Chapter 1: From bla to bla

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Online Chapter B
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The Traditional Approach to Requirements
Online Chapter B

• Systems Analysis and Design in a Changing World
  6th Ed
• Satzinger, Jackson Burd

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Online Chapter B Outline

• Traditional and Object-Oriented Views of
  Activities and Use Cases
• Data Flow Diagrams
• Documentation of DFD Components
• Locations and Communication through Networks

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Learning Objectives

• Explain how the traditional approach and the
  object-oriented approach differ when modeling the
  details of a use case
• List the components of a traditional system and
  the symbols representing them on a data flow
  diagram
• Describe how data flow diagrams can show the
  system at various levels of abstraction
• Develop data flow diagrams, data element
  definitions, data store definitions, and process
  descriptions
• Develop tables to show the distribution of
  processing and data access across system locations

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Overview

• Chapters 3, 4, and 5 described two key concepts
  associated with modeling functional requirements
  in the newer (OO) approaches to information
  systems development the use cases and the domain
  classes involved in users work
• This chapter describes an older and more
  traditional approach to representing requirements
• Entity-relationship diagrams represent things in
  the users work domain
• Data flow diagrams (DFDs) and associated concepts
  model system processes instead of use case
  diagrams and system sequence diagrams

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Traditional vs. OO Approach
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Requirements Models for Traditional vs. OO
Approach
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Data Flow Diagram Symbols
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DFD for process Look up item availability
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DFD Decomposes To show different levels of
abstractionContext DiagramDiagram 0Diagram 1
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DFD FragmentsOne for each use case
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Combine DFD FragmentsTo Create Diagram 0
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RMO Data Flow DiagramsContext Diagram
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RMO Subsystems and Use Cases
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Context Diagram for RMO Order Entry Subsystem
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DFD Fragments for RMO Order Entry Subsystem
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Diagram 0 for RMO Order Entry Subsystem
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Decomposing Process 2 from DFD Fragment 2
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Physical DFDsSometimes useful for modeling
processes
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Evaluating DFD Quality

• Minimize complexity
• Information overload difficulty in
  understanding that occurs when a reader receives
  too much information at one time
• Rule of 7 2 (Millers number) the rule of
  model design that limits the number of model
  components or connections among components to no
  more than nine
• Minimization of interfaces a principle of model
  design that seeks simplicity by limiting the
  number of connections among model components

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Evaluating DFD Quality

• Ensuring Data Flow Consistency
• Three Common Errors
• Differences in data flow content between a
  process and its process decomposition
• Data outflows without corresponding data inflows
• Data inflows without corresponding outflows
• Balancing equivalence of data content between
  data flows entering and leaving a process and
  data flows entering and leaving a process
  decomposition DFD
• Black hole a process or data store with a data
  input that is never used to produce a data output
• Miracle a process or data store with a data
  element that is created out of nothing

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Process Descriptions

• Structured English a method of writing process
  specifications that combines structured
  programming techniques with narrative English
• Decision table a tabular representation of
  processing logic containing decision variables,
  decision variable values, and actions or formulas
• Decision tree a graphical description of
  process logic that uses lines organized like
  branches of a tree

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Structured English Example
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RMO Process 2.1 Structured English
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Structured English to determine delivery charges
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Decision Table calculating shipping charges
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Decision Tree calculating shipping charges
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Decision Table with multiple action rows
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Data Flow Definitions

• Data flow definition a textual description of a
  data flows content and internal structure
• Can be simple list of data elements that make up
  the data flow. For new order

• Can use an algebraic notation to indicate
  elements and structure

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Data Flow Definitions

• RMO products and items report defined using
  algebraic notation. Note nested repeating groups
  like typical control break report.

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Data Dictionary and Other Definitions

• Data dictionary a repository for definitions of
  data flows, data elements, and data stores
• A data store on the DFD represents a data entity
  on the ERD, no separate definition is typically
  needed
• Data element definitions
• Describe a data type, such as string, integer,
  floating point, or Boolean
• Each element should also be defined to indicate
  specifically what it represents and how it is
  validated
• Use the person from Mars conceptwould someone
  from Mars know what element means?

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Data Element Definitions
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Locations and Communication Through Networks

• Location diagram a diagram or map that
  identifies all the processing locations of a
  system
• Activity-location matrix a table that describes
  the relationship between processes and the
  locations in which they are performed
• Activity-data matrix a table that describes
  stored data entities, the locations from which
  they are accessed, and the nature of the accesses
• CRUD acronym for create, read, update, and
  delete

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RMO Location Diagram
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RMO Activity-Location Matrix
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RMO Activity-Data Matrix
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Summary

• Data flow diagrams (DFDs) are used in combination
  with the use cases and entity-relationship
  diagram (ERD) to model system requirements
• DFDs model a system as a set of processes, data
  flows, external agents, and data stores
• Many types of DFDs are developed, including
  context diagrams, DFD fragments, subsystem DFDs,
  diagram 0, and process decomposition DFDs
• Each process, data flow, and data store requires
  a detailed definition

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Summary (continued)

• Analysts may define processes in a number of
  ways, including a structured English process
  specification, a decision table, a decision tree,
  or a process decomposition DFD
• Data flows are defined in terms of their
  component data elements and their internal
  structure.
• Data elements may be further defined in terms of
  their type and allowable content
• Data stores correspond to entities on the ERD and
  thus require no additional definition
• The location diagram, activity-location matrix,
  and activity-data matrix describe important
  information about system locations