1. Data Flow Diagrams

2. Introduction Data Flow Diagrams describe
how information is used to produce the functions that are required by the current system
what information is provided to the system and what information is provided from the system
who is using the system
what data will be stored
the boundaries of the system to be built

3. Data Flow Diagrams (DFD)
DFDs describe the flow of data or information into and out of a system
what does the system do to the data?
A DFD is a graphic representation of the flow of data or information through a system
Use example of borrow library book here to illustrate the different components.
Student is external passes in request (book and student details) into validate student process output valid student request into reserve book process. Two data stores, students and loans. Output back to student is book;
Case study - Just A Line system
BCS - GP1 3

4. Case study - Just A Line system
Use example of borrow library book here to illustrate the different components.
Student is external passes in request (book and student details) into validate student process output valid student request into reserve book process. Two data stores, students and loans. Output back to student is book;
Case study - Just A Line system
BCS - GP1 3

5. 4 Main Elements
external entity - people or organisations that send data into the system or receive data from the system
process - models what happens to the data i.e. transforms incoming data into outgoing data
data store - represents permanent data that is used by the system
data flow - models the actual flow of the data between the other elements

6. Notation Data Flow Process External Entity Data Store Data Flow
Process box
External
Entity D
Data Store

7. Case study - Just A Line system
Where/who
Process identifier
Process name
Use example of borrow library book here to illustrate the different components.
Student is external passes in request (book and student details) into validate student process output valid student request into reserve book process. Two data stores, students and loans. Output back to student is book;
Case study - Just A Line system
BCS - GP1 3

8. Data store description
Data store identifier
Data store description
Use example of borrow library book here to illustrate the different components.
Student is external passes in request (book and student details) into validate student process output valid student request into reserve book process. Two data stores, students and loans. Output back to student is book;
Case study - Just A Line system
Applicant’s name
Employee record
Payment
Customer details 3
BCS - GP1 3

9. Levelled DFDs
Even a small system could have many processes and data flows
DFDs could be large and messy
use levelled DFDs - view system at different levels of detail
one overview and many progressively greater detailed views
sides of context process box = system boundary
Example context diagram for library, inputs= book request, book return and output = book; external entity = student.
Go through the case study - context Fig 3.4 pg 32; level 1- Fig 3.5 pg 32 level 2 - Fig 3.6 pg 33 4
BCS - GP1 4

10. Context diagram (level 0 DFD)
Box represents whole system boundary
Context diagram (level 0 DFD)
Level 1 DFD
Level 2 DFD

11. Level 0 - Context Diagram
models system as one process box which represents scope of the system
identifies external entities and related inputs and outputs
additional notation - system box

12. Level 0 Diagram
Shows all the processes that comprise the overall system
Shows how information moves from and to each process

14. Level 1 - overview diagram
gives overview of full system
identifies major processes and data flows between them
identifies data stores that are used by the major processes
boundary of level 1 is the context diagram

15. Level 1 Diagrams
Shows all the processes that comprise a single process on the level 0 diagram
Shows how information moves from and to each of these processes
Shows data stores
Shows in more detail the content of higher level process

16. Case study - Just A Line system
Use example of borrow library book here to illustrate the different components.
Student is external passes in request (book and student details) into validate student process output valid student request into reserve book process. Two data stores, students and loans. Output back to student is book;
Case study - Just A Line system
BCS - GP1 3

17. Level 2 - detailed diagram
level 1 process is expanded into more detail
each process in level 1 is decomposed to show its constituent processes
boundary of level 2 is the level 1 process

18. Level 2 Diagrams
Shows all processes that comprise a single process on the level 1 diagram
Shows how information moves from and to each of these processes
Level 2 diagrams may not be needed for all level 1 processes
Correctly numbering each process helps the user understand where the process fits into the overall system

21. Types of DFD Data Flow Diagrams are either Logical or Physical
Logical DFD - This type of DFD concentrates on the system process, and flow of data in the system. A logical diagram shows how the business operates but not how the system will be constructed. The logical diagram omits details of how the more physical aspects are implemented.
Physical DFD - This type of DFD shows how the data flow is actually implemented in the system. Physical model deals with the way that information is stored, the manual procedures used, the necessity for temporary data collections etc.

22. DFD Samples

25. Level 0 – Food Ordering System
Level 0 – Food Ordering System

26. Level 1 – Food Ordering System