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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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Introduction• Data-flow diagramming technique (DFD) is used

to understand the business processes of a system.

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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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• 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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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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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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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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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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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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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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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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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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Relationships Between The Data Flow Diagram and The Systems Design Activities

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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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Illustration of the Relationship Between The DFD and Data Dictionary

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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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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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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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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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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.