PROCESS MODELLING USING DATA FLOW DIAGRAMS

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IMS9001 - Systems Analysis and Design

• Topic 4
• PROCESS MODELLING USING DATA FLOW DIAGRAMS
• DETAILED PROCESS DEFINITIONS
• THE DATA DICTIONAARY

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Logical and physical DFDs

• Data flow diagrams may focus on either
• the physical view of the systems processing
• OR
• the logical view of the systems processing

3
Physical DFDs

• represent a particular way of implementing the
  processes and data in a system
• they are technology dependent
• they show how the processing takes place and how
  the data is implemented

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Logical DFDs

• represent what a system must do regardless of how
  it is implemented
• they are technology independent
• they show what processing, data movements and
  data storage must occur in a system
• they show the essential aspects of a system

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Using Logical and Physical DFDs

• physical DFDs help systems analysts become
  familiar
• with how a business or system operates
• physical DFDs help systems analysts understand
  and
• document problems with existing systems
• users can relate to physical DFDs more readily
  because
• they contain implementation details
• landmarks e.g. people or roles, actual
  locations
• implementation details can be removed
• from physical DFDs

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Physical to Logical DFDs

• use names for data flows and data stores which
  indicate their content, not their physical form
  or location
• use names for processes that indicate what, not
  how

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Physical to Logical DFDs
2.1
checked AZ104 form
AZ104 form
Bill checks form
Master File
valid sales order
2.1
sales order
Validate sales order
Sales orders
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Physical to Logical
eliminate physical processes that refer to
physical activities only and do not transform data
1.1.1
1.1.2
daily mail delivery
opened mail
1.1.3
mail orders
Retrieve mail orders
Open mail
Register mail orders
registered mail orders
1.1.1
customer order
received customer order
Record customer order
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Physical to Logical
remove any data stores that are implementation
dependent
1.1.2
1.1.1
Update master file
Validate trans- actions
transactions
Transaction file
Master file
1.1.1
Validate employee details
employee details
Employees
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Physical to Logical
avoid arbitrary sequencing
2.1.3
2.1.2
A
B
only show this sequence if process B requires
data produced by process A
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Physical to Logical

• consider the implications for processing cycles
  of
• showing data stores

1.1.2
1.1.1
customer invoice
Produce customer invoice
Validate sales order
sales order
Sales orders
OR
1.1.2
1.1.1
Produce customer invoice
customer invoice
valid sales order
Validate sales order
sales order
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Example Physical DFD and Logical DFD
an example physical DFD for part of an order
processing system
AZ4-order form
Sorted orders file
2..1.1
2.1.3
2..1.2
checked order forms
Run the orders program
Orders clerk
Sort into order number
reject
Stock file
processed orders
a logical DFD derived from the physical DFD above
Products
2..1.3
2..1.1
2..1.2
sales orders
valid sales orders
Complete sales orders
Check sales orders
Check stock available
accepted sales orders
reject
filled sales orders
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Logical and Physical DFDs
Physical DFDs Logical DFDs
View How processing is What the system
does implemented Processes
Actual sequence Essential sequence
Naming Forms, locations, Underlying
data and people/roles
activities Data flows
Excess/duplicated data Only essential
inputs for implementation
and outputs of the needs
processes
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Building a Set of Data Flow Diagrams

• originally there were four different types of
  DFDs
• used in a four stage modelling process
• current physical model
• build a set of physical DFDs representing
• the current system
• current logical model
• derive a logical equivalent

• new logical model
• incorporate new system requirements
  identified
• new physical model
• add physical implementation details to reflect
  the
• selected implementation option

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Building a Set of Data Flow Diagrams
disadvantages of the four stage modelling
approach

• unnecessary emphasis on detailed modelling

of existing system

• inhibits creative problem solving and redesign

preferred approach

• create a set of physical DFDs for the current
  system
• which are detailed enough to understand and
  grasp
• any problems

• focus on the essential model of the system, i.e.
  the
• new logical model which identifies what the
  new
• system must do

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Data Dictionary (Repository)

• the data dictionary is a database or repository
  of information about objects identified during
  systems development
• every object (and each of its components) must
  have a definition in the data dictionary
• the data dictionary is a major source of
  documentation about the information system

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Data Dictionary Entries for Components of DFDs

• the data dictionary must contain precise
  definitions of all components of all data flow
  diagrams
• to fully explain the meaning of the DFDs
• to describe the contents of all data flows and
  data stores
• to describe the processing that occurs in
  primitive processes
• to ensure that names and meanings of components
  are used consistently (a common vocabulary)

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Data Dictionary Entries

• a data dictionary entry must be included for each
  
• data flow
• data store
• higher level process
• primitive process
• external agent (source/sink)

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Data elements

• each data flow consists of a series of data
  elements
• a data element is a unit of data that cannot be
  further broken down into meaningful units of data
• each data element should also have an entry in
  the data dictionary
• data flows and data stores are made up of data
  elements

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Data Elements

• a data dictionary entry for a data element should
  include any aliases (alternative names), a brief
  description of its meaning, the kinds of values
  it can take, and the range of values it can have
  (if appropriate)
• E.g. Data element product code
• Alias product number
• Description identifies a product held in
  the warehouse
• Values must be a positive integer
• Range between 1000 and 5000

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Data Flows

• a data dictionary entry for a data flow describes
  the sequence of data elements and data structures
  in the data flow using the following connectors
• is equivalent to
• and
• select one of
• iterations of
• ( ) optional
• comments

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Example Data Flow Entry

• sales order sales order no.
• sales order date
• customer number
• account customer
• cash customer
• customer name
• customer address
• (customer telephone no)
• item no item desc item price
• item qty
• sales order total amount

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Data Stores

• a data store is made up of data flows and data
  elements
• where a data store consists of a collection of
  data flows it is described as repetitions of that
  data flow
• e.g. Data Store customer invoice
• data stores may also be described by listing
  their data elements
• e.g customer deposit account no
• deposit date
• deposit amount
• account balance

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Describing Processes

• each process in higher level DFDs is defined by
  the DFD that decomposes the process at the next
  level down
• these are parent processes
• each such process should have a data dictionary
  entry which includes a brief description of the
  overall nature and purpose of the process

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Describing Processes

• example data dictionary entry for a process
• Treat patients Patient consultations are carried
  out to determine the causes of patients
  illnesses/ medical problems. Further
  treatment/ follow up is recommended if
  appropriate. Details of consultations are
  recorded.
• specific process description (minispecs)

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Describing External Agents

• each external agent (source or sink) should have
  a data dictionary entry which describes its
  relationship with the system
• e.g.
• Referring Doctors
• These are doctors who refer their patients to a
  specialist medical practitioner for treatment.
  They are usually general practitioners.

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Building and Maintaining the Data Dictionary

• determine standard formats and information
  content for all types of data dictionary entries
• have a standard means of organising and storing
  the entries in the data dictionary
• ensure that all components of the DFDs have
  entries in the data dictionary and that they are
  kept up-to-date
• cross-referencing of entries in the data
  dictionary can help to check the completeness and
  consistency of the DFDs and other types of models

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Detailed Process Definitions

• the processing that occurs within the bottom
  level (primitive) processes in DFDs needs to be
  defined
• detailed process descriptions are also known as
  minispecs
• detailed process descriptions form part of the
  data dictionary they define the contents of
  primitive processes

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Detailed Process Definitions

• many techniques can be used to define the
  details of processing
• e.g. narrative text
• Structured English
• decision tables
• decision trees
• flow charts

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Detailed Process Definitions

• detailed process descriptions should
• express what the process does (i.e. policy), not
  how the process is carried out (i.e. procedure)
• be in a form that can be easily understood and
  verified by both users and systems analysts
• be in a form that can be easily communicated to
  all potential stakeholders
• e.g. end-users, systems analysts, managers,
  system designers, project leaders, programmers

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Structured English

• Structured English is a modified form of English
  with some major restrictions on vocabulary and
  structure
• only action (imperative) verbs such as get, put,
  add, calculate, find, delete are used
• only nouns/noun phrases which refer to components
  of the DFDs should be used, i.e. data flows, data
  stores, data elements

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Structured English

• sentences consist of action verbs and DFD
  components
• sentences are combined to form process
  descriptions using the control structures of
  sequence, condition, and repetition

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Control Structures

• Condition uses If... Then...Else
• or
• Select Case Case 1...Case 2...End Case
• E.g.
• If qty-in-stock is less than minimum-order-qty
• Then update stock-reorder indicator
• Else do nothing

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Control Structures

• e.g.
• Select Case
• CASE 1 qty-in-stock is less than
  minimum-order-qty
• Do update stock-reorder indicator
• CASE 2 qty-in-stock is equal to
  minimum- order-qty
• Do nothing
• CASE 3 qty-in-stock is greater than
  minimum- order-qty
• Do nothing
• End Case

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Control Structures

• Sequence is represented with one sentence
  following another in sequence
• Add student to class list
• Decrease available-places
• Calculate class-fee

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Control Structures

• Repetition uses Do-Until or Do-While loops
• Do
• Accept customer-account-details
• Calculate daily-interest daily balance
  daily interest rate
• Add daily-interest to monthly-interest-due
• Until
• no more customer-accounts

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Example Structured English

• Accept sales-order
• Find customer-details
• If customer-details not found
• Then reject sales-order
• Else
• Create sales-order-header
• Do while more sales-order-items
• find item-details
• calculate sales-order-item price
• item price order-qty
• Enddo
• Authorise sales-order
• Endif

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Guidelines for Structured English

• use indentation to indicate control structures
  and their scope assists readability and
  understanding
• avoid more than three levels of nesting
  complicated logic can be represented using other
  techniques
• Stuctured English descriptions should illustrate
  the logic of the processing, not the
  implementation of the processing

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Decision Tables

• decision tables are useful for describing
  processes where several different conditions
  apply and the particular actions that are taken
  are determined by combinations of the values of
  the conditions
• decision tables are useful where the process
  logic is complex
• decision tables show all the possible choices and
  the conditions they depend on in a tabular form

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Decision Tables
decision tables have three stubs (four quadrants)
combinations of condition values
conditions
rules
actions
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Example Decision Table
Y
Y
Y
Y
N N N N
approve
X X
X
X
reject
X
X X X
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Decision Trees

• decision trees are an alternative graphical
  representation of a decision situation as a
  connected series of nodes and branches

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local item
10
imported item
local item
12
7
imported item
Determine Customer Discount
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Selecting Techniques for Process Descriptions

• Structured English is useful where a process has
  a sequence of activities and there is no more
  than three levels of nesting of decisions
• decision trees and decision tables are useful
  where a process involves a decision based on
  combinations of values of several conditions

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Selecting Techniques for Process Descriptions

• decision trees visually distinguish the decision
  conditions and their values from the actions
• they show the paths that decisions can follow
  but soon become cluttered if each condition has
  several possible values
• decision tables are able to show decisions
  involving many conditions each with many possible
  values

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Overview of Process Modelling

• several techniques are available for representing
  the processing within systems
• the aim of process modelling in systems analysis
  is to define the processing that occurs within a
  system

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Process Modelling Techniques

• models representing
• an overview of the system processing
• the structure of the system processing
• the flow of data into and out of the processes
• the detailed logic of the processes
• can be constructed

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References

• HOFFER, J.A., GEORGE, J.F. and VALACICH (2005)
  Modern Systems Analysis and Design, (4th
  edition), Pearson Education Inc., Upper Saddle
  River, New Jersey, USA. Chapters 7, 8
• WHITTEN, J.L., BENTLEY, L.D. and DITTMAN, K.C.
  (2001) 5th ed., Systems Analysis and Design
  Methods, Irwin/McGraw-HilI, New York, NY.
  Chapter 8