Chap 002

download Chap 002

of 48

  • date post

    15-Aug-2014
  • Category

    Documents

  • view

    575
  • download

    26

Embed Size (px)

Transcript of Chap 002

Slide 2.1

An Introduction to Object-Oriented Systems Analysis and Design with UML and the Unified ProcessMcGraw-Hill, 2004

Stephen R. Schachsrs@vuse.vanderbilt.eduCopyright 2004 by The McGraw-Hill Companies, Inc. All rights reserved.

CHAPTER 2

Slide 2.2

HOW INFORMATION SYSTEMS ARE DEVELOPED

Copyright 2004 by The McGraw-Hill Companies, Inc. All rights reserved.

Chapter Overviewq q q q q q q q q q

Slide 2.3

Information System Development in Theory Winburg Mini Case Study Lessons of the Winburg Mini Case Study Teal Tractors Mini Case Study Iteration and Incrementation Iteration: The NewtonRaphson Algorithm The Winburg Mini Case Study Revisited Other Aspects of Iteration and Incrementation Managing Iteration and Incrementation Maintenance RevisitedCopyright 2004 by The McGraw-Hill Companies, Inc. All rights reserved.

Information System Development in Theoryq

Slide 2.4

Ideally, an information system is developed as described in Chapter 1 Linear Starting from scratch

Copyright 2004 by The McGraw-Hill Companies, Inc. All rights reserved.

Information System Development in Practiceq

Slide 2.5

In the real world, information system development is very different We make mistakes The clients requirements change while the information system is being developed

Copyright 2004 by The McGraw-Hill Companies, Inc. All rights reserved.

Winburg Mini Case Studyq

Slide 2.6

Episode 1: The first version is implemented Episode 2: A mistake is found The system is too slow because of an implementation fault Changes to the implementation are begun

q

q

Episode 3: The requirements change A faster algorithm is used

q

Episode 4: A new design is adopted Development is complete

q

Epilogue: A few years later, these problems recurCopyright 2004 by The McGraw-Hill Companies, Inc. All rights reserved.

Evolution Tree Modelq

Slide 2.7

Winburg Mini Case Study

Copyright 2004 by The McGraw-Hill Companies, Inc. All rights reserved.

Waterfall Modelq

Slide 2.8

The linear life cycle model with feedback loops The waterfall model cannot show the order of events

q

Copyright 2004 by The McGraw-Hill Companies, Inc. All rights reserved.

Return to the Evolution Tree Modelq

Slide 2.9

The explicit order of events is shown At the end of each episode We have a baseline, a complete set of artifacts (constituent components)

q

q

Example: Baseline at the end of Episode 4: Requirements3, Analysis3, Design4, Implementation4

Copyright 2004 by The McGraw-Hill Companies, Inc. All rights reserved.

Lessons of the Winburg Mini Case Studyq

Slide 2.10

In the real world, information system development is more chaotic than the Winburg mini case study Changes are always needed An information system is a model of the real world, which is continually changing Information technology professionals are human, so we make mistakes

q

q

Faults must be fixed quicklysee next slide

Copyright 2004 by The McGraw-Hill Companies, Inc. All rights reserved.

Relative Cost to Detect and Correct a Fault

Slide 2.11

Copyright 2004 by The McGraw-Hill Companies, Inc. All rights reserved.

Relative Cost to Detect and Correct a Fault (contd)Slide 2.12

q

If a fault is not detected and corrected, it will be carried over into the next phase Correcting the fault means Fixing the fault itself; and Fixing the effects of the fault in subsequent phases

q

q

Between 60 percent and 70 percent of all detected faults are requirements, analysis, and design faults These faults must be detected and corrected earlyCopyright 2004 by The McGraw-Hill Companies, Inc. All rights reserved.

q

Teal Tractors Mini Case Studyq

Slide 2.13

While the Teal Tractors information system is being constructed, the requirements change The company is expanding into Canada Changes needed include: Additional sales regions must be added The system must be able to handle Canadian taxes and other business aspects that are handled differently Third, the system must be extended to handle two different currencies, USD and CADCopyright 2004 by The McGraw-Hill Companies, Inc. All rights reserved.

q

q

Teal Tractors Mini Case Study (contd)q

Slide 2.14

These changes may be Great for the company; but Disastrous for the information system

Copyright 2004 by The McGraw-Hill Companies, Inc. All rights reserved.

Moving Target Problemq

Slide 2.15

A change in the information system while it is being developed Even if the reasons for the change are good, the information system can be adversely impacted Dependencies will be induced

q

Copyright 2004 by The McGraw-Hill Companies, Inc. All rights reserved.

Moving Target Problem (contd)q

Slide 2.16

Any change made to an information system can potentially cause a regression fault A fault in an apparently unrelated part of the system

q

If there are too many changes The entire information system may have to be redesigned and reimplemented

Copyright 2004 by The McGraw-Hill Companies, Inc. All rights reserved.

Moving Target Problem (contd)q

Slide 2.17

Change is inevitable Growing companies are always going to change If the individual calling for changes has sufficient clout, nothing can be done about it

q

There is no solution to the moving target problem

Copyright 2004 by The McGraw-Hill Companies, Inc. All rights reserved.

Iteration and Incrementationq

Slide 2.18

The basic information system development process is iterative To iterate means to repeat

q

Each successive version is intended to be closer to its target than its predecessor

Copyright 2004 by The McGraw-Hill Companies, Inc. All rights reserved.

Millers Lawq

Slide 2.19

At any one time, we can concentrate on only approximately seven chunks (units of information) To handle larger amounts of information, use stepwise refinement Concentrate on the aspects that are currently the most important Postpone aspects that are currently less critical Every aspect is eventually handled, but in order of current importance

q

q

This is an incremental process To increment means to increaseCopyright 2004 by The McGraw-Hill Companies, Inc. All rights reserved.

Iteration and Incrementation

Slide 2.20

q

(a) Iteration and (b) incrementationCopyright 2004 by The McGraw-Hill Companies, Inc. All rights reserved.

Iteration and Incrementation (contd)q

Slide 2.21

Iteration and incrementation are used in conjunction with one another There is no single requirements phase or design phase Instead, there are multiple instances of each phase

q

q

Copyright 2004 by The McGraw-Hill Companies, Inc. All rights reserved.

Iteration and Incrementation (contd)

Slide 2.22

Copyright 2004 by The McGraw-Hill Companies, Inc. All rights reserved.

Iterative and Incremental Life-Cycle Model

Slide 2.23

q

Sample life cycle of an information systemCopyright 2004 by The McGraw-Hill Companies, Inc. All rights reserved.

Traditional Phases and Workflowsq

Slide 2.24

Sequential phases do not exist in the real world Instead, the five core workflows (activities) are performed over the entire life cycle Requirements workflow Analysis workflow Design workflow Implementation workflow Test workflow

q

Copyright 2004 by The McGraw-Hill Companies, Inc. All rights reserved.

Workflowsq

Slide 2.25

All five core workflows are performed over the entire life cycle However, at most times one workflow predominates Examples: At the beginning of the life cycle The requirements workflow predominates

q

q

At the end of the life cycle The implementation and test workflows predominate

Planning and documentation activities are performed throughout the life cycleCopyright 2004 by The McGraw-Hill Companies, Inc. All rights reserved.

Iteration and Incrementation (contd)q

Slide 2.26

Iteration is performed during each incrementation

Copyright 2004 by The McGraw-Hill Companies, Inc. All rights reserved.

Iteration Exampleq

Slide 2.27

We use the NewtonRaphson algorithm (1690) to compute the square root of a number N using iteration 1 N newValue = currentValue + 2 currentValue

q

Suppose N = 123456 The initial guess is 100 (initial currentValue)

q

Copyright 2004 by The McGraw-Hill Companies, Inc. All rights reserved.

Iteration Example (contd)q

Slide 2.28

Plug this into the formula to get the first iteration1 654321 newValue = + 100 = 3321.605 2 100

q

Now our estimate of

123456

is 3321.605

This becomes our next currentValueq

We plug the value 3321.605 for currentValue into the right-hand side of the formula

Copyright 2004 by The McGraw-Hill Companies, Inc. All rights reserved.

Iteration Example (contd)q

Slide 2.29

Our second iteration is then1 654321 newValue = 3321.605 + = 1759.2972036147 2 3321.605

q

Our estimate of

123456

is now 1759.2972036147

This becomes our next currentValueq

Again we plug this value into the right-hand side of the formula for our third iteration