ACG 6415

Data Modeling:Data Flow DiagramsFlow Charts

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Logical DFD

A logical data flow diagram is a graphical representation of a system showing the system’s processes (as bubbles), data stores, and the flows of data into and out of the processes and data stores. WHAT tasks the system is doing— without having

to specify how, where, or by whom the tasks are accomplished.

The advantage of a logical DFD (versus a physical DFD) is that we can concentrate on the functions that a system performs.

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Data Flow Diagrams

Logical model of ? Data gathered from requirements determination

RD determined what? What the new system should do AND What the old system does

Depiction of processes and data flows of a system

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Benefits of DFD’s

Uncover misunderstandings of system processes

Help communicate analyst system understanding to management/end users

Helps to take the system out of context Eases the problem of thinking outside of the

box

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Fed Ex Process (Physical DFD)

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Data Flow Diagrams

Modeling tool Graphically represent a system as

network of functional processes connected to one another by

“pipelines” and “holding tanks” of data. The components of a typical

dataflow diagram: the process, the flow, the store, and the terminator/Entity.

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Basic DFD Symbols

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Level-1 DFD

1.0

CreateWeeklyPayrollRecord

Employee Data FileS1

Employee Tax Form

Payroll Data FileS2

2.1

ComputeGrossPay

2.3

CreatePayrollCheck

2.2

ComputeNetPay

3.2

PrintWeeklyPayroll

Summary

3.1

SortPayroll

Records byEmployee

4.2

PrintEmployee

TaxForm

4.1

CalculatePayroll

Deductions

Employee Paycheck

PayrollDeductions

Employee PayrollRecord

Time Sheet

Employee Record

Gross PayData

Net PayData

Hours Worked

Payroll Record

Sorted Records

Payroll Report

Employee

Accounting Department

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The Process

the process name will describe what the process does.

Use Verb-Object Phrase That is, the process sometimes describes

who or what is carrying out the process, rather than describing what the process is.

In 6415 we will only describe what the process does -

logical vs. physical DFD

Process

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The Flow

A flow is represented graphically by an arrow into or out of a process

we wont use double sided arrows in class used to describe the movement of chunks, or packets of

information from one part of the system to another part. Thus, the flows represent data in motion

For many complex, real-world systems, the DFD will show the flow of materials and data.

The name represents the meaning of the packet that moves along the flow. A corollary of this is that the flow carries only one type of packet, as indicated by the flow name.

Note also that the flows show direction: indicates whether data (or material) are moving into or out

of a process

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Diverging and Converging Flows

Diverging flow: duplicate copies of a packet of data are being

sent to different parts of the system or that a complex packet of data is being split

into several more elementary data packets, each of which is being sent to different parts of the system

Converging flow: several elementary packets of data are joining

together to form more complex, aggregate packets of data.

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Diverging Data Flow

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The Store

The store is used to model a collection of data packets at rest. a store can be data stored on:

punched cards, microfilm, microfiche, or optical disk, or a variety of other electronic forms, a 3-by-5 index cards in a card box, or names and addresses in an address book, or several file folders in a file cabinet, or a variety of other non-computerized forms.

The notation for a store is two parallel lines Typically, the name chosen to identify the store is

the plural of the name of the packets that are carried by flows into and out of the store.

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The Terminator or External Entity

Terminators represent external entities with which the system communicates.

graphically represented as a rectangle They are outside the system we are modeling; the

flows connecting the terminators to various processes (or stores) in our system represent the interface between our system and the outside world. These are Inputs and Outputs to/from the System Entities providing Inputs are AKA Sources Entities receiving Outputs are AKA Sinks Entities can be both Sources and Sinks

Any relationship that exists between terminators will not be shown in the DFD model.

If there are relationships between the terminators, and if it is essential for these to be modeled then the terminators are actually part of the system and

should be modeled as processes.

Entity

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DFD Guidelines

1. Choose meaningful names for processes, flows, stores, and terminators.

2. Number the processes.

3. Redraw the DFD as many times as necessary for esthetics.

4. Avoid overly complex DFDs.

5. Make sure the DFD is internally consistent and consistent with any associated DFDs (ie Balanced).

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DFD Levels

DFD models are organized in a series of levels so that each level provides successively more detail about a portion of the level above

Top Level: Context Diagram consists of only one bubble representing the entire

system the data flows show the interfaces between the

system and the external entities Successive Levels:

Figure/Level-0 highest-level view of the major functions within the

system the major interfaces between those functions

Level-1 ... Level n

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How Many Levels should you have?

each DFD figure should have no more than half a dozen bubbles and related stores.

Simple Systems 2-3 Levels

Medium Sized System 3-6 Levels

Large System 5-8 Levels

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Balancing DFD Levels

To ensure that each figure is consistent with its higher-level figure, we follow a simple rule: the data flows coming into and going

out of a bubble at one level must correspond to the data flows coming into and going out of an entire figure at the next lower level which describes that bubble.

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How do you show stores at the various levels?

show a store at the highest level where it first serves as an interface between two or more bubbles

then show it again in EVERY lower-level diagram that further describes (or partitions) those interface

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Context Diagram

One Process Describes System / Process

At least one Entity Outside of System / Process

At least one Source & one Sink

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Logical DFD

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Context-Level Diagram

0

EmployeePayrollSystem

Employee Tax Form

Payroll Report

Time Sheet

Employee Payroll Check

Employee

Accounting Department

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Level-0 DFD

1.0

CreateWeeklyPayrollRecord

Employee Data File

S1Employee Payroll Data File

S2

2.0

ComputeWeekly

Paycheck

3.0

PrepareWeeklyPayroll

Summary

4.0

PrepareEmployee

TaxForm

AccountingDepartment

Employee Record Paycheck Data

Payroll Record

Payroll Report

Employee Paycheck

Employee Tax Form

Time Sheet

Hours Worked

Employee Payroll Record

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Level-1 DFD

1.0

CreateWeeklyPayrollRecord

Employee Data FileS1

Employee Tax Form

Payroll Data FileS2

2.1

ComputeGrossPay

2.3

CreatePayrollCheck

2.2

ComputeNetPay

3.2

PrintWeeklyPayroll

Summary

3.1

SortPayroll

Records byEmployee

4.2

PrintEmployee

TaxForm

4.1

CalculatePayroll

Deductions

Employee Paycheck

PayrollDeductions

Employee PayrollRecord

Time Sheet

Employee Record

Gross PayData

Net PayData

Hours Worked

Payroll Record

Sorted Records

Payroll Report

Employee

Accounting Department

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Functional Primitive

4.1.2

CalculateNon-tax

Deductions

4.1.3

CalculateRetirementAllowance

4.2.1

CalculateEmployeeVacation

Pay

4.1.1

CalculatePayroll

TaxDeductions

4.2.2

PrintEmployeeTax Form

EmployeeData

EmployeeData

EmployeeData

Employee Tax Form

EmployeePayrollRecord

Employee Data

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Balanced DFDs

The next slide depicts “balanced” data flow diagrams.

Balanced DFDs exist when the external data flows are equivalent.

DFD (a) is a context diagram and (b) is an “explosion” of it into a level 0 logical DFD.

DFD (c), (d) and (e) are “explosions” of the logical level 0 DFD, and so on.

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Balanced DFDs

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The Rules of DFDs

In a nutshell…

All data flows must begin and/or end at a process, because data flows either initiate or result from a process.

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Rules Governing DFD Construction

A process cannot have only outputs – “miracle.” A process cannot have only inputs – “black hole.” The inputs to a process must be sufficient to

produce the outputs from the process - (gray hole). All data stores must be connected to at least one

process.

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Rules Governing DFD Construction

A data store cannot be connected to a source or sink. A data flow can have only one direction of flow. Multiple data

flows to and/or from the same process and data store must be shown by separate arrows.

If the exact same data flows to two separate processes, it should be represented by a forked arrow.

Data cannot flow directly back into the process it has just left. All data flows must be named using a noun phrase.

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Three Basic Illegal Data Flows

Black HoleBlack Hole

MiracleMiracle

Gray HoleGray Hole

EntityEntityNameName

IdentifierIdentifier

ProcessProcessNameName

IdentifierIdentifier

ProcessProcessNameName

FileFileNameName

EmployeeEmployee

1.01.0

CreateCreateEmployeeEmployeePaychecksPaychecks

EmployeeEmployeeFileFile

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Other Illegal Flows

EmployeeEmployeeFileFileD1

OrderOrderFileFileD2

Data Must Be Data Must Be Connected to a Connected to a Process Process

EmployeeEmployee

1.01.0

CreateCreateEmployeeEmployeePaychecksPaychecks

No Bi-Directional No Bi-Directional FlowsFlows

CustomerCustomerCustCustFileFileD1D1

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One Final Illegal Flow

2.1.02.1.0

ProcessProcessNameName

Data Cannot FlowData Cannot FlowDirectly Back IntoDirectly Back IntoThe Process It LeftThe Process It Left

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A Debate

2.2.02.2.0

ProcessProcess

2.3.02.3.0

ProcessProcess

2.1.02.1.0

ProcessProcess

2.2.02.2.0

ProcessProcess

2.3.02.3.0

ProcessProcess

2.1.02.1.0

ProcessProcess

Which is Right?Which is Right?

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Balancing Data Flows

SourceSource

1.01.0

FormatFormatDataData

SinkSink

2.02.0

SendSendReportsReports

SourceSource22

A

Formatted A

Formatted C

C

B

SourceSource

00

ContextContextLevelLevel

SystemSystem

SinkSinkA B

A BSourceSource

00

ContextContextLevelLevel

SystemSystem

SinkSink

SourceSource22

C

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Preparing Data Flow Diagrams

Read narrative carefully - number lines and paragraphs in text

Prepare table of entities and activities

Draw context diagram - use DFD guidelines

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Steps for Creating E&A Table

1. Read Narrative

2. Identify all Verb phrases (circle them)

3. Identify all Noun phrases (rectangles or data store)

4. Line by line insert noun as Entity and Verb as Activity along with data flow noun if present

5. Identify External Entities

6. Identify Internal Entities

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Entities and Activities Table

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DFD Guidelines

1. Include on the systems documentation all (and only) activities and entities described in the systems narrative - no more, no less.

2. When multiple entities operate identically, depict only one to represent all.

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Causeway Context Diagram

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DFD Guidelines (cont.)

3. For clarity, draw a data flow for each flow into and out of a data store. You may, also for clarity to help you determine that you have included all necessary flows, label each flow with the activity number that gives rise to the flow or with a description of the flow.

4. If a data store is logically necessary (that is, because of a delay between processes), include a data store in the diagrams, whether or not it is mentioned in the narrative.

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DFD Guidelines (Level-0 and higher)

5. Group activities if they occur in the same place and at the same time.

6. Group activities if they occur at the same time but in different places.

7. Group activities that seem to be logically related.

8. To make the DFD readable, use between five and seven bubbles.

9. On a logical DFD, data flows cannot go from higher- to lower-numbered bubbles.

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Causeway Logical Level 0 DFD

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Standard Flowchart Symbols

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Common system flowcharting routines

The following slides show several common ways of showing processing using system flowcharting.

Pay particular attention to the way the columns are set up to communicate the flow of activities between processing entities.

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The data-entry clerk keys in an input document into the computer. The computer accesses data in a data store 1 and in data store 2 to edit/validate the input. The computer displays the input, including any errors. The clerk compares the input document to the display, keys corrections as necessary, and accepts the input. The computer updates the table in data store 2 and notifies the clerk that the input has been recorded.

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User queries the computer. The computer accesses the table(s) in one of more data stores & presents a response to the user.

Update sequentialdata store from magnetic disk are input to the computer, along with the existing (old) master data – becoming the new master data

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The user batches the input documents, prepares batch totals, and keys the documents into the computer. The computer records the inputs on disk and notifies the user as each input is accepted. The user files the input documents in numerical sequence. At the end of the batch, the computer prepares an exception & summary report that includes batch totals. The user compares the computer batch totals to those prepared earlier.

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A data entry clerk enters documents into a networked PC system. The system records the inputs on a disk and notifies the user of the acceptance of each input. The documents are then forwarded to a different clerk who keys the documents again. Differences are resolved and the event data are updated to reflect the verifications and corrections.

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A clerk scans a document into the computer. Use the data from the scanned document, the computer updates the data store(s)

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A clerk scans a document into the computer. The computer routes an image of the scanned input to a data entry clerk, who keys data from the documents image into the computer. The computer records the data with the scanned document

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Preparing Systems Flowcharts

1. Divide the flowchart into columns; one column for each internal entity and one for each external entity. Label each column.

2. Flowchart columns should be laid out so that the flowchart activities flow from left to right, but you should locate columns so as to minimize crossed lines and connectors.

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System flowcharting guidelines (cont.)

3. Flowchart logic should flow from top to bottom and from left to right. For clarity, put arrows on all flow lines.

4. Keep the flowchart on one page. If you can’t, use multiple pages and connect the pages with off-page connectors. Computerized flowcharting packages will print your flowcharts only on paper that will fit in your printer!

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System flowcharting guidelines (cont.)

5. Within each column, there must be at least one manual process, keying operation, or data store between documents. That is, do not directly connect documents within the same column.

6. When crossing organizational lines (one column to another), show a document at both ends of the flow line unless the connection is so short that the intent is unambiguous.

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System flowcharting guidelines (cont.)

7. Documents or reports printed in a computer facility should be shown in that facility’s column first. You can then show the document or report going to the destination unit.

8. Documents or reports printed by a centralized computer facility on equipment located in another organizational unit ( warehouse, shipping dept.) should not be shown within the computer facility.

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System flowcharting guidelines (cont.)

9. Processing within an organizational unit on devices such as a PC or computerized cash register should be shown within the unit or as a separate column next to that unit, but not in the central computer facility column.

10. Sequential processing steps (computerized or manual) with no delay between them (and resulting from the same input) can be shown as one process or as a sequence of processes.

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System flowcharting guidelines (cont.)

11. The only way into a computer data storage unit is through a computer processing rectangle.

12. A manual process is not needed to show the sending of a document. The sending of the document should be apparent from the movement of the document itself.

13. Do not use a manual process to file a document. Just show the document going in to a file.

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Causeway’s Systems Flowchart

The systems flowchart for Causeway is presented again on the next slide

This flowchart for causeway is again presented without an enterprise database

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Documenting Enterprise Systems

Moving from a file-based system to an enterprise database changes the system flowchart An enterprise database replaces

transaction and master data Other flows may change depending on

the system implementation

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