DATA FLOW

DIAGRAMS
(DFDs)
Data Flow Diagrams
A graphical tool, useful for communicating with
users, managers, and other personnel.
A relatively simple technique to learn and use.
DFD elements

Source/Sinks (External entities)

Data flows
Processes
Data Stores
External Entities
External
A Rectangle represents Entities
an external entity
They either supply or
receive data • Source – Entity that
They do not process data supplies data to the
system.
• Sink – Entity that
receives data from the
system.
Data Flows
Marks movement of data
through the system - a pipeline
to carry data.
Connects the processes, external Data Flow
entities and data stores.
 Generally unidirectional, If same
data flows in both directions,
double-headed arrow can be
used.
 Delivery Slip

Processes Stores demand

note
1.
STORES
Issue Slip

A circle represents a process

Straight line with incoming arrows are input data
flows
Straight lines with outgoing arrows are output data
flows
Labels are assigned to Data flow.
Data Stores
D1 Data Stores D1 Data Stores D1 Data Stores

Writing Reading

A Data Store is a repository of data

Data can be written into the data store. This is
depicted by an incoming arrow
Data can be read from a data store. This is depicted
by an outgoing arrow
External entity cannot read or write to the data store
Rules of Data Flow
Data can flow from Data cannot flow from

External entity to process External entity to

Process to external entity external entity
Process to store and back External entity to store
Process to process Store to external entity
Store to store
Data Flow Diagrams
An alternate notation is often used:

3 Label
A Process Store
Name
Issue

A Data Store D1 Data Stores Name

Label
Good Style in Drawing DFD
Use meaningful names for data flows, processes and
data stores.
Use top down development starting from context
diagram and successively levelling DFD
A process can only transfer input to output.
Decomposition of DFDs
Decomposition is the iterative process of exploding
data flow diagrams to create more detail.
Level 0 data flow diagrams may be exploded into
successive low levels of detail. The next level of detail
would be a level 1 data flow diagram.
The DFDs become linked together in
a hierarchy, which would fully
document the system.
Why Level DFD
Start from a broad overview. Expand to details
Each DFD must deal with one aspect of a big
system
Levels of DFD
Context diagram
Level-0 diagram (System diagram)
Level-n diagram
- Detail of one process from next
highest level

Primitive diagram (Lowest level DFD)

Levelling Rules
If a process p is expanded, the process at the next level
are labelled as p.1, p.2 etc.
All data flow entering or leaving p must also enter or
leave it’s expanded version
Expanded DFD may have data stores
No external entity can appear in expanded DFD
Keep the number of processes at each level less than 7
Creating Level 0 Diagram
Combine the set of
DFD fragments into
one diagram.
Generally move from
top to bottom, left to
right.
Minimize crossed lines.
When to stop decomposing
DFDs?
Ideally, a DFD has at least
three levels.
When the system becomes
primitive i.e. lowest level
is reached and further
decomposition is useless.
DFD for University Admission System
Context Diagram

Student Information
0
0
University
Student
University
Student
Admission
Admission
System
System
Admission Approval
or Rejection
 Level 1

Student Report Request

Information Student
Name & ID
0
0 22
Admission Approval Perform Generate
Perform Generate
Student
Student Staff
Staff
or Rejection Intake Reports Data
Intake Reports
Procedure
Procedure Items

Approved Data
Application
D1
D1 Student
Student Data
Data
Verified
Approved
11 Application
Maintain
Maintain
Student
Student
Information
Information
Request for Student
Information Maintenance
 Level 2 Process 1, Perform Intake Procedure

Student
Information
Admission Application
1.1
1.1 1.2
1.2
Student
Student
Receive
Receive Verify
Verify
Admission
Admission Student Name Admission
Admission
Application
Application and ID Application
Application

Verified
Admission
D1
D1 Student
Student Data
Data Application

Application
Request
Application Approval 1.3
1.3
or Rejection
Review
Review
Admission
Admission
Application
Application
Logical and Physical DFD
DFDs considered so far are called logical DFDs
A physical DFD is similar to a document flow diagram
It specifies who does the operations specified by the
logical DFD
Physical DFD may depict physical movements of the
goods
Physical DFDs can be drawn during fact gathering
phase of a life cycle
Physical DFD for Cheque Encashment
Cash

Token

Clerk
Clerk
Cheque Cashier
Cashier
Verify
Verify A/C
A/C
CUSTOMER Verify
Verify Token
Token
CUSTOMER Signature
Signature Update
Update Cheque with Take Signature
Take Signature
Balance
Balance Token number
Token

Bad Cheque

Customer Accounts Store cheques Entry in Day Book

User Interface Design
Characteristics of a user interface
1. Speed of learning: A good user interface should be
easy to learn and it should not require its users to
memorize commands.
2. Attractiveness. A good user interface should be
attractive to use. In this respect, graphics-based user
interfaces have a definite advantage over text-based
interfaces.
3. Consistency. The commands supported by a user
interface should be consistent. Consistency facilitates
speed of learning
Characteristics of a user interface
4. Feedback. A good user interface must provide feedback to
various user actions.
5. Error recovery (undo facility). While issuing commands,
even the expert users can commit errors. Therefore, a good
user interface should allow a user to undo a mistake
committed by him while using the interface.
6. User guidance and on-line help. Users seek guidance and
on-line help when they either forget a command or are
unaware of some features of the software. Whenever users
need guidance or seek help from the system, they should be
provided with the appropriate guidance and help.
Graphical User Interface vs. Text-based User
Interface
In a GUI multiple windows with different information
can simultaneously be displayed on the user screen. This
is perhaps one of the biggest advantages of GUI over
text-based interfaces.
Iconic information representation and symbolic
information manipulation is possible in a GUI
A GUI usually supports command selection using an
attractive and user-friendly menu selection system.
In a GUI, a pointing device such as a mouse or a light
pen can be used for issuing commands. The use of a
pointing device increases the efficacy issue procedure.
On the other hand, a text-based user interface can be
implemented even on a cheap alphanumeric display
terminal.
Graphics terminals are usually much more expensive
than alphanumeric terminals.
Types of user interfaces
User interfaces can be classified into
the following three categories:
1. Command language based interfaces
2. Menu-based interfaces
3. Direct manipulation interfaces
Command Language-based Interface
is based on designing a command language which the user
can use to issue the commands.
The user is expected to frame the appropriate commands in
the language and type them in appropriately whenever
required.
Command language-based interfaces allow fast interaction
with the computer and simplify the input of complex
commands.
Examples
Menu-based Interface
An important advantage of a menu-based interface
over a command language-based interface is that a
menu-based interface does not require the users to
remember the exact syntax of the commands.
A menu-based interface is based on recognition of the
command names, rather than recollection.
Examples
Direct Manipulation Interfaces
Direct manipulation interfaces present the interface to the
user in the form of visual models (i.e. icons or objects).
For this reason, direct manipulation interfaces are
sometimes called as iconic interface.
In this type of interface, the user issues commands by
performing actions on the visual representations of the
objects, e.g. pull an icon representing a file into an icon
representing a trash box, for deleting the file.
Examples
Examples
Important advantages of iconic interfaces include the
fact that the icons can be recognized by the users very
easily, and that icons are language-independent.
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