Structured Design and Modeling

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Structured Designand Modeling

• CS 123/CS 231

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Design Models Revisited

• UML Modeling Language for OO Systems
• For the Procedural (Structured) Paradigm
• Data and processes are separately considered
• Data Model Entity-Relationship Diagram
• Process Model Data-Flow Diagram
• Some modeling techniques apply to both paradigms

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ERDs

• Entity-Relationship Diagrams
• Used to produce a data model for an enterprise
• Models data entities, their attributes, and
  relationship among data entities
• Database Design

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Entities and Attributes

• Entity
• thing, person, place
• examples Book, Sales Invoice, Student,
  Customer, Employee, Department, Airport
• Attribute
• feature of an entity
• examples Name, Address, Age, QCA, Title,
  Author, Amount, City

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Relationships

• Relationship
• an association between (at least) two entities
• examples student borrows a book
• Cardinality
• One-to-one, one-to-many, many-to-many
• Intersection Data
• attribute resulting from relationship
• example date-borrowed

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The Notation

• Rectangles - Entities
• Ellipses - Attributes
• Diamonds - Relationships
• Links
• Labels
• Other models omit the diamonds and use the
  lines and other notation to denote relationships

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Sample Notation
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Example
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Example The University Setting

• Description
• Students enroll in classes. Each class is a
  section of a particular course.
• Entities
• Student, Section, Course

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address
name
year
id number
qca
Student
11
room
index
sched
M
M
Student
enrolls
Section
M
grade
1
Course
has
cat num
desc
title
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Example 2 Library

• Student borrows book
• many to many relationship between students and
  books
• see first diagram
• Further analysis reveals there could be several
  copies of a book
• student borrows copy, book has copy
• see second diagram

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M
M
Student
borrows
Book
14
M
M
Student
borrows
Copy
M
1
Book
has
Exercise Complete Diagram by filling in
attributes
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ERDs and the Software Life Cycle

• Used at the design phase of the software life
  cycle for database systems, although it is often
  used as early as the analysis phase
• In design, it precedes table definition

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From ERD to Tables

• Create a table for each entity
• attributes associated to entity are columns of
  the table (a primary key should exist per table)
• For each 1M relationship
• add the primary key of the 1 participant as an
  attribute of the M participant (foreign key)
  intersection data is also added to the M
  participant

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ERD to Tables, continued

• For each 11 relationship
• add primary key of one table plus intersection
  data to the other table
• For each MM relationship
• create a new table
• attributes are the primary keys of the
  participants plus intersection data

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University Setting Example

• Student
• id number, name, address, year, qca
• Section
• index, room, sched, cat num
• Course
• cat num, title, desc
• Enroll
• id number, index, grade

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DFDs

• Data Flow Diagrams
• Models Operations (Processes)
• Also used in analysis and design

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Traditional approach

• System is a collection of processes
• Processes interact with data entities
• Processes accept inputs and produce outputs

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DFD Definition

• A graphical representation of data processes in
  the organization
• Depicts the broad overview of system inputs,
  processes and outputs
• Dataflow approach emphasizes the logic underlying
  the system
• A tool for system analysis - aid to
  conceptualization

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Processes

• An operation or function in a system
• example Borrow Book
• Involves
• data flow (input and results)
• Data stores, sources and sinks
• Example student and book-copy info are data
  that flow into the process results stored in
  borrow table

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Notation

• Circle (Bubble) or Square with rounded corners -
  Process
• Arrows - Data Flows
• Rectangles Stores and Entities (Sources and
  Sinks)
• Labels

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1 Borrow Book
id num, copy num
borrow confirmation
Student
Borrow
Copy
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Conventions - Symbols
External entity that can send data or receive
data Source/Origin or Sink/Destination - Noun
Movement of data from one point to another - Noun
Transforming process representing work
being performed within the system -
Verb-Adjective-Noun
Repository of data or data store which could be
manual or computerized - Noun
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(No Transcript)
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Comparison of DeMarco Yourdon and Gane Sarson
DFD Symbol Sets
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Illegal Data Flows
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DFD Levels

• A process may involve sub-processes
• provides detail about the process
• DFD levels emerge
• Process hierarchy also depicted using a structure
  chart

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1.1 Check Student Status
id num
borrower status
Student
1.2 Check Book Copy Status
copy num
copy status
Copy
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borrower status, copy status
1.3 Record Borrow
borrow confirmation
id num, copy num
Borrow
Copy
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Developing DFDs - Top Down Approach

• 1. List business activities and determine
• external entities
• data flows
• processes
• data stores
• 2. Create context diagram
• show external entities and data flows in and out
  of the system
• no detailed processes or data stores

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Developing DFDs

• 3. Draw Diagram 0
• show processes, keep them general
• show data stores at this level
• 4. Create the Child Diagram/s for all processes
  of Diagram 0, as necessary
• 5. Check for errors and make sure labels are
  meaningful

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Context Diagram

• Highest level and contains only one process
  representing the entire system
• Broadest possible conceptualization of the system
  - birds eye view of data movement
• Contains the external entities and data flow to
  and from the system
• No data stores
• Numbered 0

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Context Diagram

• Top-level view of an information system that
  shows the systems boundaries and scope.
• Start by placing the process symbol at the
  center place external entities at the perimeter

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Context Diagram of Hoosier Burgers Food Ordering
System
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Diagram 0 - Next Level

• Context diagram explosion into sub-processes
  entities and data flows intact
• Contains new lower level data flows and data
  stores shows details inside black box
• Handling of exceptions is ignored
• Up to 9 processes in single sheet
• Numbered by integers, left to right
• Data stores numbered sequentially - D

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Level 0 DFD of Hoosier Burgers Food Ordering
System
Diagram 0 Food Ordering System
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Child Diagram - Low Level

• Parent Process in Diagram 0 is exploded to one or
  more Child Diagrams
• Vertical balancing - Diagram 0 data flows intact
  called interface data flow
• New data flows error line included
• No external entities
• New data stores Diagram 0 data stores are
  optional

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Child Diagram - Low Level

• Numbering follows Parent Process - 3.1
• Primitive Process
• If Parent Process is not exploded and is
  functionally primitive
• If Child Diagram Process cannot be further broken
  down
• Logic is written as Process Specifications

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Level 1 Diagram Showing Decomposition of Process
1.0 from the Level 0 Diagram
Diagram 1 Receive and Transform Customer Order
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Level 1 Diagram Showing the Decomposition of
Process 4.0 from the Level 0 Diagram
Diagram 4 Produce Management Reports
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Level 2 Diagram Showing the Decomposition of
Process 4.3 from the Level 1 Diagram for Process
4.0
Diagram 4.3 Prepare Management Reports
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About DFDs

• Data flows should balance between levels
• Number label format X.X.X provides level
  information
• Context Diagram
• Level 0 (or system-level) DFD
• Notational Variations

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Checking for Errors

• Forgetting to include a data flow or pointing the
  arrow in the wrong direction
• shows all inputs and no outputs or vice versa
• Connecting data stores and entities directly -
  should only connect to a process
• Incorrect label for process and data flow
• Not too many processes in a diagram
• cluttered diagram and hinders communication

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Checking for Errors

• Omitting data flow linear flows with single
  input and single output are rare and may indicate
  missing data
• Creating unbalanced decomposition in child
  diagrams interface data flows should be same as
  parent
• Exception - error lines only in child diagram

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DFD Summary

• DFDs are structured analysis tools for better
  understanding of the logical movement of data and
  its transformation presented in visual form
• DFDs are used to document systems independent of
  its analyst

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Summary

• Modeling techniques and notation
• needed in analysis and design
• models data and processes of a system
• Other techniques and notation
• state diagrams, structure charts, flowcharts,
  document flow diagrams, more...
• Structured vs OO Paradigm
• some techniques specific to the paradigm