Relationships Between The Data Flow Diagram
and The Systems Design Activities
Mohammad A. Rob
School of Business and Public Administration
University of Houston-Clear Lake
Houston, Texas 77058
10/12/2001
Data Structure
1
Introduction
• Data-flow diagramming technique (DFD) is used
to understand the business processes of a system.
10/12/2001
Data Structure
2
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
Processes
Inputs
Outputs
– output
– program modules
– data storage
Data
10/12/2001
Data Structure
3
• 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.
10/12/2001
Data Structure
4
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 .
10/12/2001
Data Structure
5
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.
10/12/2001
Data Structure
6
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.
10/12/2001
Data Structure
7
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.
Data Store
10/12/2001
Entity
Data Structure
8
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.
10/12/2001
Data Structure
9
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.
10/12/2001
Data Structure
10
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 are commonly used to illustrate the
relationships between the program modules of a system
• The levels of a structure chart typically correspond to the
levels of a data flow diagram
Context-level
0-level
10/12/2001
Data Structure
Sub
Main
Sub
Sub
11
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
10/12/2001
Data Structure
12
Relationships Between The Data Flow Diagram
and The Systems Design Activities
10/12/2001
Data Structure
13
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
10/12/2001
Data Structure
14
10/12/2001
Data Structure
15
Illustration of the Relationship Between The DFD
and Data Dictionary
10/12/2001
Data Structure
16
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.
10/12/2001
Data Structure
17
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.
10/12/2001
Data Structure
18
10/12/2001
Data Structure
19
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.
10/12/2001
Data Structure
20
10/12/2001
Data Structure
21
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.
10/12/2001
Data Structure
22
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.
10/12/2001
Data Structure
23