Lecture Requirement engineering Chapter 4 Requirement analysis

 DFD

definition
 DFD components
 DFD Key definition
 DFD level


 Data

flow diagram shows business processes
and the data that flows between them.
 Data flow modeling takes a functional
decomposition approach to systems analysis,
breaking complex problems into progressive
levels of detail.


2

type of DFD

Logical process models describe processes without

suggesting how they are conducted
Physical models include information about how the
processes are implemented

 The

DFD is a diagram that consists principally
of four symbols, namely the external entity,
the data flow, the process and the data store

 Additionally,

a physical flow can be shown on
the DFD of the current system


 External

entity: are those things that are
identified as needing to interact with the
system under consideration

Gane and Sarson
Symbol

DeMarco and
Yourdan Symbol

Example


 Process:

an activity that receives data and

carries out some form of transformation or
manipulation before outputting.
 Naming convention of a process:
Name of

a system
Name of a subsystem
A verb


 Process:

an activity that receives data and
carries out some form of transformation or
manipulation before outputting

Gane and
Sarson Symbol

DeMarco and
Yourdan Symbol


 Data

flow: Data move in a specific direction
from an origin to a destination

Gane and Sarson
Symbol

DeMarco and Yourdan
Symbol


 Data

store: places where data may be stored

Gane and Sarson
Symbol

DeMarco and Yourdan
Symbol


 Decomposition

is the process of modeling the
system and its components in increasing levels
of detail.
 Balancing involves insuring that information
presented at one level of a DFD is accurately
represented in the next level DFD.


 DFDs

exist in a hierarchy, beginning with the
simplest, highest, system level and ending

with the lower level diagrams with detailed
processes
Context diagram: DFD at the system level illustrates

the cotext
DFD level 0
DFD level 1
DFD level 2


 Shows

the context into which the business
process fits
 Shows the overall business process as just one
process
 Shows all the outside entities that receive
information from or contribute information to
the system


 Context

Diagram can be drawn by following

steps:
Step 1 - List the documents used in the system.
Step 2 - List all the sources & recipients
Step 3 - Draw a box representing the system and

show the flow of documents from these sources
and recipients. Those areas which are known to be
inside the system are hidden within the box.


 Example


 Shows

all the processes that comprise the
overall system
 Shows how information moves from and to
each process
 Adds data stores



 Shows

all the processes that comprise a single
process on the level 0 diagram
 Shows how information moves from and to
each of these processes
 Shows in more detail the content of higher
level process
 Level 1 diagrams may not be needed for all
level 0 processes









Shows all processes that comprise a single process
on the level 1 diagram
Shows how information moves from and to each of
these processes
Level 2 diagrams may not be needed for all level 1
processes
Correctly numbering each process helps the user
understand where the process fits into the overall
system


Build the context diagram
 Create DFD fragments for each scenario
 Organize DFD fragments into level 0
 Decompose level 0 DFDs as needed
 Validate DFDs with user


 Use

case definition
 Use case diagram