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Chapter 7Structuring System Process
Requirements
2011 by Prentice Hall: J.A.Hoffer et.al., Modern Systems Analysis & Design, 6th Edition 2/68
Content9 Understand the logical (&physical) process modeling
by using data flow diagrams (DFDs)9 Draw DFDs & Leveling9 Balance higher-level and lower-level DFDs9 Explain the differences among four types of DFDs:
current physical, current logical, new physical, and new logical
9 Use DFDs as a tool to support the analysis of ISs9 Discuss process modeling for e-Commerce
applications9 Process Description **
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1. Process Modeling
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Process Modeling (Cont.)
Graphically represent the processes that capture, manipulate, store, and distribute databetween a system and its environment and among system components
Utilize information gathered during requirements determination
Processes and data structures are modeled
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1.1 Deliverable and Outcome 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
DFDs of new logical system Technology independent Show data flows, structure, and functional requirements of
new system DFDs of new physical system Description of each DFD component
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2. Data Flow Diagramming Mechanics Represent both physical and logical ISs Only four symbols are used
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Definitions and Symbols (Cont.)
Processwork or actions performed on data
Data storedata at rest (inside the system)
Source/sink (destination); terminator; actor; boundary external entity that is origin or destination of data
(outside the system) Data flowarrows depicting movement of data
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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
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Different between Sources/ Sinks and Processes
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2.1 Developing DFDs: an Example
Context diagram is an overview of an organizational system that shows the system boundariesexternal entities that interact with the systemMajor information flows between the entities
and the system Note: only one process symbol, and no data
stores shown
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Context Diagram
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Developing DFDs (Cont.)
Level-0 diagram is a data flow diagram that represents a systems major processes, data flows, and data stores at a high level of detailProcesses are labeled 1.0, 2.0, etc. These will be
decomposed into more primitive (lower-level) DFDs.
Leveling & Balancing
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Level-0 Diagram
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2.2 Data Flow Diagramming Rules
Two DFD guidelinesThe inputs to a process are different from
the outputs of that processProcesses purpose is to transform inputs into
outputsObjects on a DFD have unique namesEvery process has a unique name
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Data Flow Diagramming Rules (Cont.)
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Data Flow Diagramming Rules (Cont.)
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B2
Illegal Data Flows
Whitten 18/68
2.3 Decomposition of DFDs Functional decomposition is an iterative
process of breaking a system description down into finer and finer detail
PROJECTGOAL
0
PHASE
2PHASE
3PHASE
1
ACTIVITY
2.2ACTIVITY
2.1ACTIVITY
2.3
TASK
2.2.2TASK
2.2.1TASK
2.2.3
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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 a n nested decompositions from a process on a level-0 diagram.
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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.
Balancing!!!
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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.
Not Necessary for this level!!
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2.4 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
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Balancing DFDs (Cont.)
Balanced means (in general):Number of inputs to lower level DFD equals
number of inputs to associated process of higher-level DFDNumber of outputs to lower level DFD
equals number of outputs to associated process of higher-level DFD
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Balancing DFDs (Cont.)
This is unbalanced because the process of the context diagram has only one input but the Level-0 diagram has two inputs.
1 input1 output
2 inputs1 output
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Balancing DFDs (Cont.)
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
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Balancing DFDs (Cont.)
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Balancing DFDs: More DFD Rules
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Exercise
Document your project requirements by usingContext diagram (scope)L-0 DFD (main activities)++ Leveling & Balancing ++ Process description
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3. 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
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Four Different Types of DFDs (Cont.)
New Logical Include additional functionsRemove obsolete functions Reorganize inefficient data flows
New PhysicalRepresents the physical implementation of the new
system
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System Development Model (Classical Model)
AS IS (Current)
1 Physical Model
2 Logical Model
TO BE (New)
4 Physical Model
3 Logical Model
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4. Using Data Flow Diagram in the Analysis Process
4.1 Guidelines for Drawing DFDsCompleteness
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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Guidelines for Drawing DFDs (Cont.)
TimingTime is not represented well on DFDsBest to draw DFDs as if the system has never
started and will never stop Iterative DevelopmentAnalyst should expect to redraw diagram several
times before reaching the closest approximation to the system being modeled
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Primitive DFDs
Lowest logical level of decomposition Decision has to be made when to stop
decomposition
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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 each 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 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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4.2 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
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4.3 Using DFDs in BPR
FIGURE 7-16IBM Credit Corporations primary work process before BPR(Source: Based on Hammer and Champy, 1993.) 38/68
Using DFDs in BPR (Cont.)
FIGURE 7-17IBM Credit Corporations primary work process after BPR(Source: Based on Hammer and Champy, 1993.)
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5. e-Commerce Application: Process Modeling using Data Flow Diagram
Process modeling for Pine Valley Furnitures Webstore
Completed JAD session Began translating the Webstore system
structure into data flow diagram Identified six high-level processes
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e-Commerce Application: Process Modeling using Data Flow Diagrams (Cont.)
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e-Commerce Application: Process Modeling using Data Flow Diagrams (cont.)
Figure 7-22 Level-0 data flow diagram for the WebStore42/68
6. Process Description 9 Use Structured English as a tool for representing
steps in logical processes in DFDs9 Use decision tables to represent the logic of
choice in conditional statements9 Select among Structured English and decision
tables for representing processing logic9 Understand how logic modeling techniques
apply to the development of your project study
2011 by Prentice Hall: J.A.Hoffer et.al., Modern Systems Analysis & Design, 6th Edition
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Logic Modeling: process description
Data flow diagrams (DFDs) do not show the logic inside the processes
Logic modeling involves representing internal structure and functionality of processes depicted on a DFD
Logic modeling can also be used to show when processes on a DFD occur
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Deliverable and Outcome
Structured English representation of process logic
Decision Tables representation OOT Interaction diagram Sequence diagram &
Communication diagramActivity diagram
Plus: decision tree & flowchart
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Structured English
Structured English: modified form of English language used to specify the logic of IS processes
Typically relies on action verbs and noun phrases and contains no adjectives or no specific standards
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Structured English
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Structured English
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Structured English Constructs (Part 1)
Whitten
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Structured English Constructs (Part 2)
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Process Description: Modeling Logic * * * Process #: Process NameDescription: .......................Created Date: .....................
InputActor
formP#: ...
attribute, ...D#: ...
(attribute, ...)
OutputActor
reportP#: ...
attribute, ...D#: ...
(attribute, ...)
Tasks and Activities (show logic not brief description)Use:
Structured English orActivity diagramFlowchartDecision table/ tree etc.
P# process
boundary
o
u
t
p
u
t
i
n
p
u
t
D# data store
p p
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Decision Table
Decision table: a matrix representation of the logic of a decision which specifies the possible conditions for the decision and the resulting actions
Best used for complicated decision logic
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Decision Table
1
2
3
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Decision Table
Condition stubs: list conditions that relevant to the decision
Action stubs: list actions that result for a given set of conditions
Rules: define which actions are to be followed for a given set of conditions
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Modeling Logic with Decision Table
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Logic Modeling (Cont.)
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Logic Modeling (Cont.)
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Summary9 Understand the logical process modeling by
using data flow diagrams (DFDs)9 Draw DFDs & Leveling9 Balance higher-level and lower-level DFDs9 Explain the difference among four types of
DFDs: current physical, current logical, new physical, and new logical
9 Process Description
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Questions & Answers
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Exercise
What is a data flow diagram? Why do system analysts use data flow diagrams (DFDs)?
Explain the rules of drawing good DFDs. What unique rules apply to drawing context
diagram?
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Case Study
A simple banking system1 Open new a/c2 Deposit and withdraw3 Close a/cDraw the context diagram and DFD of this
simple banking system.
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Main Activity New customers are registered; and accounts are
closed by bank manager When accounts are closed, customers are notified
by a system Cash deposits into an account are made by the
customer, and salary are paid by the employer The bank clerk performs a balance check before
allowing a withdrawal by a customer at the bank counter
List of System Logical ElementsWichian 62/68
List of system logical elements
Wichian
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Context Diagram :A simple banking system
Wichian 64/68
Data Flow Diagram : a simple banking system
Wichian
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Data Flow Diagram : a simple banking system
Wichian 66/68Wichian
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Problems of System Development
Poor system thinking Poor system integration Poor verifying the modelFinding missing algorithmSyntax checkingTracing data elementCross-referencingTracing objective
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Physical Data Flow Diagrams (PDFDs)
Technical and human decisions to be implemented as part of an information system
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Example: Classify the following aspects into functional (F), non-functional (NF), or not be a requirement.
1. How information about flights, passengers and booking are entered.
2. What information appears on tickets and reports.
3. How fares are calculated.4. What information must be stored in the
database that travel agents and others access.5. The system should be designed so it can be
extended to handle a frequent flier plan.6. The system must be available at all times.
Only 2 minutes downtime a week is to be permitted.
7. A merge sort algorithm must be used to sort the flights by departure time.