1. Software Engineering-II
Data Flow Diagrams
Software Engineering-II
Sir Zubair Sajid

2. Introduction SSADM What is a Data Flow Diagram? Why do we use DFDs?
Levelling
Conventions
Decomposition and Abstraction
The Elements
Process and Data Stores
Outside Entity
Data Flow
The Levels
Rules
The Procedure for Constructing DFD’s
The Document Flow Diagram
The Context Diagram
Draw the external entities and data stores
Level 1 Physical DFD - Complete

3. S.S.A.D.M. Data Flow Diagrams
S.S.A.D.M. - Structured Systems Analysis and Design Method
Uses different techniques to model a system
Data Flow Diagrams
Entity Relational Model (Logical Data Stores)
Normalisation

4. What is a Data Flow Diagram?
Known as DFDs
A way to model a real world situation
They are the interface between the real world activities and an understanding of how this can be converted into a computer system.

5. Why do we use DFDs?
It is a way of taking the physical view and converting it into a logical view.
The physical view - all documents involved
The logical view - the data they contain
Their main purpose is to communicate with the user, the analyst’s understanding of the scope of the required system

6. Levelling DFDs are expanded or decomposed into levels.
Separating each process into sub processes
Uncovers more and more detail

7. Conventions Balancing
Process at lower level should have identical data flows if they flow out of a process
Modelling Data Stores
Only use DATA STORES used within this process on the diagram
Numbering
Labels
Should carry as much meaning as possible

8. Decomposition and Abstraction
Decomposition - Divide and subdivide into manageable size problems
Abstraction - Concentrate on the important issues and ignore the irrelevant

9. The Elements Data Flows, with a label to indicate what data is flowing
The four main elements of DFDs notation
Data Flows, with a label to indicate what data is flowing
Processes, that handle the data
Data stores, within the system (diary, filing cabinet or computer file)
Outside entities, outside sources of data

10. Process and Data Stores
A process is made up of
Data Stores
Destination (Place or Name)
Process Number
Process description Should be descriptive, starting with a verb.
Can be M for manual or D for computer base data stores.
Name of Store M1

11. Outside Entity
Is anything outside the system that is of interest to the system. Can be a person, a company or another system.
Customer a
Outside entity shows the Name and a lowercase alpha character is used to uniquely identify it.
Customer a
If an outside entity is repeated for the purpose of neat layout a line is added across the top.

12. Data Flow
Is shown by a line with an arrowhead, indicating the direction of the flow of data. Each data flow should be named to indicate what data is being passed. Nouns or adjectives only no verbs are permitted.

13. The Levels Context - Overview - contains only one process
Level 1 - Utilises all four elements
Level 2 - A breakdown of a level 1 process
Level 3 - A breakdown of a level 2 process
There is no rule as to how many levels of DFD that can be used.

14. Rules
Sequence not important - getting the Process correct is
Context or Level 0 - Identifies the system/ boundary/External Links
Level 1 - Overview of function
Level 2 - Breakdown to Understand
Hard to know where to stop
Rule of Thumb
If there are more than 8 data flows break it
Process of Identifying major Processes

22. Process 3 Level 2 3 Hair/Reception Client a Clie a 3.1 Hairdresser
Appointment Details
Conduct Appointment
Hair Details M2
Diary
3.2
Hairdresser
Inform Reception
Change of Hair Details
3.3
Receptionist
Complete Appointment M3
Client Card

23. Naming of DFD processes

24. Summary
SSADM
What a DFD is & Why we use them
The different conventions
What the elements are
Example
Next Week:- Entity Relational Model