Chapter 7software engneeringand system development life cycle.ppt

Chapter 7
Structuring System Process
Requirements
Modern Systems Analysis
and Design

2
Chapter 7
Learning Objectives
 Understand the logical modeling of processes by
studying examples of data flow diagrams
(DFDs).
 Draw data flow diagrams following specific rules
and guidelines that lead to accurate and well-
structured process models.
 Decompose data flow diagrams into lower-level
diagrams.
 Balance higher-level and lower-level data flow
diagrams.

3
Chapter 7
Learning Objectives (Cont.)
 Use data flow diagrams as a tool to
support the analysis of information
systems.
 Discuss process modeling for electronic
commerce applications.
 Use decision tables to represent the logic
of choice in conditional statements.

Process Modeling
4
Chapter 7
FIGURE 7-1
Systems development life cycle with the analysis phase highlighted

5
Chapter 7
Process Modeling (Cont.)
 Graphically represent the processes
that capture, manipulate, store, and
distribute data between a system and
its environment and among system
components.

6
Chapter 7
Process Modeling (Cont.)
 Utilize information gathered during
requirements determination.
 Processes and data structures are
modeled.

7
Chapter 7
Deliverables and Outcomes
 Context data flow diagram (DFD)
Scope of system
 DFDs of current physical system
Adequate detail only
 DFDs of current logical system
Enables analysts to understand current
system

8
Chapter 7
Deliverables and Outcomes (Cont.)
 DFDs of new logical system
Technology independent
Show data flows, structure, and
functional requirements of new system
 Thorough description of each DFD
component

9
Chapter 7
Data Flow Diagramming
Mechanics
 Represent both physical and logical
information systems
 Only four symbols are used

10
Chapter 7
Data Flow Diagramming
Mechanics (Cont.)
 Useful for depicting purely logical
information flows
 DFDs that detail physical systems
differ from system flowcharts which
depict details of physical computing
equipment

11
Chapter 7
Definitions and Symbols
FIGURE 7-2
Comparison of DeMarco
and Yourdon
and Gane and Sarson
DFD symbol sets

12
Chapter 7
Definitions and Symbols (Cont.)
 Process: work or actions performed
on data (inside the system)
 Data store: data at rest (inside the
system)

13
Chapter 7
Definitions and Symbols (Cont.)
 Source/sink: external entity that is
origin or destination of data (outside
the system)
 Data flow: arrows depicting
movement of data

Developing DFDs
 Context diagram is an overview of an
organizational system that shows:
the system boundaries.
external entities that interact with the system.
major information flows between the entities
and the system.
 Note: only one process symbol, and no
data stores shown
14
Chapter 7

Context Diagram
15
Chapter 7
FIGURE 7-4
Context diagram of Hoosier Burger’s food-ordering system

Developing DFDs (Cont.)
 Level-0 diagram is a data flow diagram
that represents a system’s major
processes, data flows, and data stores at
a high level of detail.
Processes are labeled 1.0, 2.0, etc. These will
be decomposed into more primitive (lower-
level) DFDs.
16
Chapter 7

Level-0 Diagram
17
Chapter 7
FIGURE 7-5
Level-0 DFD of Hoosier Burger’s
food-ordering system

18
Chapter 7
Data Flow Diagramming Rules
 There are two DFD guidelines that apply:
The inputs to a process are different from the
outputs of that process.
 Processes purpose is to transform inputs into
outputs.
Objects on a DFD have unique names.
 Every process has a unique name.

Data Flow Diagramming Rules (Cont.)
19
Chapter 7
TABLE 7-2 Rules Governing Data Flow Diagramming

Data Flow Diagramming Rules (Cont.)
20
Chapter 7
TABLE 7-2 Rules Governing Data Flow Diagramming (cont.)

21
Chapter 7
Decomposition of DFDs
 Functional decomposition is an iterative
process of breaking a system description
down into finer and finer detail.
Creates a set of charts in which one process
on a given chart is explained in greater detail
on another chart.
Continues until no subprocess can logically
be broken down any further.

22
Chapter 7
Decomposition of DFDs (Cont.)
 Primitive DFD is the lowest level of a DFD.
 Level-1 diagram results from
decomposition of Level-0 diagram.
 Level-n diagram is a DFD diagram that is
the result of n nested decompositions from
a process on a level-0 diagram.

23
Chapter 7
Level-1 DFD
Level-1 DFD shows
the sub-processes
of one of the
processes in the
Level-0 DFD.
This is a Level-1
DFD for Process
4.0.
Processes are labeled 4.1, 4.2, etc.
These can be further decomposed
in more primitive (lower-level)
DFDs if necessary.
FIGURE 7-8
Level-1 diagram showing the decomposition
of Process 4.0 from the level-0 diagram for
Hoosier Burger’s food-ordering system

24
Chapter 7
Level-n DFD
Level-n DFD shows
the sub-processes
of one of the
processes in the
Level n-1 DFD.
This is a Level-2
DFD for Process
4.3.
Processes are labeled 4.3.1, 4.3.2, etc. If this is the
lowest level of the hierarchy, it is called a primitive DFD.
FIGURE 7-9
Level-2 diagram showing the decomposition of
Process 4.3 from the level-1 diagram for Process
4.0 for Hoosier Burger’s food-ordering system

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Chapter 7
Balancing DFDs
 Conservation Principle: conserve
inputs and outputs to a process at the
next level of decomposition
 Balancing: conservation of inputs
and outputs to a data flow diagram
process when that process is
decomposed to a lower level

26
Chapter 7
Balancing DFDs (Cont.)
 Balanced means:
Number of inputs to lower level DFD
equals number of inputs to associated
process of higher-level DFD
Number of outputs to lower level DFD
equals number of outputs to associated
process of higher-level DFD

27
Chapter 7
This is
unbalanced
because the
process of the
context diagram
has only one
input but the
Level-0 diagram
has two inputs.
1 input
1 output
2 inputs
1 output
FIGURE 7-10 An unbalanced set of data flow diagrams
(a) Context diagram
(b) Level-0 diagram

28
Chapter 7
 Data flow splitting is when a
composite data flow at a higher level is
split and different parts go to different
processes in the lower level DFD.
 The DFD remains balanced because the
same data is involved, but split into two
parts.

29
Chapter 7
FIGURE 7-11
Example of data flow splitting
(a) Composite data flow
(b) Disaggregated data flows

30
Chapter 7
Balancing DFDs: More DFD Rules

31
Chapter 7
Four Different Types of DFDs
 Current Physical
 Process labels identify technology (people or
systems) used to process the data.
 Data flows and data stores identify actual name of the
physical media.
 Current Logical
 Physical aspects of system are removed as much as
possible.
 Current system is reduced to data and processes that
transform them.

32
Chapter 7
Four Different Types of DFDs
(Cont.)
 New Logical
Includes additional functions.
Obsolete functions are removed.
Inefficient data flows are reorganized.
 New Physical
Represents the physical implementation of
the new system.

33
Chapter 7
Guidelines for Drawing DFDs
 Completeness
DFD must include all components necessary
for system.
Each component must be fully described in
the project dictionary or CASE repository.
 Consistency
The extent to which information contained on
one level of a set of nested DFDs is also
included on other levels

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Chapter 7
(Cont.)
 Timing
Time is not represented well on DFDs.
Best to draw DFDs as if the system has never
started and will never stop.
 Iterative Development
Analyst should expect to redraw diagram
several times before reaching the closest
approximation to the system being modeled.

35
Chapter 7
(Cont.)
 Primitive DFDs
Lowest logical level of decomposition
Decision has to be made when to stop
decomposition

36
Chapter 7
(Cont.)
 Rules for stopping decomposition
When each process has been reduced
to a single decision, calculation or
database operation
When each data store represents data
about a single entity

37
Chapter 7
(Cont.)
 Rules for stopping decomposition, cont.
When the system user does not care to
see any more detail
When every data flow does not need to
be split further to show that data are
handled in various ways

38
Chapter 7
(Cont.)
 Rules for stopping decomposition, cont.
When you believe that you have shown
each business form or transaction,
online display and report as a single
data flow
When you believe that there is a
separate process for each choice on all
lowest-level menu options

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Chapter 7
Using DFDs as Analysis Tools
 Gap Analysis is the process of
discovering discrepancies between two or
more sets of data flow diagrams or
discrepancies within a single DFD.
 Inefficiencies in a system can often be
identified through DFDs.

Using DFDs in BPR
40
Chapter 7
FIGURE 7-16
IBM Credit Corporation’s primary work process before BPR
(Source: Based on Hammer and Champy, 1993.)

Using DFDs in BPR (Cont.)
41
Chapter 7
FIGURE 7-17
IBM Credit Corporation’s primary work process after BPR
(Source: Based on Hammer and Champy, 1993.)

Electronic Commerce Application: Process
Modeling using Data Flow Diagrams
 Process modeling for Pine Valley
Furniture’s Webstore
Completed JAD session.
Began translating the Webstore system
structure into data flow diagrams.
 Identified six high-level processes.
42
Chapter 7

Modeling using Data Flow Diagrams (Cont.)
43
Chapter 7

Modeling using Data Flow Diagrams
44
Chapter 7
FIGURE 7-22
Level-0 data flow
diagram for the
WebStore

Chapter 7software engneeringand system development life cycle.ppt

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