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Structured Systems Analysis. What we will cover. System modelling generally What is a model? Why model a system A summary of modelling approaches. The System. System modelling generally. O u t p u t s. Source 1. Recipient 1. I n p u t s. And lo …. Source 2. Recipient 2.

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What we will cover
What we will cover

  • System modelling generally

  • What is a model?

  • Why model a system

  • A summary of modelling approaches


System modelling generally

The System

System modelling generally

O

u

t

p

u

t

s

Source 1

Recipient 1

I

n

p

u

t

s

And lo …

Source 2

Recipient 2

a miracle happens!

Source 3

Recipient 3


What is a model
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 …


Why model a system
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”


Why model a system summary
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


Modelling development approaches
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


Structured modelling
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


Process modelling
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


Data modelling
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


Useful structured modelling tools and techniques
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!


Aim of a system
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”


Aim of a system examples
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”


Levelling
Levelling

  • 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

Something

less complex

Something

less complex

Something

less complex

Something

less complex

Something

simpler

Something

simpler

Something

simpler

Something

simpler

Something

even simpler

Something

even simpler

Something

even simpler

Something

even simpler

Levelling - graphically

Something

complex


Problem requirements list
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


What we have covered
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


References
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


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