Data Flow Diagrams. 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
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What is a Data Flow Diagram?
Why do we use DFDs?
Decomposition and Abstraction
Process and Data Stores
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
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
1 - 1.1 - 1.1.1
1.2 - 1.2.1
Should carry as much meaning as possible
The four main elements of DFDs notation
A process is made up of
Destination (Place or Name)
Process description Should be descriptive, starting with a verb.
Can be M for manual or D for computer base data stores.
Name of Store
Is anything outside the system that is of interest to the system. Can be a person, a company or another system.
Outside entity shows the Name and a lowercase alpha character is used to uniquely identify it.
If an outside entity is repeated for the purpose of neat layout a line is added across the top.
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.
Sequence not important - getting the Process correct is
Hard to know where to stop
Rule of Thumb
If there are more than 8 data flows break it
Process of Identifying major Processes
Change of Hair Details
What a DFD is & Why we use them
The different conventions
What the elements are
Next Week:- Entity Relational Model