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Information Systems

Data Flow Diagrams

Data Flow Diagrams

Objectives:• To define what is meant by structured analysis• To define the symbols used in a Data Flow

Diagram (DFD)• To draw a context diagram (Level 0 DFD) for a

given scenario• To refine a DFD to a more detailed (lower-

level) view

Structured Analysis

• A widely-used top-down method for defining system inputs, processes and outputs.

• It shows how information flows through a system, using several diagrams showing progressively more and more detail at each level.

• The primary tool of structured analysis is the Data Flow Diagram (DFD).

Data Flow Diagrams

• Data Flow Diagrams– Model how information flows around a system, how it

is processed and stored

– Partition a system into independent units of more manageable size that is verifiable, concise and easily understood

– Hierarchical

Drawing Data Flow Diagrams

• Cannot be achieved without– Paper or Computer– Cross-checking at every stage – Pain and Confusion!

• Eventually – Allow understanding of New System– Produce Invaluable Document

• Steps are for Guideline purposes only!

DFD Symbols - External Entity

• An external entity is often the source or destination of information e.g. – Customer

Entity Name

DFD Symbols - Data Flow

Description of Data Flow

NOT A VERB !!!

DFD Symbols - Data Stores

D1 Data Store name

The system files

File identifier (Data store)

DFD Symbols - Processes

1

Process Name

Location

Sequence

Process Description (NOT WHERE or WHO)

No more than 7 per diagram

DFD’s - Valid Connections

Process

(Event or activity)

External Entity (Data Source)

Data Flow

Another Process(Event or activity)

Data FlowA Process

(Event or activity)

A Process

(Event or activity)

Data FlowD1 Data store

DFD’s - Invalid Connections

D B Data Store D A Data Store

Invalid

OutsideSystem

D A Data StoreAlso Invalid

OutsideSystem

AlsoOutsideNot only invalid

but nothing to do with system

Levels of DFD

Order Processing

Customer

Order Rejection

OOS Letter

GDN

Order

Level 0

Order Processing

Update fileMake up Order

Check Stock Level

Produce OOS letter

Validate Order

Customer

M Stock

M Parts

Orderb Details

Stock Detail

Stock Level

In Stock Orders

OOS Orders

Valid Order

Parts Details

Order

GDN

OOS Letter

Order Rejection

Level 1

Validate Order

Return Order

Check Parts Validity

Check Form Completion

Check Stock LevelCustomer

M Parts

Invalid Order

Incorrect Form

Correct Form

Order

Parts Details

Valid Order

Order Rejection

Level 2

Context Diagrams

Customer

Order

Invoice

Process

orderOut-of-stock

notice

Picking ListWarehouse

Order rejection

notice

This is a Level 0 DFD or Context DiagramIt shows the least amount of detail

Creating a Context Diagram – step 1

• Producing Context Diagram ( Level 0)

– Identify External Entities– Identify Inputs

• Data Flows

– Identify Outputs• Data Flow

– Draw Context Diagram• Possibly combine data flows at this stage• e.g. Orders = Phone Orders and Mail Orders

Context Diagram

Order Processing

Customer

Order Rejection

OOS Letter

GDN

Order

Step 2• Producing Level 1 Diagram

– Identify up to seven major processes• Draw a Level 1 DFD

– Associate data flows and external entities from Context Diagram

• Update Level 1 DFD

– Identify the data flows that link the identified processes• Update Level 1 DFD

– Identify files associated with identified processes• Update Level 1 DFD

– Check Level 1 DFD• You will have made a mistake !!

– Possibly correct Level 1 DFD and Context Diagram

DFD Example Functionality

• Companies send in requests for references

• Requests are checked and either returned (if invalid) or retained for processing

• Request Processing involves reading data from the Assessment file and forwarding the Collated Reference Information for Reference writing

Level 1 DFD Example

Company

3

Write Reference

Process Request

2

1

Check Request

D1 Address

D3 Reference File

Address

Invalid Request

Valid Request

Reference (copy)

Collated Assessment

Assessment records

Reference

Reference Request

D2 Assessment File

DFD Level 1 Example 2

3

Convert File

2

Process Enquiry

Standardise VacancyAgency

Client

D1 Client Vacancies

D1 Vacancy File

Converted Vacancy details

Suitable Vacancies

Vacancy detailsVacancy

Enquiry

Vacancy Details

Matched Vacancies

Vacancy

Step 3

• Producing Level 2 Diagrams• For each Level 1 process

– Identify up to seven major processes (Draw a Level 2 DFD)– Associate data flows, processes, files and external entities from

Level 1 DFD (Update Level 2 DFD)– Identify the data flows that link the identified processes (Update

Level 2 DFD)– Identify files associated with identified processes

(Update Level 2 DFD)– Check Level 2 DFD (You will have made a mistake !!)– Possibly correct Context Diagram, Level 1 & Level 2 DFD.

DFD Exercise

• Draw a Level 1 DFD for the following scenario– Customers send in orders. The orders are validated by

the Sales Department using information from the Parts file. Invalid orders are returned to customers. Valid orders are passed to Despatch Dept, where the orders are made up and forwarded to the customer

• Now have a go at the DFD scenarios worksheet

• Level 0 or context diagram shows a system as a single process with inputs and outputs flowing to or from external entities.

• Level 1 DFD will split up that single process into

subsystems and show more detail about the data flows and data stores.

• Level 2 DFD may decompose a single subsystem even further.

DFD Levels - Summary