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10/12/2001 Data Structure 1
Relationships Between The Data Flow Diagram and The Systems Design Activities
Mohammad A. RobSchool of Business and Public Administration
University of Houston-Clear LakeHouston, Texas 77058
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10/12/2001 Data Structure 2
Introduction• Data-flow diagramming technique (DFD) is used
to understand the business processes of a system.
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10/12/2001 Data Structure 3
Relationships Between The Data Flow Diagram and The Systems Design Activities
• The data flow diagrams also contain information on the four essential components of a systems, namely – the input – output – program modules – data storage
ProcessesProcessesInputsInputs
DataData
OutputsOutputs
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10/12/2001 Data Structure 4
• We modify the system concept such that its components are within the dotted-line boundary and the external entities (of DFD) interact with the system through the input and output components.
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10/12/2001 Data Structure 5
Relationships Between The Data Flow Diagram and The Systems Design Activities
• The goal of a systems analyst is to extract necessary information on the input, program modules, data storage, and output components of a system from the data flow diagrams and then design these components .
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10/12/2001 Data Structure 6
Relationships Between The Data Flow Diagram and The Systems Design Activities
Data Dictionary• First, a data dictionary is created that describes all
data flows, data stores, processes, and external entities of the data-flow diagrams.
• The data-flows and data stores are then expanded to describe the data structure associated with each of these components.
• Each data structure is further expanded to describe the data elements that comprise the data structure.
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10/12/2001 Data Structure 7
Relationships Between The Data Flow Diagram and The Systems Design Activities
Data Storage Design
• Once the data-structures associated with all data stores in the data flow diagrams are defined, they are organized to understand the relationships between the data elements of various data stores.
• Files and databases that comprise the data storage component of a system, are designed on the basis of these data stores.
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10/12/2001 Data Structure 8
Relationships Between The Data Flow Diagram and The Systems Design Activities
Data Storage Design
• Entity-relationship diagrams are commonly used for data modeling or database design.
• The data stores in the data-flow diagram become the entities in the entity-relationship diagram.
EntityData Store
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10/12/2001 Data Structure 9
Relationships Between The Data Flow Diagram and The Systems Design Activities
Input Design
• The data flows originated from the external entities and ending to the processes in a data flow diagram carry information on the input components of a system.
• The data structures associated with the input data flows are described in the data dictionary.
• These data structures are used to design the data-entry forms or screens of a system.
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10/12/2001 Data Structure 10
Relationships Between The Data Flow Diagram and The Systems Design Activities
Output Design
• The data flows originated from the processes and ending to the external entities in a data flow diagram carry information on the output components of a system.
• The data structures associated with the output data flows are described in the data dictionary.
• These data structures are used to design the output reports or statements of a system.
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10/12/2001 Data Structure 11
Relationships Between The Data Flow Diagram and The Systems Design Activities
Program Modules Design
• The business processes in the data flow diagrams become
the program modules of a system.
• Structure charts Structure charts are commonly used to illustrate the relationships between the program modules of a system
• The levels of a structure chart typically correspond to the The levels of a structure chart typically correspond to the levels of a data flow diagramlevels of a data flow diagram
Main
Sub Sub Sub
Context-level
0-level
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10/12/2001 Data Structure 12
Relationships Between The Data Flow Diagram and The Systems Design Activities
• It should be clear that the data dictionary of a system is solely based on the data flow diagram, but the design activities are based on both the “data flow diagrams” and the “data dictionary”
• Understanding this relationship is not obvious when the individual design activities and their associated tools are presented as independent activity
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10/12/2001 Data Structure 13
Relationships Between The Data Flow Diagram and The Systems Design Activities
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10/12/2001 Data Structure 14
Illustration of the Relationship Between The DFD and Data Dictionary
• The data dictionary of a system requires three stages of descriptions:– Describe all components of the data flow
diagrams– Describe the data structures associated with the
data flow and data store– Describe data elements associated with the data
structure
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10/12/2001 Data Structure 15
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10/12/2001 Data Structure 16
Illustration of the Relationship Between The DFD and Data Dictionary
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10/12/2001 Data Structure 17
Illustration of the Relationships Between The DFD and The Systems Components
• There are many components in a data flow diagram.
• All these components collectively participate in the design activities of the system components
• But only a set of these data flow diagram components are used to design an individual component of a system.
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10/12/2001 Data Structure 18
Illustration of the Relationships Between The DFD and The Systems Components
• In order to clarify the relationship between the DFD and the system components, we rearrange the data flow diagram components to fit into the system model.
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10/12/2001 Data Structure 19
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10/12/2001 Data Structure 20
Illustration of the Relationships Between The DFD and The Systems Components
• We move all data store components of the DFD and enclose within the boundary of the data storage component of a system.
• All external entities of the DFD are enclosed in two external entities of the system.
• The rest of the DFD components, which include all the processes and the data flows are enclosed in a boundary of program modules component of a system.
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10/12/2001 Data Structure 21
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10/12/2001 Data Structure 22
Illustration of the Relationships Between The DFD and The Systems Components
• With the illustration, the knowledge of the
design activities becomes simple.
• All data stores in the data flow diagram participate in the data storage design – for files and/or database.
• All data flows originating from the external entities and leading towards the program modules are considered for the input design.
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10/12/2001 Data Structure 23
Illustration of the Relationships Between The DFD and The Systems Components
• Similarly, all data flows originating from the program modules and leading to the external entities are considered for the design of the output components.
• All the processes and data flows enclosed by the outer boundary of the program modules are considered for program design.