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Data and Process Modeling. Chapter 5. Introduction. In chapter 5, you use data and process modeling techniques to develop a logical model of the proposed system and document the system requirements Logical model shows what the system must do

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introduction
Introduction
  • In chapter 5, you use data and process modeling techniques to develop a logical model of the proposed system and document the system requirements
    • Logical model shows what the system must do
    • Physical model describes how the system will be constructed
overview of data and process modeling tools
Overview of Data and Process Modeling Tools
  • Systems analysts use many graphical techniques to describe an information system
  • Three main tools for data and process modeling:
    • DFD (data flow diagram)
    • Data dictionary
    • Process descriptions
data flow diagrams
Data Flow Diagrams
  • A data flow diagram (DFD) uses various symbols to show how the system transforms input data into useful information
  • A data flow diagram (DFD) shows howdata moves through an information system but does not show program logic or processing steps
  • A set of DFDs provides a logical model that shows whatthe system does, not howit does it
data flow diagrams1
Data Flow Diagrams
  • DFD Symbols
    • DFDs use four basic symbols that represent processes, data flows, data stores, and entities
      • Gane and Sarson symbol set
      • Yourdon symbol set
    • Symbols are referenced by using all CAPITAL LETTERS for the symbol names
data flow diagrams2
Data Flow Diagrams
  • DFD Symbols
    • Process symbol
      • Receives input data and produces output that has a different content, form, or both
      • Referred to as a black box, where the inputs, outputs, and general function of the process are known, but the underlying details are not shown.
      • The process name identifies a specific function and consists of a verb followed by a singular noun
data flow diagrams3
Data Flow Diagrams
  • DFD Symbols
    • Data flow symbol
      • Represents one or more data items
      • The symbol for a data flow is a line with a single or double arrowhead
      • Spontaneous generation(noinput)
      • Black hole (no output)
      • Gray hole (給不足的input)
      • Incorrect examples (p. 201)
data flow diagrams4
Data Flow Diagrams
  • DFD Symbols
    • Data store symbol
      • Is a flat rectangle that is open on the right side and closed on the left side
      • Used to represent data that the system stores for later use.

Ex. Store students’ test scores to calculate the final grade later

      • The physical characteristics of a data store are unimportant because you are concerned only with a logical model
      • A data store MUST be connected to a process with a data flow.
      • See p. 202 for correct and incorrect examples
      • Exception for data store without input dataflow
        • Ex. A data store called TAX TABLE, 儲存國稅局規定的扣繳比率
data flow diagrams5
Data Flow Diagrams
  • DFD Symbols
    • Entity Symbol
      • Symbol is a rectangle, which may be shaded to make it look three-dimensional
      • 通常是人或事
      • Name of the entity appears inside the symbol
      • Also called Terminators: because they are data origins or final destination
      • Source:來源
      • Sink:結束
      • Each entity must be connected to a process by a data flow
      • Ex. P. 203
creating a set of dfds
Creating a Set of DFDs
  • Create a graphical model of the information system based on your fact-finding results
  • ReviewasetofguidelinesfordrawingDFDs.
  • Performing three main tasks
    • Step 1: Draw a context diagram
    • Step 2: Draw a diagram 0 DFD
    • Step 3: Draw the lower-level diagrams
creating a set of dfds1
Creating a Set of DFDs
  • Drawing Guidelines
      • Draw the context diagram so it fits on one page
      • Use the name of the information system as the process name in the context diagram
      • Use unique names within each set of symbols
      • Do not cross lines
      • Provide a unique name and reference number for each process
      • Obtain user input and feedback
  • Draw a Context Diagram
creating a set of dfds2
Creating a Set of DFDs
  • Step1:Draw a Context Diagram (全景圖)
      • More examples: p. 206
creating a set of dfds3
Creating a Set of DFDs
  • Step2:Draw a Diagram 0 DFD
    • Diagram 0, P. 208
    • Zooms in (放大) on the context diagram and shows major processes, data flows, and data stores
    • Must retain all the connections that flow into and out of process 0 (balancing)
    • Each process has a reference number
    • Diverging(分岔) data flow
creating a set of dfds4
Creating a Set of DFDs
  • Draw a Diagram 0 DFD
    • If same data flows in both directions, you can use a double-headed arrow
    • Diagram 0 represents exploded viewof process 0
    • Parent diagram:上一層
    • Child diagram:下一層
    • Functional primitive: a process that consists of a single function that can not be exploded further.
creating a set of dfds5
Creating a Set of DFDs
  • Draw the Lower-Level Diagrams
    • Whentodrawadatastore?
      • Onlywhen2ormoreprocessesusethatdatastore
    • Must use leveling and balancing techniques
    • Leveling
      • Uses a series of increasingly detailed DFDs to describe an information system
      • Exploding, partitioning, or decomposing
      • Example: p. 210-212
creating a set of dfds6
Creating a Set of DFDs
  • Draw the Lower-Level Diagrams
    • Balancing
      • Ensures that the input and output data flows of the parent DFD are maintained on the child DFD
  • Figure 5-19, 5-20,21
creating a set of dfds7
Creating a Set of DFDs
  • Draw the Lower-Level Diagrams
data flow diagrams6
Data Flow Diagrams
  • Strategies for Developing DFDs
    • A set of DFDs is a graphical, top-down model
    • With a bottom-up strategy, you first identify all functional primitives, data stores, entities, and data flows
    • The main objective is to ensure that your model is accurate and easy to understand
data flow diagrams7
Data Flow Diagrams
  • Strategies for Developing DFDs
    • General rule of thumb is that a diagram should have no more than nine process symbols
    • To construct a logical model of a complex system, you might use a combination of top-down and bottom-up strategies
    • The best approach depends on the information system you are modeling
    • Exercise: what is wrong in this diagram 0 DFD?

p. 215, figure 5-22

data dictionary
Data Dictionary
  • A data dictionary, or data repository, is a central storehouse of information about the system’s data
  • An analyst uses the data dictionary to collect, document, and organize specific facts about the system
  • Also defines and describes all data elements and meaningful combinations of data elements
data dictionary1
Data Dictionary
  • A data element, also called a data item or field, is the smallest piece of data that has meaning
  • Data elements are combined into records, also called data structures
  • A record is a meaningful combination of related data elements that is included in a data flow or retained in a data store
data dictionary2
Data Dictionary
  • Documenting the Data Elements
    • You must document every data element in the data dictionary
    • The objective is the same: to provide clear, comprehensive information about the data and processes that make up the system
data dictionary3
Data Dictionary
  • Documenting the Data Elements
    • The following attributes usually are recorded and described
      • Data element name or label
      • Alias
      • Type and length
      • Default value
      • Acceptable values - Domain and validity rules
data dictionary4
Data Dictionary
  • Documenting the Data Elements
    • The following attributes usually are recorded and described
      • Source
      • Security
      • Responsible user(s)
      • Description and comments
data dictionary5
Data Dictionary
  • Documenting the Data Flows
    • The typical attributes are as follows
      • Data flow name or label
      • Description
      • Alternate name(s)
      • Origin
      • Destination
      • Record(a group of related data elements)
      • Volume and frequency
data dictionary6
Data Dictionary
  • Documenting the Data Stores
    • Typical characteristics of a data store are
      • Data store name or label
      • Description
      • Alternate name(s)
      • Attributes
      • Volume and frequency
data dictionary7
Data Dictionary
  • Documenting the Processes
    • Typical characteristics of a process
      • Process name or label
      • Description
      • Process number
      • Process description
data dictionary8
Data Dictionary
  • Documenting the Entities
    • Typical characteristics of an entity include
      • Entity name
      • Description
      • Alternate name(s)
      • Input data flows
      • Output data flows
data dictionary9
Data Dictionary
  • Documenting the Records
    • Typical characteristics of a record include
      • Record or data structure name
      • Definition or description
      • Alternate name(s)
      • Attributes
data dictionary10
Data Dictionary
  • Data Dictionary Reports
    • Many valuable reports
      • An alphabetized list of all data elements by name
      • A report describing each data element and indicating the user or department that is responsible for data entry, updating, or deletion
      • A report of all data flows and data stores that use a particular data element
      • Detailed reports showing all characteristics of data elements, records, data flows, processes, or any other selected item stored in the data dictionary
process description tools
Process Description Tools
  • A process description documents the details of a functional primitive, and represents a specific set of processing steps and business logic
process description tools1
Process Description Tools
  • Modular Design
    • Based on combinations of three logical structures, sometimes called control structures, which serve as building blocks for the process
      • Sequence
      • Selection
      • Iteration - looping
process description tools2
Process Description Tools
  • Structured English
    • Must conform to the following rules
      • Use only the three building blocks of sequence, selection, and iteration
      • Use indentation for readability
      • Use a limited vocabulary, including standard terms used in the data dictionary and specific words that describe the processing rules
process description tools3
Process Description Tools
  • Structured English
    • Might look familiar to programming students because it resembles pseudocode
process description tools4
Process Description Tools
  • Decision Tables
    • Shows a logical structure, with all possible combinations of conditions and resulting actions
    • It is important to consider every possible outcome to ensure that you have overlooked nothing
process description tools5
Process Description Tools
  • Decision Tables
    • Can have more than two possible outcomes
    • Often are the best way to describe a complex set of conditions
    • Simplify tables: p. 225
process description tools6
Process Description Tools
  • Decision Trees
    • Graphical representation of the conditions, actions, and rules found in a decision table
    • Whether to use a decision table or tree often is a matter of personal preference
logical versus physical models
Logical Versus Physical Models
  • While structured analysis tools are used to develop a logical model for a new information system, such tools also can be used to develop physical models of an information system
  • A physical model shows how the system’s requirements are implemented
dfd supplement
DFD supplement
  • More on DFD