MELJUN CORTES System Modelling Data flow Diagram (DFD)

SystemsModelingSystemsModeling

MELJUN CORTESMELJUN CORTES

System ModelSystem Model

A representation of an in-place or proposed systemthat describes the data flow throughout the structure. A representation of an in-place or proposed systemthat describes the data flow throughout the structure.

The model describes the points where dataor information enters a system and places where it will be processed, as well as the actions taken and the points where data will be output.

The model describes the points where dataor information enters a system and places where it will be processed, as well as the actions taken and the points where data will be output.

A system model is documented through a variety of A system model is documented through a variety of DESIGN DIAGRAMDESIGN DIAGRAM

a graphic or visual representation of a structure. a graphic or visual representation of a structure.

It includes data flow diagrams, structured charts, decisiontrees, and other items.It includes data flow diagrams, structured charts, decisiontrees, and other items.

Advantages of a Design DiagramAdvantages of a Design Diagram

• Serves as a communications tool. • Serves as a communications tool.

It provides a concrete, visual medium for describing complex plans or problem solutionIt provides a concrete, visual medium for describing complex plans or problem solution

• Serves as a planning tool.• Serves as a planning tool.

It helps the SA to visualize the relationships and movements that will exist within a system while it isstill in the planning stages

It helps the SA to visualize the relationships and movements that will exist within a system while it isstill in the planning stages

• Provide an overview of a system.• Provide an overview of a system.

Makes it possible to see the important elementsand relationships in a system , free ofextraneous details - birds-eye view of the major steps and processes.

Makes it possible to see the important elementsand relationships in a system , free ofextraneous details - birds-eye view of the major steps and processes.


• Define roles.• Define roles.

It spells out in pictorial forms the roles played by personnel and workstations and processes in a systemwhere it shows data, forms and reports generated andthe end results produced.

It spells out in pictorial forms the roles played by personnel and workstations and processes in a systemwhere it shows data, forms and reports generated andthe end results produced.

• Demonstrate relationships.• Demonstrate relationships.

Points out relationships that may not be readily obvious from verbal or written descriptions. It showshow data elements relate to each other, as well as the relationships between parts of a system.

Points out relationships that may not be readily obvious from verbal or written descriptions. It showshow data elements relate to each other, as well as the relationships between parts of a system.


• Promote logical procedures.• Promote logical procedures.

It helps SA clarify and refine their thinking and to describe explicitly all paths, branches and flows in a system. With this, SA can identify incorrect branches and missing elements and may discover more efficient way to implement a part of a system.

It helps SA clarify and refine their thinking and to describe explicitly all paths, branches and flows in a system. With this, SA can identify incorrect branches and missing elements and may discover more efficient way to implement a part of a system.

• Facilitate troubleshooting.• Facilitate troubleshooting.

It serves as a blue print of the inner working of a system, to which the troubleshooter can refer whendiagnosing a system failure.

It serves as a blue print of the inner working of a system, to which the troubleshooter can refer whendiagnosing a system failure.


• Document a system.• Document a system.

It records graphically, clearly, and permanently the important elements of a system. This facilitates makingchanges, revisions, and modifications at a later date.It also enables those unfamiliar with a system to grasp its elements easily and quickly.

It records graphically, clearly, and permanently the important elements of a system. This facilitates makingchanges, revisions, and modifications at a later date.It also enables those unfamiliar with a system to grasp its elements easily and quickly.

System DesignSystem Design

the evaluation of alternative solutions andthe specification of a detailed computer-based solution

the evaluation of alternative solutions andthe specification of a detailed computer-based solution

this phase includes resolving any user design issues and developing the technical, detailed design of each specific program

this phase includes resolving any user design issues and developing the technical, detailed design of each specific program

System DesignSystem Design

review system requirements, then to consider someof the major aspects of a system.review system requirements, then to consider someof the major aspects of a system.

Preliminary DesignPreliminary Design

“Should the system be centralized or distributed?”“Should the system be centralized or distributed?”

“Should the system be on-line?”“Should the system be on-line?”“Should packaged software be purchased as opposed to having programmers write new software?”“Should packaged software be purchased as opposed to having programmers write new software?”“Can the system be run on the user’s personal computer?”“Can the system be run on the user’s personal computer?”“How will input data be captured?”“How will input data be captured?”“What kind of reports will be needed?”“What kind of reports will be needed?”

Modeling ToolsModeling Tools

Data Flow DiagramData Flow Diagram

a graphic illustration that shows the flow of dataand logic within a system. a graphic illustration that shows the flow of dataand logic within a system.

the shape of the symbol indicates to the analyst that a specific operation is performed. The arrows that connectthe symbol shows the different direction in which the data flows.

the shape of the symbol indicates to the analyst that a specific operation is performed. The arrows that connectthe symbol shows the different direction in which the data flows.

Data Flow DiagramData Flow Diagram

The detail and complexity of data flow diagram varieswith the system being described. As you work The detail and complexity of data flow diagram varieswith the system being described. As you work

As you work with the systems, you will find yourselfcontinually relying upon diagrams such as these to showspoints where data originates, is processed or transformed,and is output.

As you work with the systems, you will find yourselfcontinually relying upon diagrams such as these to showspoints where data originates, is processed or transformed,and is output.

Four Basic SymbolsFour Basic Symbols

External EntityExternal Entity

a square box that specifies either the sourceor the destination of data. It shows where data originatesoutside a system or where it will be transmitted after processing, always outside the system.

a square box that specifies either the sourceor the destination of data. It shows where data originatesoutside a system or where it will be transmitted after processing, always outside the system.

• sometimes called sources and sinks.• sometimes called sources and sinks.

Source - a point outside the system that generates dataSource - a point outside the system that generates data

Sink - a point outside the system that receives dataSink - a point outside the system that receives data

• both are external entities, drawn assquare boxes

• both are external entities, drawn assquare boxes


ProcessProcess

drawn as a rectangle with rounded corners.It represents the transformation or processing of informationwithin a system. The process symbols shows those places ina system where calculations are made or where informationis changed in character.

drawn as a rectangle with rounded corners.It represents the transformation or processing of informationwithin a system. The process symbols shows those places ina system where calculations are made or where informationis changed in character.

1.51.5

The top of the process box is usually ruled off, leaving spaceto enter a reference number. The reference number is usedto key more detailed data flow diagrams to the box

The top of the process box is usually ruled off, leaving spaceto enter a reference number. The reference number is usedto key more detailed data flow diagrams to the box


Data StoreData Store

a point in a system where information is permanentlyor temporary stored or held. It shown as a rectangle with one end open.

a point in a system where information is permanentlyor temporary stored or held. It shown as a rectangle with one end open.

The left side of the box may be ruled off to enter a reference number, which keys the box to other diagrams

The left side of the box may be ruled off to enter a reference number, which keys the box to other diagrams

1.71.7

data may be stored on filing cards, ledger sheets,floppy disk or even on the check stubs in a checkbook. While stored, the information remains intact and is not changed or modified in any way.

data may be stored on filing cards, ledger sheets,floppy disk or even on the check stubs in a checkbook. While stored, the information remains intact and is not changed or modified in any way.


FlowlineFlowline

sometimes called pipes or vectors, connect externalentities, process, and data store elements.

sometimes called pipes or vectors, connect externalentities, process, and data store elements.

Information flow may be one-way or two-way. One or two arrows are drawn between boxes to show which waythe information flows

Information flow may be one-way or two-way. One or two arrows are drawn between boxes to show which waythe information flows

these lines, always drawn with a arrowhead, trace theflow of information throughout the system.

these lines, always drawn with a arrowhead, trace theflow of information throughout the system.

How To Draw Data Flow Diagrams

How To Draw Data Flow Diagrams

How to draw data flow diagramsHow to draw data flow diagrams

1. Do not mix levels of detail on one chart. Instead,draw several data flow diagrams, each with a different level of detail.

2. Select either the Gane and Sarson or Yourdon*notations and use them consistently.

1. Do not mix levels of detail on one chart. Instead,draw several data flow diagrams, each with a different level of detail.

2. Select either the Gane and Sarson or Yourdon*notations and use them consistently.

*Yourdon uses a circle for a process, while Ganeand Sarson uses a rectangle with rounded corners.Yourdon’s data store symbol is a rectangle open ateach end. Gane and Sarson show a data store as arectangle closed at one end.

*Yourdon uses a circle for a process, while Ganeand Sarson uses a rectangle with rounded corners.Yourdon’s data store symbol is a rectangle open ateach end. Gane and Sarson show a data store as arectangle closed at one end.


3. Use a template to draw uniformly sized and shaped symbols for permanent system documentation. Symbols may be drawn freehand for rough or temporary diagrams.

4. Connect symbols with flowlines. Draw connectionsusing pipes or lines with arrowheads to show the flow of data between symbols. Place an arrowheadat each end of the pipe to show a two-way flow of data between symbols. Place an arrowhead at one end of the pipe to show a one-way flow of data.

3. Use a template to draw uniformly sized and shaped symbols for permanent system documentation. Symbols may be drawn freehand for rough or temporary diagrams.



5. Name and label all symbols and connectors. Selectnames that are descriptive and reflect what is being done. Place text within each symbol describing the function or transformation takingplace. Place textual labels next to flowlines or pipes to describe the movement or transformationof data taking place.

6. Correlate symbols to other data flow diagrams,using reference numbers to show relationships

5. Name and label all symbols and connectors. Selectnames that are descriptive and reflect what is being done. Place text within each symbol describing the function or transformation takingplace. Place textual labels next to flowlines or pipes to describe the movement or transformationof data taking place.

6. Correlate symbols to other data flow diagrams,using reference numbers to show relationships


7. Desk check all data flow diagrams to make sure each symbols is logically connected to another


7. Desk check all data flow diagrams to make sure each symbols is logically connected to another


MELJUN CORTES System Modelling Data flow Diagram (DFD)

Technology

system model

proposed system

system failure

flow of data

data elements

data flows

data sink

destination of data