Post on 09-May-2015
description
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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