System Analysis & DesignSystem Analysis & Design

Where do they fit in?

Analysis (What do we do?) Fact finding

investigate business process and the current system

modelling the current and required systems

deliverables - requirements specification logical models of the

required system

Life Cycle Phases Planning Feasibility Study Analysis Design Code and Unit test

3

DFDs describe the flow of data or information into and out of a system what does the system do to the data?

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

Data Flow Diagrams (DFD)

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

4 Main Elements

Notation

Process box

D Data Store

ExternalEntity

Data Flow

• Data Flow

• Process

• External Entity

• Data Store

DFD Shapes from Version

F r o m F l o w C h a r t /D a t a F l o w D i a g r a m

P r o c e s s

D a t a S t o r e

E x t e r n a l E n t i t y

F r o m S o f t w a r e D i a g r a m /G a n e - S a r s o n D F D

P r o c e s s

I D #

I D#

E x t e r n a lE n t i t y

D a t a S t o r e1

External Entity

Data Store

Process

From Flow Chart /Data Flow Diagram

Version 5.x Version 2000

4

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

detail one overview and many progressively greater detailed

views

Levelled DFDs

models system as one process box which represents

scope of the system identifies external entities and related inputs and

outputs Additional notation - system box

Level 0 - Context Diagram

System boxExternalentity

Data flow out

Data flow in

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 1 - overview 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

Level 2 - detailed diagram

Duplicates marked by diagonal line in corner System Boundary Elementary Processes - star in corner Process that is levelled - dots on top

Other Notation

5

Numbering Labelling Balancing

Rules for DFDs

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...

Numbering

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

Labelling

Balancing

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

Data stores data stores that are local to a process need not be

included until the process is expanded

Balancing and data stores

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

Data Flows

Find the people who send data into the system

Often data is part of a PHYSICAL transaction When 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 Diagram

Design StrategyContext Diagram

Design Strategy

DFD

Design Strategy

DFD

Design Strategy

DFD

Design Strategy

DFD

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SAD Documentation

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