CSC3070 - Lecture 19 (Data Flow Diagram - I).pptx

Software Requirements Engineering
CSC3070
Lecture No. 19
Muhammad Shahid
Department of Computer Science
National Textile University
shahid.abdullah@hotmail.com

 Activity Diagrams
 Activity Diagrams - Symbols
 Activity Diagrams - Examples
 Swinlane vs. Non-Swinlane
 Activity Diagrams - Task
Last Lecture Review
2 Software Requirements Engineering - CSC3070

Agenda – What will you Learn Today?
Data Flow Diagram

Data Flow Diagram (DFD)

Elements of the Analysis Model
Structured Analysis
Object-Oriented Analysis
Scenario-Based Modeling
Use Case Text
Use Case Diagrams
Activity Diagrams
Swim Lane Diagrams
Flow-Oriented Modeling
Data Structure Diagrams
Data Flow Diagrams
Control-flow Diagrams
Processing Narratives
Class-Based Modeling
Class Diagrams
Analysis Packages
CRC Models
Collaboration Diagrams
Behavioral Modeling
State Diagrams
Sequence Diagrams

 A way to model a real world situation
 They are the interface between the real world
activities and an understanding of how this
can be converted into a computer system
What is a Data Flow Diagram?

 It is a way of taking the physical view (all
documents involved) and converting it into a
logical view (the data they contain)
 Their main purpose is to communicate with
the user, the analyst’s understanding of the
scope of the required system
Why do We Use DFDs?

 Life Cycle Phases
 Planning
 Feasibility Study
 Analysis
 Design
 Code and Unit test
Where Do They Fit In?

 Life Cycle Phases
 Planning
 Feasibility Study
 Analysis
 Design
 Code and Unit test
Where Do They Fit In?
 Analysis (What do we
do?)
– Fact finding
o investigate business
process and the current
system
– modelling the current and
required systems
– deliverables -
o requirements specification
o logical models of the
required system

 DFDs describe the flow of data or
information into and out of a system
 What does the system do to the data?
 A DFD is a graphic representation of the flow
of data or information through a system
Data Flow Diagrams (DFD)
 What does the system do to the data?

 Decomposition - Divide and subdivide into
manageable size problems
Decomposition & Abstraction
 Abstraction - Concentrate on the important
issues and ignore the irrelevant

 External Entity - people or organisations
that send data into the system or receive data
from the system
Four Main Elements
 Process - models what happens to the data
i.e. transforms incoming data into outgoing
data
 Data Store - represents permanent data that
is used by the system
 Data Flow - models the actual flow of the
data between the other elements

 Data Flow
 Process
 External Entity
 Data Store
Four Main Elements - Notation
Process
Flow of data / information
External
Agent
Data Source

 Is anything outside the system that is of
interest to the system
 Can be a person, a company or another
system
Outside Entity
Outside entity shows the Name
and a lowercase alpha character
is used to uniquely identify it.
Customer
a
If an outside entity is
repeated for the purpose of
neat layout a line is added
across the top.
Customer
a

 Shown by a line with an arrowhead, indicating
the direction of the flow of data
 Each data flow should be named to indicate
what data is being passed
 Nouns or adjectives only no verbs are
permitted
Data Flow

Illegal Data Flows
Patient Doctor
Patient Doctor
A process is
needed to
exchange data
between
external agents

Illegal Data Flows
Patient
Patient
A process is
needed to
update a data
store
Data
Source
Data
Source

Illegal Data Flows
Data
Source
Data
Source
Data
Source
Data
Source
A process is
needed to
copy data from
one data store
to another

 Context
 Level 1
 Level 2
 Level 3
DFD Levels
 There is no rule as to how many levels of
DFD that can be used

 Models system as one process which
represents scope of the system
 Identifies external entities and related inputs
and outputs
Level 0 - Context Diagram

Level 0 - Context Diagram

 Utilizes all four elements
 Gives overview of full system
 Identifies major processes and data flows
and data stores that are used by the major
processes
 Boundary of level 1 is the context diagram
Level 1 - overview diagram

Level 1 - overview diagram

 A breakdown of a level 1 process
 Level 1 process is expanded into more detail
 Each process in level 1 is decomposed to
show its constituent processes
 Boundary of level 2 is the level 1 process
Level 2 - detailed diagram

 Draw document flow diagram of the current
situation
 Draw a systems boundary around the agencies
that are part of the system
 Draw a Context Diagram
 Identify processes in the system
 Complete the level 1 Current Physical DFD
Procedure for Constructing DFD’s

 Numbering
– On level 1 processes are numbered 1,2,3…
– On level 2 processes are numbered x.1, x.2,
x.3… where x is the number of the parent level 1
process
– Number is used to uniquely identify process not
to represent any order of processing
– Data store numbers usually D1, D2, D3...
Rules for DFDs

 Labelling
– Process label - short description of what the
process does, e.g. Price order
– Data flow label - noun representing the data
flowing through it e.g. Customer payment
– Data store label - describes the type of data
stored
– Make labels as meaningful as possible
Rules for DFDs

 Balancing
– Any data flows entering or leaving a parent level
must by equivalent to those on the child level
 Data Stores
– Data stores that are local to a process need not
be included until the process is expanded
Rules for DFDs

 Data Flow Diagrams (DFD)
 Four Main Elements
– Data Flow
– Process
– External Entity
– Data Store
 DFD Levels
 Procedure for Constructing DFD’s
 Rules for DFDs
Recap

Questions

CSC3070 - Lecture 19 (Data Flow Diagram - I).pptx

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