’s mark book and the graded paper finally returned to the student.

6. The Steps in Drawing a DFD
The first level is the context level. The context level only contains a single process (a consolidation of all processes) along with all the external entities. This assist the database designer ‘discover’ the data that will be passing through the system with no detail given to all the processes.
Context Level:
Unmarked test paper 0.
Process test paper
Student
Graded test

7. The Steps in Drawing a DFD
Level 0 shows all general processes, external entities and data storage. For all of our databases, this is a sufficient level of detail.
1.0
Process test paper
Unmarked test paper
Student
Marked test paper
Graded test paper
2.0
Calculate grade
Calculated grade
Teacher’s markbook D

8. DFD Rules A data flow line must have a source
A data flow line must have a destination
Data flow lines must not cross. It may be necessary to draw some entities or data stores more than once.
The name of the context level process identifies the main purpose of the information system.
A data store must have at least one incoming data flow.
Data cannot flow directly from an entity to another entity or a data store without a process in between.
Data cannot flow directly from a data store to another data store or entity without a process in between.
A process cannot have outputs that are greater than the sum of its inputs. That is, its inputs must be able to produce the outputs shown.
A process must have an output

9. Limitations of a DFD
Paraphrase the below notes into your exercise books.
The data flow diagram is one of the best techniques available for developing a comprehensive collection of data items in a problem. Eventually the diagram will become substantial system documentation.
A DFD however does not provide specification for process sequence or process control. The diagram gives no indication in what order the processes are to take place, nor does it give information as to how or why decisions might be made. Both have no place or meaning in a DFD. There are other modelling tools that cater for these requirements.
A DFD shows what an information system does, but not the how, who, when or why. Processes are not described in detail on a DFD. A database designer would need to provide the details in a separate document called a process specification.

10. Lemonade Stand Example DFD (I do)
Creating a context and level 0 DFD for a lemonade stand

11. Bus Garage Repairs DFD (We do)
Buses come to a garage for repairs.
A mechanic and helper perform the repair, record the reason for the repair and record the total cost of all parts used on a Shop Repair Order.
Information on labor, parts and repair outcome is used for billing by the Accounting Department, parts monitoring by the inventory management computer system and a performance review by the supervisor.

12. Restaurant Order Processing System DFD (You do)
At a restaurant, a customer orders what they want from the menu. The order is given to the kitchen where it is prepared. At the end of the evening, the bill is calculated and given to the customer. All bills are kept in duplicate in the order book.
Draw a simple DFD (Level 0) for this flow of information.

13. Check what you have learnt
Most often, an information system is designed to take the place of an existing system. Why would it be useful to make a DFD of the old system first?
Two errors that are sometimes made with data flow lines are called ‘miracles’ and ‘black holes’. What do you suppose these errors are?

14. Check what you have learnt
The following is a DFD (level 0) for making a loan from a school library
Add to the diagram the process ‘Make a return’, including appropriate data flows.
Take the ‘Make a loan’ process to the next level of decomposition using the processes that occur in your library system.
Student data
Student records D
Student
ID Card
1.0
Make a loan
Resource data
Resource records D
Resource
New loan data
Barcode
Loan records D
Current loan data

15. Learning Objective: Success Criteria: Learn process of a DFD
Know the components of a DFD
Understand the rules and conventions of creating DFD’s
Create 3 DFD’s
Success Criteria:
Notes about DFD’s in books
3 DFD’s created in books

16. Lemonade Stand – DFD (Context Level)
0.0
Lemonade prepared
Customer
Order and money
Product and change (if required)

17. Lemonade Stand – DFD (Level 0)
1.0
Take order
Order + Money
Order + Money
Customer
Change
2.0
Calculate change
Money
Till D
Change
Purchase
Order
3.0
Prepare order

18. Bus Garage Repairs – DFD (Context Level)
0.0
Bus prepared
Bus given to Mechanic
Bus
Bus returned/repaired

19. Bus Garage Repairs – DFD (Level 0)
1.0
Bus processed
Bus
Bus delivered
Bus goes to garage
Mechanic
2.0
Bus repaired
Mechanic identifies issue
Parts and Time used
Accounting Dept
3.0
Invoice processed
Invoice details
Parts Time used
MYOB D
Bus
Bus/Invoice provided