Systems Analysis and Design

Plan

• Introduction

• Structured Methods– Data Flow Modelling

– Data Modelling

– Relational Data Analysis

• Feasibility

• Maintenance

Plan

• Introduction

• Structured Methods– Data Flow Modelling

– Data Modelling

– Relational Data Analysis

• Feasibility

• Maintenance

Data Flow Modelling

What are Data Flow Models?

• A model of the system– Software modelling is unlike other types of modelling

• A communication tool– Should be clear and unambiguous

• An analysis tool– Should help us to understand systems

• A design tool– Should help us to develop and improve systems

What are Data Flow Models?

• They only represent a single view of the system

• We cannot describe the system fully in a single diagram

• Represents the Process View– What processes act on data as it flows

through our system

When do we use them?

• Feasibility– High Level of abstraction– Enough Detail to estimate Data/Processing

requirements

• Analysis– Analysis of Current System and Requirements

• Design– Used as source for design of new system

Structured Analysis + Design

Existing System ERD

Repository

Existing Physical

System DFD

New System ERD

Repository

New Logical System DFD

Convert to Logical View

User Requirements

Technical Options +

Constraints

New Physical System

What do they look like?

• As usual there are a number of “standards” for drawing DFDs

• They share a set of common elements– Data Flows– Processes– Entities– Data Stores

Example 1 – Gane and Sarson

Example 2 – DeMarco/Yourdon

Example 3 - SSADM

What do they look like?

• We will use the SSADM style

• Used in Lejk and Deeks, 2002

• You should be able to read diagrams using another style

How do we build one?

• Top down– More likely to be done in Tutorials– Good way of learning the technique– More detail added in successive iterations

• Bottom up– More likely to be done in practice– Interviews provide localised detail not context

Building a Data Flow Model

• Understand the system• Establish the context• Identify major processes• Identify Data Flows:

– What do processes produce as output– What Data is required to produce output

• Identify other elements– Where does data come from/go to

• Check against System Description

Document Flow Diagram (DocFD)

• A precursor to DFDs• A good way of establishing the context• Identify the entities in the system (departments)• Identify documents flowing between them• Establish system boundary

• Bottom up and top down?

DocFD Elements

• Entity (also called ‘Agencies’)– Symbol

– Duplicate symbol

acustomer

acustomer

DocFD Elements

• Document Flow

Sales Order

Example Diagram (CD Hotel)

aguest

bbooking

dept

caccounts

dept

dbar/

restaurant

booking+depositpayment

weekly statement

final statement

confirmation

vouchers

weekly statement (copy)

deposit

Identifying the system boundary

• This is rarely an easy task• There may be different opinions on this

boundary• Unless it is finalised development will fail

• Once identified, the boundary will be clearly stated in the terms of reference for the project

Example Diagram (CD Hotel)

aguest

bbooking

dept

caccounts

dept

dbar/

restaurant

booking+depositpayment

weekly statement

final statement

confirmation

vouchers

weekly statement (copy)

deposit

Example Diagram (CD Hotel)

aguest

bbooking

dept

caccounts

dept

d

bar/restaurant

booking+depositpayment

weekly statement

final statement

confirmation

vouchers

weekly statement (copy)

deposit

Example Diagram (CD Hotel)

aguest

bbooking

dept

caccounts

dept

dbar/

restaurant

booking+deposit payment

weekly statement

final statement

confirmation

vouchers weekly statement

(copy)

deposit

Document Definitions

• It is important at this stage to identify the information in the documents that flow across the system boundary

• These will inform us of the interface between the system and external entities– Data Interface and possibly GUI

• These will form the basis of our data dictionary

Data Dictionary Entry

(Mason and Wilcocks, 1994)

DFD Context Diagram

• Now that the system boundary has been established we can show the context in which our system operates– All internal details are removed– System boundary is changed to a process

box– All input and output data flows have been

identified

Example Diagram (CD Hotel)

aguest

bbooking

dept

caccounts

dept

dbar/

restaurant

booking+depositpayment

weekly statement

final statement

confirmation

vouchers

weekly statement (copy)

deposit

Example Diagram (CD Hotel)

aguest

bbooking

dept

caccounts

dept

d

bar/restaurant

booking+depositpayment

weekly statement

final statement

confirmation

vouchers

weekly statement (copy)

deposit

Example Diagram (CD Hotel)

aguest

bbooking

dept

caccounts

dept

d

bar/restaurant

booking+depositpayment

weekly statement

final statement

confirmation

vouchersweekly statement

(copy)

deposit

Example Diagram (CD Hotel)

aguest

bbooking dept

caccounts

dept

dbar/

restaurant

booking+deposit

weekly statement

final statement

confirmation

vouchersweekly statement

(copy)

deposit

CD HotelBookings

DFD Context Diagram

We now know what Systemwe are going to develop

Adding More Detail

• Now we have established the system context we can start adding more detail

• To do this we create a Data Flow Diagram (DFD)– Identify Processes– Identify Data Flows– Identify Data Stores– Connect them to External Entities

• Process Box– Symbol

• Active Component• What goes in must come out or be transformed• Physical or Logical?

Create Reservation

1 Sales Assistant

Maintain Sales Account

2 Accounts Dept

*

Elements of a DFD

Indicates Process at lowest level of detail

Elements of a DFD

• Data Flow

• Does not change data from end to end– Changes imply there is a process – so add

one

• Used to create the data model – must be labelled

Sales Order

Elements of a DFD

• Data Store– Symbol

– Duplicate

• Passive Component• Does not process data• Manual or Digital (M/D)

Sales FileM1

Sales FileM1

Elements of a DFD

• External Entity – also called ‘Agencies’, source and sink– Symbol

– Duplicate symbol

acustomer

acustomer

Example - SSADM

Example (CD Hotel)

• When a guest sends us a booking, with deposit, we change the vacancy list to show one less and enter the customer details into the reservation file. The cash from the deposit is sent to the accounts department. A letter of confirmation is sent to the customer and we also open a guest account in our ledger

Example (CD Hotel)

• When a guest sends us a booking, with deposit, • …we change the vacancy list to show one less • …and enter the customer details into the

reservation file• The cash from the deposit is sent to the

accounts department• A letter of confirmation is sent to the customer • …and we also open a guest account in our

ledger

Example (CD Hotel)

aguest

bbooking dept

caccounts

dept

dbar/

restaurant

booking+deposit

weekly statement

final statement

confirmation

vouchersweekly statement

(copy)

deposit

CD HotelBookings

Identifying Processes

• High Level Processes can often be identified as business functions– Accounts– Sales– Purchasing

• Lower level Processes will be implied by verbs such as– Create– Sort– Compile– Change

Example (CD Hotel)

• When a guest sends us a booking, with deposit, • …we change the vacancy list to show one less • …and enter the customer details into the

reservation file• The cash from the deposit is sent to the

accounts department (splitting documents)• A letter of confirmation is sent to the customer • …and we also open a guest account in our

ledger

Example (CD Hotel)

aguest

bbooking dept

caccounts

dept

dbar/

restaurant

booking+deposit

weekly statement

final statement

confirmation

vouchersweekly statement

(copy)

deposit

CD HotelBookings

Example (CD Hotel)

CD Hotel Bookings

Create Booking

1 Reception

aguest

aguest

baccounts

dept

Identifying data Stores

• These may be obvious:– File– Catalogue– Tray– Folder– Database

• ..or implied– Check against– Put into– Read

Example (CD Hotel)

• When a guest sends us a booking, with deposit, • …we change the vacancy list to show one less • …and enter the customer details into the

reservation file• The cash from the deposit is sent to the

accounts department• A letter of confirmation is sent to the customer • …and we also open a guest account in our

ledger

Example (CD Hotel)

CD Hotel Bookings

Create Booking

1 Reception

aguest

aguest

baccounts

dept

Example (CD Hotel)

CD Hotel Bookings

Create Booking

1 Reception

M1 Vacancy List

M2 Reservation file

M3 Guest account

aguest

acustomer

baccounts

dept

Identifying data flows

• These will be present wherever there is movement of data between:– A data store and a process– An entity and a process– Two processes

• But Never:– Between data Stores– From an entity to a data store

Example (CD Hotel)

• When a guest sends us a booking, with deposit, • …we change the vacancy list to show one less • …and enter the customer details into the reservation

file• The cash from the deposit is sent to the accounts

department• A letter of confirmation is sent to the customer • …and we also open a guest account in our ledger

Example (CD Hotel)

CD Hotel Bookings

Create Booking

1 Reception

M1 Vacancy List

M2 Reservation file

M3 Guest account

aguest

acustomer

baccounts

dept

Example (CD Hotel)

CD Hotel Bookings

Create Booking

1 Reception

booking+deposit M1 Vacancy List

M2 Reservation file

M3 Guest account

aguest

acustomer

baccounts

dept

guest details

room details

depositconfirmation

deposit

Errors

• Do not join data stores to data stores• Do not join data stores to entities• Any change in data implies a process – so

add one• If data comes out of a process – it must

have got in somehow• Do not give any impression of time• Do not show triggers (e.g. start)

Guidelines

• Draw one to throw away – you’ll never get it right first time

• Identifying the entity that starts the system process can help

• Develop one process at a time

• A process is always on

• Read Lejk and Deeks from cover to cover

Plan

• Introduction

• Structured Methods– Data Flow Modelling

– Data Modelling

– Relational Data Analysis

• Feasibility

• Maintenance

References

• Whiteley, D. (2004) Introduction to Information Systems, Palgrave, 2004.

• Lejk, M. and D. Deeks (2002) Systems Analysis Techniques, Addison Wesley 2002

• Mason, D. and L. Willcocks (1994), Systems Analysis, Systems Design, Alfred Waller, 1994.

References

• Yeates, D. and T. Wakefield (2004) Systems Analysis and Design, FT/Prentice Hall 2004

• Gane, C. and T. Sarson (1979) Structured Systems Analysis, Prentice Hall, 1979

• Eva, M (1994) SSADM Version 4: A users guide, McGraw hill, 1994

References

• DeMarco, T. (1979) Structured Analysis and System Specification, Yourdon, 1979

• Royce, W. (1970) Managing the development of large software systems, In: Proceedings of IEEE WESCON, 1970 pp1-9.