DATA FLOW DIAGRAM
DATA FLOW DIAGRAM:
"a network representation of a system.
The system may be automated, manual,
or mixed. The DFD portrays the system
in terms of its component pieces, with all
interfaces among the components
indicated."
- Tom DeMarco
hence DFDs:
focus on the movement of data between
external entities and processes, and
between processes and data stores
WHERE DO THEY FIT IN?
Planning
 Analysis
 Design
 Implementation
 Maintenance


Analysis (What do we do?)

Fact finding


investigate business process
and the current system
WHERE DO THEY FIT IN?
Planning
 Analysis
 Design
 Implementation
 Maintenance



Design
Describes how the system will
fulfill the user requirements

OVERVIEW

The Data flow diagram is a form of
This is a technique for organising and documenting
the structure and flow of data through a system’s
processes, and the procedures to be implemented by a
system’s processes.
 DFD help to represent and communicate

Information and data flows
 Processes that handle or change data
 Stored data
 Sources and recipients of information and data

WHY CONDUCT PROCESS MODELING?

The reason we do process modeling is to improve
the operation of a system, not just to change its
physical form.
Start with understanding (Understand components of
current logical or physical system for purpose of
rebuilding in a different physical form/technology,
possibly with some changed functionality)
 Then analyze for improvements or where to add new
functionality (Find inefficiencies in current system)
 Then specify re-engineered system

COMPONENTS
•
A DFD is made up of four components:
processes, external entities, data flows
and data stores.
Processes
• External entities
• Data flows
• Data stores
•
COMPONENTS
1.
External
Entity
Entities

These are the places which provide the organisation with
data, or have data sent to them by the organisation (e.g.
customers, partners, government bodies).
 Form the boundaries of the system.
 The system and external entities exchange data in the
form of data flows.
 Must be named, titles preferred to names of individuals 
COMPONENTS
2. Process
 Jobs that are done with the data.

Process box
COMPONENTS
Data Flow
3. Data Flows

Connects the processes, external entities and data
stores
 Arrows show how data flows from one place to another
 Label the arrows with the name of the data that moves
through it
e.g. Customers logs into systems
Logs in

COMPONENTS
4. Data Stores

Represents holding areas for collection of data,
processes add or retrieve data from these stores
 Only processes are connected to data stores
 They can be manual, digital or temporary.

M1
Data Store
D1
Data Store
Digital Data Store = D1
T1
Data Store
Temporary Data Store = T1
Manual Data Store = M1
NOTATION
Data Flow
• Data Flow
Process box
• Process
• External Entity
External
Entity
• Data Store
D1
Data Store
BENEFITS OF DFD

Provide a pictorial, non-technical representations


Quick to produce and easy to amend


Concise system descriptions
LOGICAL VERSUS PHYSICAL MODELS

What versus How

Shows what a system is or does
 Focuses on how the business operates
 Is implementation-independent


Shows how a system operates
 Is implementation-dependent because it shows how a
system is physically and technically implemented



Systems analysts use logical models to depict
business requirements, and physical models to depict
technical designs
System analysis activities tend to focus on the logical
models
CONTEXT LEVEL DFD
The Context Level DFD attempt to show the
workings of the entire system at a glance.
 It is the
and the first to be
drawn
 This level consists of

The process description usually outlines in general
terms what the system is or does, for example, ‘Order
Processing System’, ‘Video Rental Systems’.
 The single process represents the entire system

Depicts the interaction between the system
and external entities.
 This level

CONTEXT LEVEL DATA FLOW DIAGRAM
a.
Entity
Process
b.
Entity
CONTEXT DATA FLOW DIAGRAM EXAMPLE
a.
Personnel Department
New Staff Details
Hours Worked
Pay Cheque
c.
Employee
b.
Management
Salary Changes
Cash List
Payroll System
Name/Address Change
RULES FOR DRAWING DFDS...
1.
External entities cannot flow directly to each other
II. A data flow cannot link a data store to an external entity
III. Data cannot move between data stores without first
being processed
I.
x
a.
Entity
Communicates
b.
Entity
a.
Entity
Communicates
D1
Data Store
x
RULES FOR DRAWING DFDS...
1. All data flows must flow to or from a
process continued..
RULES FOR DRAWING DFDS...
2. A process must have at least one input flow
and one output flow
I.
II.
A
arises when a process has
input flows but no output flows
A
is a situation which occurs
when a process has outputs but no input flows.
Process
Process
RULES FOR DRAWING DFDS...
3. The inputs to a process must be sufficient
to produce output flows.
I.
If a process has outputs that are greater than the
sum of its inputs, a
is said to occur
4. The inputs to a process must be sufficient
to produce output flows.
I.
When naming data flows, adjectives should be used
which show how processing has changed the data
flow.
DFD – COMMON ERRORS
Black Hole
Miracle
Gray Hole
RULES FOR DRAWING DFDS...
5.
Data flows cannot cross each other
To overcome this problem, data stores and entities
can be
.
However, processes must never be duplicated
Data flows must be unidirectional



Additional rules
•
•
Entities must be
Title = “Context Level DFD” and System or
Company Name
RULES FOR CONNECTING PROCESSES,
DATA STORES, AND ENTITIES IN A DFD
STEPS IN CONSTRUCTING CONTEXT
LEVEL DFD






You will be supplied with a piece of narrative text
describing a particular system. Read and re-read
this text.
Extract and state a set of assumptions from the
narrative text.
Identify the entities from the text – these will be
depicted by nouns.
Highlight the process for the context level DFD
Identify data flows from the text – data flows are
usually nouns or noun phrases.
Construct the Context Level DFD using all the
preceding information. Name and label the entities,
data flows and name the main process.
CREATING A SET OF DFDS

Draw a Context Diagram
Put a title on the page = Context Level
DFD for ‘name of company’.
2. Draw the context diagram so it fits on one
page
1.
3.
4.
Use unique names within each set of
symbols
5.
6.
Provide a unique name and reference
number for process
LAB WORK: PAYROLL SYSTEM CONTEXT LEVEL DFD




XYZ LTd. is a nationwide retail chain. Management is considering conducting
Systems Analysis and Design with a view to computerising the payroll system.
Hours worked per employee per month are forwarded by the Personnel Department.
This is used to create an employee time record for each employee and this is held in
the Employee Time File. This is used to calculate Gross Pay, and Gross Pay is used to
determine net pay. Pay slips are issued on the last Friday of every month.
The employee Master File must be updated on a regular basis. One instance which
necessitates updating the system is when a new employee joins the firm. The
Personnel Department gives the payroll system all new staff details. When an
Employee gets married or moves house, details of the name/address change are given
to the payroll system. Pay cheques are then prepared.
Management is in charge of salary changes – these are input to the Employee Master
File when the time arises. Every month, management is sent a cash list with details
of salary payments for each employee for the month previous.
Required:


Draw up a reasonable set of assumptions, based on the narrative above.
You are now required to construct a Context Level DFD based on those assumptions,
to help the company in its efforts computerise the payroll system.
ASSUMPTIONS
Assumptions 1(a):
 It is assumed that when employee details change,
such as when he/she gets married or moves
house, it is the responsibility of the individual
employee to notify the payroll system as to
name/address changes.

CONTEXT LEVEL

Identify Process


Payroll System
Identify entities
a. Personnel Dept
b. Management
c. Employee

Identify data flows
• Hours Worked
• Salary Changes
• Cash List
• Pay Cheque
• Name/Address
Change
• New Staff Details
LAB WORK: PAYROLL SYSTEM CONTEXT LEVEL DFD




XYZ LTd. is a nationwide retail chain. Management is considering conducting
Systems Analysis and Design with a view to computerising the payroll system.
Hours worked per employee per month are forwarded by the Personnel Department.
This is used to create an employee time record for each employee and this is held in
the Employee Time File. This is used to calculate Gross Pay, and Gross Pay is used to
determine net pay. Pay slips are issued on the last Friday of every month.
The employee Master File must be updated on a regular basis. One instance which
necessitates updating the system is when a new employee joins the firm. The
Personnel Department gives the payroll system all new staff details. When an
Employee gets married or moves house, details of the name/address change are given
to the payroll system. Pay cheques are then prepared.
Management is in charge of salary changes – these are input to the Employee Master
File when the time arises. Every month, management is sent a cash list with details
of salary payments for each employee for the month previous.
Required:


Draw up a reasonable set of assumptions, based on the narrative above.
You are now required to construct a Context Level DFD based on those
assumptions, to help the company in its efforts computerise the payroll system.
PAYROLL SYSTEM (A): CONTEXT LEVEL
DATA FLOW DIAGRAM
a.
Personnel Department
New Staff Details
Hours Worked
Pay Cheque
c.
Employee
b.
Management
Salary Changes
Cash List
Payroll System
Name/Address Change
ASSUMPTIONS
Assumptions 1(b):
 It is assumed that when employee details change,
such as when he/she gets married or moves
house, the personnel department is responsible
for informing the payroll system. The personnel
department updates the payroll system directly.
 The Employee is notified when changes are
made by the Personnel Dept.
 The Personnel Dept receives an
acknowledgement that states Employee file is
updated.

PAYROLL SYSTEM (B): CONTEXT LEVEL
DATA FLOW DIAGRAM
a.
Personnel Department
New Staff Details
b.
Management
Salary Changes
Name/Address Change
Hours Worked
Cash List
Pay Cheque
Payroll System
Acknowledgement of changes
a.
Personnel Department
c.
Employee
Confirmation of Detail Changes
VALIDATING THE DFD
 Syntax

errors – diagram follows the rules
Assure correct DFD structure
For each DFD:
Check each process for:
•A unique name: action verb phrase; number;
description
•At least one input data flow
•At least one output data flow
•Output data flow names usually different than
input data flow names
•Between 3 and 7 processes per level 1+ DFD
VALIDATING THE DFD

For each DFD:

Check each data flow for:
A unique name: noun; description
Connects to at least one process
Shown in only one direction (no two-headed arrows)


Check each external entity for:
A unique name: noun; description
At least one input or output data flow
Check each data store for:
A unique name: noun; description
At least one input data flow
At least one output data flow
EXERCISE
Precision Tools sells a line of high-quality woodworking
tools. When customers place orders on the company’s
Web site, the system checks to see if the items are in
stock, issues a status message to the customer, and
generates a shipping order to the warehouse, which fills
the order. When the order is shipped, the customer is
billed. The system also produces various reports, such as
inventory reports for Accounting.
 Draw a context diagram for the order system
Document any assumptions you may have made
about the system.