Systems Analysis and Design in a Changing World, 6th Edition 1

Online Chapter B

The Traditional Approach to Requirements

Systems Analysis and Design in a Changing World 6th Ed

Satzinger, Jackson & Burd

Online Chapter B

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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 user’s 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 abstraction

Context DiagramDiagram 0Diagram 1

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DFD FragmentsOne for each use case

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Combine DFD Fragments

To Create Diagram 0

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RMO Data Flow Diagrams

Context 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 DFDs

Sometimes 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 (Miller’s 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

flow’s 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 concept—would 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