Structured Systems Analysis

What we will cover System modelling generally What is a model? Why model a system A summary of modelling approaches

Slide 2 of 20

System modelling generally

Slide 3 of 20

I

n

p

u

t

s

Source 1

Source 3

Source 2

Recipient 1

Recipient 2

Recipient 3

The System

O

u

t

p

u

t

s

a miracle happens!

And lo …

What is a model? That was a model! An abstracted representation of something

that enables the identification of relevant elements, components and their inter-relationships

For example: An OS map of North Staffordshire A YHA map of North Staffordshire A cycling map of North Staffordshire A road map of North Staffordshire

All show the same territory, but …

Slide 4 of 20

Why model a system?

to find out what is and/or what should be going on

to allow system developers to understand and communicate with system users/sponsors

to provide a framework that handles complexity, allowing decomposition

it’s cheaper to model than to build enables “look ahead”

Slide 5 of 20

Why model a system? - summary Enables you to “go there” in your head

before you write any code things can be identified and sorted into the

important and the not-so-important problems can be identified (and possibly even

solved) early clarifies woolly ideas allows issues to be raised and discussed early facilitates more sensible decisions helps you to find out what you don’t know saves embarrassment later

Slide 6 of 20

Modelling/development approaches Object oriented approaches Structured approaches Collaborative approaches

RAD, Prototyping, DSDM …….

Agile approaches SCRUM, XP etc Today’s challenge – what does SCRUM stand

for? prizes next time

Other approaches

Slide 7 of 20

Structured modelling Separates the consideration of:

what a system does (or is to do) from the data and the (relational) data structures that are

required to enable it to do it Is all Edgar Codd’s fault (from 1970) Process modelling - covers the what Data modelling - covers the structure of the data

needed to support what is happening Time and Event modelling – cover things that

Process Modelling doesn’t do very well Structured modelling includes some useful

techniques that are generically applicable Slide 8 of 20

Process Modelling does what it says on the tin! for any information system, models

processes (at various levels of detail) participants data required/processed/stored/transmitted

pictures give framework for detailed descriptions

Slide 9 of 20

Data Modelling

Models the data required to enable a system to perform it’s defined functions (processes)

and how this data should be structured when persisting

Loads more of this later in the module This is an area which improved greatly with

practice and, at first, seems incomprehensible and impossible

Slide 10 of 20

Useful structured modelling tools and techniques Included here as they do not really fit

neatly into the process and/or data categories Aim of a system Levelling Problem/Requirements lists Data dictionary

But, Process and Data modelling are implicitly included!

Slide 11 of 20

Aim of a system a (short) paragraph to define succinctly what

a system does (and does not) do For a company’s retail system

“To support the correct administration and accounting of ordering, packaging and sales of goods through its shop and mail order operations”

Slide 12 of 20

Aim of a system - examples A payroll system

“To enable the correct calculation of gross pay and deductions for all weekly paid employees. Funds will be transferred electronically.”

A lift system “To transport a maximum of eight people between

floors in a safe and timely manner”

Slide 13 of 20

Levelling

Slide 14 of 20

Found in SAD structured process modelling but generically applied often

Handles complexity through grouping of low level functions and/or by decomposing high level functions

Each level lower has more detail than higher level(s)

Links high and low level functions allowing simultaneous consideration of all levels

Levelling - graphically

Slide 15 of 20

Somethingcomplex

Somethingless complex

Somethingless complex

Somethingless complex

Somethingless complex

Somethingeven simpler

Somethingeven simpler

Somethingeven simpler

Somethingeven simpler

Somethingsimpler

Somethingsimpler

Somethingsimpler

Somethingsimpler

Problem/Requirements list A numbered list of all the problems with

the current system and (usually hence) requirements of a new system

Does not differentiate between whether issue is a problem or a requirement

Includes the sublime, the ridiculous and all points between

Serves as a check-list when conceiving and designing new system

Helps to prevent problems and requirements becoming lost in the maw (mire) of analysis and design methods

Slide 16 of 20

What we have covered System modelling generally Consideration of a model and what it is Why model a system Identification of different modelling

approaches. Elements of structured approaches Assignment guidance What comes next

Slide 17 of 20

References Essentials of Systems Analysis and Design,

Valachi, George and Hoffer, Pearson Prentice Hall

Software Engineering, Ian Sommerville, Addison Wesley, 2000

Software System Development – A Gentle Introduction – Briton and Doake, McGraw Hill

Slide 18 of 20