Is a picture of the movement of data between external entities and the processes and data stores within a system.

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Data Flow Models
Data Flow Models (DFMs)
Are easy to understand and, with a little practice, reasonably quick and
straightforward to develop. They consist of two parts: a set of Data Flow
Diagrams (DFDs) and a set of associated textual descriptions. And that provide
us with the truly effective tool for understanding the information processes of a
system
Context Diagram (DFD)
Context Diagram Shows the context into which the business process fits and
Shows the overall business process as just one process and Shows all the outside
entities that receive information from or contribute information to the system.
Level 0 Diagram (DFD)
Shows all the processes that comprise the overall system and Shows how
information moves from and to each process and adds data stores.
Example2:
In this Data Flow Diagram consists of University Library BookBorrowing
System.

2. Introdaction to Python
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Data Flow Models
Figure 4.3. Context Diagram of system Library Book

3. Introdaction to Python
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Data Flow Models
Figure 4.4. Level 0 data-flow diagrams of system Library