context

context

Process0

student

student

student

Data Flow Data Flow Diagram Diagram

ProgressioProgressionn

Logical and physical DFDs

Models may focus on either:Models may focus on either:

The “physical” view of the real world –The “physical” view of the real world – howhow things are donethings are done

OROR The “logical” view of the real world – The “logical” view of the real world –

whatwhat things are done things are done

Logical Data Flow Diagrams Logical Data Flow Diagrams AdvantagesAdvantages

Advantages of logical DFDs are:Advantages of logical DFDs are: Better communication with users.Better communication with users. Increased understanding of the business Increased understanding of the business

by analysts.by analysts. The system will have increased The system will have increased

flexibility and be easier to maintain.flexibility and be easier to maintain. Elimination of redundancy.Elimination of redundancy.

Physical Data Flow Physical Data Flow DiagramsDiagrams

Physical data flow diagrams show how the Physical data flow diagrams show how the system operates or how the new system system operates or how the new system will be implemented.will be implemented.

Physical data flow diagrams include:Physical data flow diagrams include: Clarifying which processes are manual and Clarifying which processes are manual and

which are automated.which are automated. Describing processes in greater detail.Describing processes in greater detail. Sequencing processes in the order they must Sequencing processes in the order they must

be executed.be executed.

Physical Data Flow Physical Data Flow DiagramsDiagrams

Physical data flow diagrams include Physical data flow diagrams include (continued):(continued): Temporary data stores and transaction files.Temporary data stores and transaction files. Specifying actual document and file names.Specifying actual document and file names. Data flows and data stores are labeled with Data flows and data stores are labeled with

the actual name of the physical media on the actual name of the physical media on which data flow or in which data are storedwhich data flow or in which data are stored

Controls to ensure accuracy and Controls to ensure accuracy and completeness.completeness.

Customer Order

Valid salesordersCheck

Salesorders

Sales orders

reject

Logical and Physical DFDsPhysical DFDs Logical DFDs

View How processing is implemented

What the system does

Processes Actual sequence Essential sequence

Naming Forms, locations, people/roles

Underlying data and activities

Data flows Detailed/ specific/ duplicated data describing exact

Only essential inputs and outputs of the

processes implementation needs

Process modeling

Principal techniques Data flow diagrams Function decomposition

function decomposition diagrams showing only higher levels can be built early in systems analysis

Function Decomposition Function decomposition is the decomposing of a

system into its component functions and processes as a way of managing complexity

Function decomposition creates a top-down view: it shows a hierarchy of levels of increasingly detailed processes within a system

the decomposition of functions corresponds to leveling of processes in DFDs

this structure may not correspond to the structure of the organization shown in an organization chart

Function Decomposition They may be built using either a top

down or a bottom up approach

They provide a useful overview of the processing within a system