Chapter 4
Using Data Flow Diagrams
DFD
DD
2
1
Process
specification
System proposal
3
7
4
6
Kendall & Kendall
5
© 2005 Pearson Prentice Hall
7-2
Learning Objectives
• Explain the importance of DFDs
• Describe the convention used in DFD
• Draw and explode logical DFDs
• Draw and explode physical DFDs
• Identify errors in DFD
Kendall & Kendall
© 2005 Pearson Prentice Hall
7-3
Enterprise Modeling Tools
‫ادوات النمذجة‬
•
•
•
•
Systems analysts use many graphical techniques to
describe an information system
Two popular tools are entity-relationship diagrams and
data flow diagrams (DFD)
A DFD shows how data moves through an information
system but does not show program logic or processing
steps ‫تظهر انتقال البيانات خالل البرنامج لكن ال تظهر منطق البرنامج أو‬
‫خطوات المعالجة‬
A set of DFDs provides a logical model that shows what
the system does, not how it does it
Kendall & Kendall
© 2005 Pearson Prentice Hall
7-4
Advantages of the Data Flow
Diagram Approach
Four advantages over narrative
explanations of data movement:
• Understanding of the interrelationships of
systems and subsystems ‫فهم العالقات المتبادلة بين‬
‫النظم والنظم الفرعية‬.
• Communicating current system knowledge
to users ‫اتصال النظام الحالي بالمستخدم‬.
• Analysis of the proposed system ‫تحليل النظام‬
‫المقترح‬.
Kendall & Kendall
© 2005 Pearson Prentice Hall
7-5
Basic Symbols
Four basic symbols are:
• A double square for an external entity--a
source or destination (sink or terminator) of
data.
• An arrow for movement of data from one
point to another.
• A rectangle with rounded corners for the
occurrence of transforming process.
• An open-ended rectangle for a data store.
Kendall & Kendall
© 2005 Pearson Prentice Hall
7-6
Basic Symbols
Kendall & Kendall
© 2005 Pearson Prentice Hall
7-7
External Entities
customer
• External entities may be:
• A person, such as CUSTOMER or
STUDENT.
• A company or organization, such as BANK
or SUPPLIER.
• Another department within the company,
such as ORDER FULFILLMENT.
• Another system or subsystem, such as the
INVENTORY CONTROL SYSTEM.
Kendall & Kendall
© 2005 Pearson Prentice Hall
7-8
1
Add New
Customer
Processes
• Represent either:
•
• A whole system
• A subsystem
• Work being done, an activity
Names should be in the form verb-adjective-noun
• The exception is a process that represents an entire system
or subsystem.
• A process receives input data and produces output
•
that have different content, form or both
E.g. the process for CALCULATING PAY uses 2 input
(pay rate and hours worked) to produce 1 output
(total pay).
Kendall & Kendall
© 2005 Pearson Prentice Hall
7-9
Data Stores
D1
Customer
Master
• A data store represents data that the system stores
•
•
•
•
because 1 or more processes need to use thedata a
later time.
Name with a noun (plural), describing the data
Data stores are usually given a unique reference
number, such as D1, D2, D3.
Include any data stored, such as:
• A computer file or database.
• A transaction file .
• A set of tables .
• A manual file of records.
E.g: STUDENTS, DAILY PAYMENTS, PRODUCTS
Kendall & Kendall
© 2005 Pearson Prentice Hall
7-10
Data Flow
•
•
•
•
•
•
•
New Customer
Data flow is a path for data to move from one part of
IS to another.
Data flow shows the data about a person, place, or
thing that moves through the system.
Names should be a noun that describes the data
moving through the system.
Arrowhead indicates the flow direction.
Use double headed-arrows only when a process is
reading data and updating the data on the same
table or file.
Data flow represents 1 or more data items.
E.g. a student ID (1 data item) and registration dates
for a specific class (many data item)
Kendall & Kendall
© 2005 Pearson Prentice Hall
7-11
Kendall & Kendall
© 2005 Pearson Prentice Hall
7-12
DFD Diagramming Rules
Process
No process can have
only outputs or only
inputs…processes
must have both
outputs and inputs.
Process labels should be verb phrases.
Kendall & Kendall
© 2005 Pearson Prentice Hall
7-13
DFD Diagramming Rules
Data Store
All flows to or from a data store must
move through a process.
Data store labels should be noun phrases.
Kendall & Kendall
© 2005 Pearson Prentice Hall
7-14
DFD Diagramming Rules
Source/Sink
No data moves directly between external entities
without going through a process.
Interactions between external entities without
intervening processes are outside the system and
therefore not represented in the DFD.
Source and sink labels should be noun phrases.
Kendall & Kendall
© 2005 Pearson Prentice Hall
7-15
DFD Diagramming Rules
Data Flow
Bidirectional flow
between process
and data store is
represented by two
separate arrows.
Forked data flow
must refer to exact
same data item (not
different data items)
from a common
location to multiple
destinations.
Kendall & Kendall
© 2005 Pearson Prentice Hall
7-16
DFD Diagramming Rules
Data Flow (cont.)
Joined data flow
must refer to exact
same data item (not
different data items)
from multiple
sources to a
common location.
Data flow cannot
go directly from a
process to itself,
must go through
intervening
processes.
Kendall & Kendall
© 2005 Pearson Prentice Hall
7-17
DFD Diagramming Rules
Data Flow (cont.)
• Data flow from a process to a data
store means update (insert, delete or
change).
• Data flow from a data store to a
process means retrieve or use.
Kendall & Kendall
© 2005 Pearson Prentice Hall
7-18
Kendall & Kendall
© 2005 Pearson Prentice Hall
7-19
Kendall & Kendall
© 2005 Pearson Prentice Hall
7-20
Kendall & Kendall
© 2005 Pearson Prentice Hall
7-21
Kendall & Kendall
© 2005 Pearson Prentice Hall
7-22
Kendall & Kendall
© 2005 Pearson Prentice Hall
7-23
Data Flow Diagram Errors
• Incorrectly labeling data flow or objects
• Examples are:
• Labels omitted from data flow or objects .
• Data flow labeled with a verb .
• Processes labeled with a noun .
• Too many processes on a data flow
diagram.
• Nine is the suggested maximum.
• Omitting data flow from the diagram
Kendall & Kendall
© 2005 Pearson Prentice Hall
7-24
Class exercise 1
Kendall & Kendall
© 2005 Pearson Prentice Hall
7-25
Class exercise 2
Kendall & Kendall
© 2005 Pearson Prentice Hall
7-26