Chapter08 structuring system requirements

Copyright 2002 Prentice-Hall, Inc.
Modern Systems Analysis
and Design
Third Edition
Jeffrey A. Hoffer
Joey F. George
Joseph S. Valacich
Chapter 8
Structuring System
Requirements:
Process Modeling
8.1

Learning Objectives
Understand the logical modeling of
processes through studying data flow
diagrams
How to draw data flow diagrams using
rules and guidelines
How to decompose data flow diagrams
into lower-level diagrams
Balancing of data flow diagrams
8.2

Learning Objectives
Explain the differences among four types of
DFDs: current physical, current logical, new
physical and new logical
Discuss the use of data flow diagrams as
analysis tools
Compare and contrast data flow diagrams
with Oracle’s process modeling tool and with
functional hierarchy diagrams
Discuss process modeling for Internet
applications
8.3

Process Modeling
Graphically represent the processes
that capture, manipulate, store and
distribute data between a system and
its environment and among system
components
Data flow diagrams (DFD)
 Graphically illustrate movement of data
between external entities and the
processes and data stores within a system
8.4

Process Modeling
Modeling a system’s process
 Utilize information gathered during
requirements determination
 Structure of the data is also modeled in
addition to the processes
Deliverables and Outcomes
 Set of coherent, interrelated data flow
diagrams
8.5

Process Modeling
Deliverables and outcomes (continued)
 Context data flow diagram (DFD)
 Scope of system
 DFDs of current system
 Enables analysts to understand current system
 DFDs of new logical system
 Technology independent
 Show data flows, structure and functional
requirements of new system
8.6

Process Modeling
Deliverables and outcomes (continued)
 Project dictionary and CASE repository
8.7

Data Flow Diagramming
Mechanics
Four symbols are used
 See Figure 8-2
 Two different standard sets can be used
 DeMarco and Yourdan
 Gane and Sarson
8.8

Figure 8-2
Comparison of DeMarco & Yourdan and Gane & Sarson
DFD symbol sets
8.9

Mechanics
Data Flow
 Depicts data that are in motion and moving
as a unit from one place to another in the
system.
 Drawn as an arrow
 Select a meaningful name to represent the
data
8.10

Mechanics
Data Store
 Depicts data at rest
 May represent data in
 File folder
 Computer-based file
 Notebook
 The name of the store as well as the
number are recorded in between lines
8.11

Mechanics
Process
 Depicts work or action performed on data
so that they are transformed, stored or
distributed
 Number of process as well as name are
recorded
8.12

Mechanics
Source/Sink
 Depicts the origin and/or destination of the
data
 Sometimes referred to as an external entity
 Drawn as a square symbol
 Name states what the external agent is
 Because they are external, many
characteristics are not of interest to us
8.13

Definitions
Context Diagram
 A data flow diagram (DFD) of the scope of an
organizational system that shows the system
boundaries, external entities that interact with the
system and the major information flows between
the entities and the system
Level-O Diagram
 A data flow diagram (DFD) that represents a
system’s major processes, data flows and data
stores at a high level of detail
8.14

Developing DFDs: An
Example
Hoosier Burger’s automated food
ordering system
Context Diagram (Figure 8-4) contains
no data stores
Next step is to expand the context
diagram to show the breakdown of
processes (Figure 8-5)
8.15

Figure 8-4
Context diagram of Hoosier Burger’s
food ordering system
8.16

Figure 8-5
Level-0 DFD of Hoosier Burger’s food ordering system
8.17

Data Flow Diagramming Rules
Basic rules that apply to all DFDs
 Inputs to a process are always different
than outputs
 Objects always have a unique name
 In order to keep the diagram uncluttered, you
can repeat data stores and sources/sinks on a
diagram
8.18

Process
 No process can have
only outputs (a
miracle)
 No process can have
only inputs (black
hole)
 A process has a verb
phrase label
Data Store
 Data cannot be moved
directly from one store to
another
 Data cannot move
directly from an outside
source to a data store
directly from a data store
to a data sink
 Data store has a noun
phrase label
8.19

Source/Sink
directly from a
source to a sink
 A source/sink has a
noun phrase label
Data Flow
 A data flow has only one
direction of flow between
symbols
 A fork means that exactly
the same data goes from
a common location to two
or more processes, data
stores or sources/sinks
8.20

Data Flow (Continued)
L. A join means that exactly the same data comes
from any two or more different processes, data
stores or sources/sinks to a common location
M. A data flow cannot go directly back to the same
process it leaves
N. A data flow to a data store means update
O. A data flow from a data store means retrieve or
use
P. A data flow has a noun phrase label
8.21

Decomposition of DFDs
Functional decomposition
 Act of going from one single system to many
component processes
 Repetitive procedure
 Lowest level is called a primitive DFD
Level-N Diagrams
 A DFD that is the result of n nested
decompositions of a series of subprocesses from
a process on a level-0 diagram
8.22

Balancing DFDs
When decomposing a DFD, you must
conserve inputs to and outputs from a
process at the next level of decomposition
This is called balancing
Example: Hoosier Burgers
 In Figure 8-4, notice that there is one input to the
system, the customer order
 Three outputs:
 Customer receipt
 Food order
 Management reports
8.23

Balancing DFDs
Example (Continued)
 Notice Figure 8-5. We have the same
inputs and outputs
 No new inputs or outputs have been
introduced
 We can say that the context diagram and
level-0 DFD are balanced
8.24

Balancing DFDs
An unbalanced example
 Figure 8-10
 In context diagram, we have one input to
the system, A and one output, B
 Level-0 diagram has one additional data
flow, C
 These DFDs are not balanced
8.25

Figure 8-10
An unbalanced set of data flow diagrams
(a) Context diagram
(b) Level-0 diagram
8.26

Balancing DFDs
We can split a data flow into separate
data flows on a lower level diagram
(see Figure 8-11)
Balancing leads to four additional
advanced rules (See Table 8-3)
8.27

Four Different Types of DFDS
Current Physical
 Process label includes an identification of
the technology (people or systems) used to
process the data
 Data flows and data stores are labeled with
the actual name of the physical media on
which data flow or in which data are stored
8.28

Current Logical
 Physical aspects of system are removed as much
as possible
 Current system is reduced to data and processes
that transform them
New Logical
 Includes additional functions
 Obsolete functions are removed
 Inefficient data flows are reorganized
8.29

New Physical
 Represents the physical implementation of
the new system
8.30

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
8.31

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
8.32

Primitive DFDs
 Lowest logical level of decomposition
 Decision has to be made when to stop
decomposition
8.33

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
 When the system user does not care to
see any more detail
8.34

Rules for stopping decomposition (continued)
 When every data flow does not need to be split
further to show that data are handled in various
ways
 When you believe that you have shown each
business form or transaction, on-line 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
8.35

Using DFDs as Analysis Tools
Gap Analysis
 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
8.36

Using DFDs in Business
Process Reengineering
Example: IBM Credit
 See Figure 8-20 – before reengineering
 Credit approval process required six days
before BPR
 Figure 8-21 depicts DFD after
reengineering
 IBM was able to process 100 times the
number of transactions in the same
amount of time
8.37

Oracle’s Process Modeler and
Functional Hierarchy Diagrams
Process Modeler
 Unique to Oracle
 Similar to DFDS but outputs and methods differ in
several ways.
 Table 8-4 illustrates differences
 Picture of various tasks performed in a business
and how they are related
 Tasks are broken down into their various parts
 Does not include data flows
8.38

Summary
Data flow diagrams (DFD)
 Symbols
 Rules for creating
 Decomposition
 Balancing
Four different kinds of DFDs
 Current Physical
 Current Logical
 New Logical
 New Physical
8.39

Summary
DFDs for Analysis
DFDs for Business Process
Reengineering (BPR)
Oracle’s Process Modeler
8.40

Chapter08 structuring system requirements

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Chapter08 structuring system requirements