University of Sunderland ISIC 1 Data Flow Diagrams - Part 2 Hierarchical DFDs.

University of Sunderland ISIC 1

Data Flow Diagrams - Part 2Data Flow Diagrams - Part 2Hierarchical DFDsHierarchical DFDs


Temporary Data storeTemporary Data store

T1• Can use T to indicate a temporary data store• usually manual• eg in tray, where orders are placed before filing


Hierarchical DFDsHierarchical DFDs• DFDs are hierarchical

• Top-down stepwise refinement is employed

• Processes are expanded into sub-processes to show more and more detail

• Each expansion is a new level

• Highest level is level 0 aka a CONTEXT DIAGRAM

• Lower levels are numbered 1,2,3 etc,

• No limit to the number of levels allowed, but

level 3 is usually enough


Components of a Data Components of a Data Flow ModelFlow Model

Overallprocess

Entity

Entity

Level 0 Level 1

Process

Process

Process

1

2

3



Overallprocess

Entity

Entity

Level 0 Level 1 Level 2

Process

Process

Process

1

2

3

2

2.1

2.2

2.3

subProcess

subProcess

subProcess



Overallprocess

Entity

Entity

Level 0 Level 1 Level 2 Level 3

Process

Process

Process

1

2

3

2 2.2

2.1

2.2

2.3

2.2.1

2.2.2

2.2.3

subProcess

subProcess

subProcess

sub-subProcess

sub-subProcess

sub-subProcess


Overallprocess

Entity

Entity

Level 0 Level 1 Level 2 Level 3 Level 4

Process

Process

Process

1

2

3

2 2.2 2.2.2

2.1

2.2

2.3

2.2.1

2.2.2

2.2.3

subProcess

subProcess

subProcess

sub-subProcess

sub-subProcess

sub-subProcess



Level 0 DFDs - Level 0 DFDs - Context DiagramsContext Diagrams

• Level 0 DFDs, aka Context diagrams are the highest level DFD, but not always required or drawn

• Level 0 DFDs depict the system as a single process

• Since there is only one process, there is no need to number the process


Level 0 DFDs –Level 0 DFDs – Context Diagrams Context Diagrams

• The process description at Level 0 gives an overview of the system i.e. the activities on the Level 1 DFD

• Level 0 diagrams do not show Data stores

• External entities should be shown and referenced


Example Example Context DiagramContext Diagram

Sales,Despatch

& Warehouse

AccountsCustomer

Supplier

Order

Delivery note

Orderacknowledgement

Weekly creditlimit list

Copy deliverynote

Re-order card

Goods receivednote

Goods receivednote

ab

c


GOODS RECEIVED GOODS RECEIVED NOTE (GRN)NOTE (GRN)

– A document produced when goods are received into the factory. It will usually accompany goods to any inspection and is used to check against invoices before payment..


delivery notedelivery note

• Definition

• Document accompanying a shipment of goods that lists the description, grade, and quantity of the goods delivered. A copy of the delivery note, signed by the buyer or consignee, is returned to the seller or consignor as a proof of delivery.


Level 1 DFDsLevel 1 DFDs

• First real DFD, i.e. always included

• Show processes, external entities, data stores physical resources if required

• Number all processes 1,2,3 etc

• External entities must have the same references as Level 0 (if it was drawn)


Example - Level 1 DFDExample - Level 1 DFD

Accounts

Customer

Accounts

Supplier

*

OrderEntry

Sales1

*

Packagegoods

& despatch

Despatch2

Stockcontrol

Warehouse3

Delivery note

Orderacknowledgement

Order

Weekly creditlimit list

Goods receivednote

Internal orderform

Despatchinstruction set

Re-order card

Goods receivednote

Copy deliverynote

a

b

c

b



• If a process cannot be expanded any further, put an asterisk in the lower right hand corner

*


Example - Level 2 DFDExample - Level 2 DFD

• In our example, just process 3 Stock control can be expanded to a Level 2 diagram

• In the Level 2 diagram, the Stock control process is broken down into 4 sub-processes

• Processes are numbered 3.1, 3.2, 3.3 etc

• Process 3 has a Data store, called Stock, which was not present at Level 1, so its ID is M3/1, the first data store within process 3


Level 2 DFD - Stock ControlLevel 2 DFD - Stock Control

Stock control3

AccountsAccounts

*

Packagegoods

& despatch

Despatch2

Supplier

*

OrderEntry

Sales1

*

Allocatestock

to order

Stock Clk3.1

*

Type thedespatchinstruction

set

Typist3.2

Monitorstocklevels

W'h men3.3

*

Check goodsreceipt note

to stock recei-ved & incre

Goods Rec3.4

StockM3/1

Accounts

Re-order card

Goods receivednote

Despatchinstruction set

Goods receivednote

Internal orderform

read & updatestock

stock level

stock additions

Internal orderform

Re-order card

b

b c



• Notice, from our Level 2 DFD, sub process 3.3 does not have an asterisk, so it can be expanded further ...


Level 3 DFD - Level 3 DFD - Monitor Stock LevelsMonitor Stock Levels

Monitor stock levels3.3

Accounts

StockM3/1

*

Monitor stocklevels andreport low

stock

Stock Clk3.3.1

*

Replacere-order

card

S/visor3.3.2

*

Extractappropriate

re-ordercard

S/visor3.3.3

Re-ordercardsM3.3/1

Re-order card

stock level

Stock receiptnote

Low stockdetails Re-order card

Re-order card

Re-order card



• Note processes are now numbered 3.3.1, 3.3.2 etc

• New data store has the ID M3.3/1

• All these Level 3 sub processes cannot be expanded further

• You will not be asked to go beyond Level 2 in this Module


GoodGood PractisePractise

• Level 0 diagram shows the whole system as just one process

• Level 1 diagram has one process for each department

• Level 2 shows what happens within each department

• Level 3 could show processes performed by a single named job role – eg warehouse supervisor

• This is not always possible, but is very effective when it can be done


Drawing DFD TipsDrawing DFD Tips

• Identify data flows, data stores and processes, as described in the last lecture

• Draw processes first, down the centre of the page

• Add external entities down the left hand side• Draw data stores down the right hand side• Finally add the data flows• Flow lines should not cross, use duplicate

symbols to avoid crossing lines and keep DFD neat and tidy ...


Multiple Instances Multiple Instances on a Single DFDon a Single DFD

For each instance of the duplicate element, add a line to the symbol, like so …

Use duplicates to keep your DFD clear and easy to follow.


Duplicate ProcessesDuplicate Processes

• Is possible, but should

be avoided• ie duplicate external entities

and data stores• Don’t duplicate Processes,

unless you really have to• Duplicated processes complicate DFDs• Which defeats the objective


Drawing DFDs in this way Drawing DFDs in this way encourages a encourages a step-by-step approach, that should step-by-step approach, that should result in diagrams which are better result in diagrams which are better laid out and are easier to read.laid out and are easier to read.You need to know the terminology and conventions for exams. Practice interpreting case studies and drawing the DFDs.

Read through the examples in Lejk and Deeks.


• Drawing DFDs in this way encourages a step-by-step approach, that should result in diagrams which are better laid out and are easier to read.

• You need to know the terminology and conventions for exams. Practice interpreting case studies and drawing the DFDs.

• Read through the examples in Lejk and Deeks.


Rules for drawing Rules for drawing DFDsDFDs

Processes:

• No process can have only output

• No process can have only input

• If you think a process has only input or output, then it must be an external entity (source/sink)

• A process name must contain a verb



Data Stores:• Data cannot move directly from one data

store to another data store - data must be moved by a Process

• Data cannot move directly from an external entity to a data store or vice versa - data must be moved by a process

• The name of a data store must contain a noun



External Entities:

• Data cannot move directly from one external entity to another. It must be moved by a process, otherwise don’t show the data flow on the diagram

• The external entity name must contain a noun


Rules for drawing Rules for drawing DFDsDFDsData flow:

• A data flow has only one direction of flow between symbols. It may flow in both directions, but these are usually shown as 2 separate arrows since they happen at different times

• A data flow cannot go directly back to the same process it leaves

• A data flow to a data store means update (append, delete or change)

• A data flow from a data store means retrieve or use• A data flow label will contain a noun

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University of Sunderland ISIC 1 Data Flow Diagrams - Part 2 Hierarchical DFDs.

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