Software Engineering-II

Sir Zubair Sajid

3

Data Flow Diagrams (DFD) DFDs describe the flow of data or

information into and out of a systemwhat does the system do to the data?

A DFD is a graphic representation of the flow of data or information through a system

4 Main Elements external entity - people or organisations

that send data into the system or receive data from the system

process - models what happens to the data i.e. transforms incoming data into outgoing data

data store - represents permanent data that is used by the system

data flow - models the actual flow of the data between the other elements

Notation

Process box

D Data Store

ExternalEntity

Data Flow

• Data Flow

• Process

• External Entity

• Data Store

4

Levelled DFDs

Even a small system could have many processes and data flows and DFD could be large and messyuse levelled DFDs - view system at different

levels of detailone overview and many progressively

greater detailed views

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.

Level 0 - Context Diagram models system as one process box

which represents scope of the system identifies external entities and related

inputs and outputs Additional notation - system box

System boxExternalentity

Data flow out

Data flow in

Level 1 - overview diagram gives overview of full system identifies major processes and data

flows between them identifies data stores that are used by

the major processes boundary of level 1 is the context

diagram

Level 2 - detailed diagram level 1 process is expanded into more

detail each process in level 1 is decomposed

to show its constituent processes boundary of level 2 is the level 1

process

Other Notation

Duplicates marked by diagonal line in corner

System Boundary Elementary Processes - star in corner Process that is levelled - dots on top

5

Rules for DFDs

Numbering Labelling Balancing

Numbering

On level 1 processes are numbered 1,2,3…

On level 2 processes are numbered x.1, x.2, x.3… where x is the number of the parent level 1 process

Number is used to uniquely identify process not to represent any order of processing

Data store numbers usually D1, D2, D3...

Labelling

Process label - short description of what the process does, e.G. Price order

Data flow label - noun representing the data flowing through it e.G. Customer payment

Data store label - describes the type of data stored

Make labels as meaningful as possible

Balancing and data stores Balancing

any data flows entering or leaving a parent level must by equivalent to those on the child level

Data storesdata stores that are local to a process need

not be included until the process is expanded

Data Flows

Allowed to combine several data flows from lower level diagrams at a higher level under one data flow to reduce clutter

Flows should be labelled except when data to or from a data store consists of all items in the data store

Context Diagram Find the people who send data into the

systemOften data is part of a PHYSICAL transactionWhen handing a bar of chocolate to a

shopkeeper, you are handing him/her a barcode.

Find the people who get data out of the system.The only data you need is data that is

transformed or sent completely out of the system – not data that is handled by an operator within the system.

Context diagramsa top-level DFD shows the least amount of detail and is known as a Level 0 DFD or context diagram.

Customer

Order

Invoice

Process orderOut-of-stock notice

Picking ListWarehouse

Order rejection notice

Context diagram

Joe'sYard

Joe Customer

Supplier

Shareholders

Docket &Payment

Signed docket

Supply needs

Supply invoiceSupply order& payment

financialreport

Remove passive verbs and queries

Passive: has stock list Buy supplies

Picks stock items (views list) Writes orders Pays joe Stamps docket

Customer then Takes docket to yard Hands it to foreman

Gets items Gives them to builder Builder signs docket Takes copy as receipt

Joe then Looks around yard and

reorders Records order in order

book Foreman

takes delivery – checks Foreman pays supplier Staples receipt to order

book Joe

Produces financial report

Level 1 current physical

Customer

Buysupplies

20

getitems

12

Reordersupplies

21

Restock

22

*

Producefinancialreport

Joe's Office5

Joe

Orderbook

M3

DocketM1

Money

stock

Shareholders

Supplier

Foreman

*

Taketo

yard

11Supply needs

financialreport

Docket &Payment

Supply order

Docket

Signed docket

Signed docket

Signed docket

Payment

required stock

required stock

completeddocket copy

completeddocket copy

Supply order

supplies

supplies

Payment Payment

Supplierreceipt

Supplierreceipt

completeddocket copy

buildersignature

Buy SuppliesBuy supplies20 DocketM1

Money

Customer

*

WritesOrder

20.1

*

PaysJoe

20.2*

StampDocket(signatu-

re)

20.3

Docket

Payment

Docket

Payment Docket

Get Items

get items12

Foreman

stock

*

Give itemsto customer

12.1*

Get buildersignature

12.2

*

Givecopy

as receipt

12.3

Customer

Signed docket

required stock

buildersignature

Signed docket

completeddocket copy completed

docket copy

Reorder suppliesReorder supplies21

OrderbookM3

Joe

*

Reorderfromyard

21.1

*

Recordorder

21.2

Supply order

Supply needs

Supply needs

Restock

Restock22

stock

OrderbookM3

Supplier

Money

*

Takedelivery

22.1

*

Paysupplier

22.2

supplies

Payment

Supplierreceipt

Supply order

supplies

Payment

Supply order