20 March

XMLExtreme Programming

Customization

Separation of customization from code Create family of applications instead

of a single one Design from the start

Currently favored: XML

XML

What is it?

Tools

How to use it?

What is XML?XML - eXtensible Markup Language

A "standard/specification" for describing data with markup tags

Tags can describe data or a structureXML - cornerstone of a set of technologies

XML can be used aloneRelated technologies for most effective use

History of XML

Built on other technologies GML and SGML – (Standard) Generalized Markup Language: 1960s Before there was Word and WYSIWYG editors

HTML - HyperText Markup Language W3C (World Wide Web Consortium) standard

.ce Introduction .ju .11 39 .ss The work of authors, publishers, and researchers involves, in varying degrees, three recognizable text Formatting commands: .ce center next line .ju justify right margin .ll line length .ss single spacing

Script (GML)Example

Tags

Focus on data description and structureElement -- a pair of tags with contentAttribute -- specific information about an element

Allow creation of XML DialectsSMIL -- for multimedia (RealPlayer Multimedia players) WML -- Wireless WAP-phones XHTML -- XMLized version of HTMLMathML -- for mathematicsHEML -- historical events

Hundreds of dialects

HTML XML Uses <tag> & </tag> style Markup describes text

Focus on presentation Data is text (limited reuse)

(Relatively) fixed set of tags

Loose syntax End tags assumed Nesting errors affect display

Simple and complete Single use - for Web

Uses <tag> & </tag> style Markup describes information

Focus on data structure Data retains meaning

Extensible - can define new tags

Stringent syntax End tags required Element nesting enforced

Uses related technologies Highly reusable documents

Related Technologies

XML schema (XSD replaced DTD) provides the syntax defines way elements and attributes represented

in a XML document eXtended Stylesheet Language (XSL)

enables data reuse transform XML into other XML HTML format XML for presentation (rendering)

Fonts, size, color, alignment, ... Rules for ordering

Tools

XML Parser XSL Processor

Lots and lots of them Should you use them?

How to Use XML Key uses

Separation of changeable decisions Need for multiple forms

Before you invent your own tags, are there relevant ones that you can appropriate?

Use of attributes versus elements continuing discussion

The Goal of Software Engineering

The right software, delivered defect free,

on time and on cost,

every time.

Software Engineering History

First key conference in 1968 Became important because of

perceived software crisis in productivity Cost and budget overruns (OS/360)

Morphed to issues of quality Financial implications (BoNY) Safety (Therac)

BoNY (1985)

BoNY (Bank of New York): Nation’s largest clearer of government securities

Software to track Federal securities transactions wrote new information on top of old.

Feds debited the bank for each transaction but bank did not know who owed it how much.

90 minutes => $32 Billion overdraft!

Cost of Bug Bank had to borrow $24 bill from federal

reserves. Interest paid ~$5 mill for 1 day. (Annual earnings of bank ~$120 mill)

BoNY share prices dropped by 25 cents Federal funds rate dropped from 8.4%

to 5.5% System down for 28 hours. Fear of financial crisis caused increase

in price of platinum!

Cause of bug

Message buffer counter at BoNY system was 16-bit long.

Counters at Fed (and other banks) 32 bit.

More than 32,000 transactions that morning! =>Counter overflow

Securities database corrupted.

The Drama continues…

Trying to correct it – they copied corrupted data over the backup.

Lost a few hours because of this. Does code for error recovery get

tested at all?

Therac-25 (1985-1987) Medical linear accelerator

Used to zap tumors with high energy beams.

Electron beams for shallow tissue or x-ray photons for deeper tissue.

Eleven Therac-25s were installed: Six in Canada Five in the United States

Therac-25 Turntable

Counterweight

Field Light Mirror

Beam Flattener (X-ray Mode)

Scan Magnet (Electron Mode)

Turntable

Therac-25

Changes from Therac-20 Used new “double pass” technique to

accelerate electrons…more deadly Machine itself took up less space Software coupled to the rest of the

system and responsible for safety checks.

Hardware safety interlocks removed. “Easier to use”

What Happened?

Six patients were delivered severe overdoses of radiation between 1985 and 1987. Four of these patients died.

Why? The turntable was in the wrong position. Patients were receiving x-rays without

beam-scattering.

What caused that to happen? Race conditions. Overflow error.

The turntable position was not checked every 256th time the “Class3” variable was incremented.

No hardware safety interlocks. Non-descriptive error messages.

“Malfunction 54” “H-tilt”

User-override-able error modes.

Source of the Bug

Incompetent engineering. Design Troubleshooting

Virtually no testing of the software. The safety analysis excluded the

software! No usability testing.

That’s Why We Care

Now, back to history…

1960’s 60’s

“Cowboys” wrote software anyway that they could

Difference between best programmers and worst as high as 28:1 (many sources)

1968 Edsger Dijkstra, “GOTO Statement Considered

Harmful” (CACM) Recognition that rules can improves the average

programmer The start of software engineering?

Structuring Software Development

Few rules helped immensely Good rules and practices developed over

the 70’s and 80’s If a few rules are good, more are

better… Late 80’s, major focus on process as a

key to quality ISO 9000 Malcolm Baldridge National Quality Award

Why not apply to software development?

ISO 9000 What is ISO?

International body 150 national standards organization

US: ANSI Primarily technical standards Recent years has broadened its scope

Generic management system standards First published in 1987 Revision in 2000

Compendium of best practices Not the process but the management of the process

http://www.iso.org/iso/en/ISOOnline.frontpage

Which brings us back to

What is part of software engineering?

The 4 P’s of Software Engineering

People: those doing it Product: what is produced Process: the manner in which it

is done Project: the doing of it

Models of Software Development

Process or people oriented Early Watts Humphrey work: people,

later process Different balances from each model

Number of iterations Waterfall Spiral Agile

Software Craftmanship

Software craftsmanship (McBreen 2001) Craft of writing software Craft of using software

Distinguish from software engineering Scope Rigor

Relevant distinction?

Product Sales brochure Use cases and requirements Design document: from architecture to

detailed design Fully documented code Test plan and tools Manuals

Users Administrators

Process

Software Engineering Steps Concept Requirements Architecture Design Implementation Unit test Integration System test Maintenance

Software Engineering Processes Reviews and Inspections

Software Engineering Processes

Differ by how often you do the steps Points on the spectrum Differences in overhead

Three fundamental models Waterfall Spiral Iterative

Two widely used models Rational Unified Process (a.k.a. Unified

Software Development Process) Extreme Programming

Integrated Product Development:The IBM Approach

Originated at GE Key principles

Cross-functional teams at all phases Phased approval

Example checkpoints Concept Plan Ship Sunset

Rational Unified Process

Iterations within phases 4 phases

Inception (interaction with stakeholders) Elaboration (architecture and functions) Construction (initial operational) Transition (completed product)

Core workflows for each iteration Requirements Analysis (part of Requirements) Design Implementation (includes Integration) Test

Unified Process Matrix

ElaborationInception Construction Transition

Requirements

Analysis

Design

Implemen-tation

Test

Extreme Programming

Complete development process First code drop is 2-3 weeks after start Customer a part of the development

team Iterative development to the max Derive requirements with customer

through hands-on experimentation Agile methodology

Why XP?

Companies started codifying their practices

Large documents and people to manage them Rise of the project manager

“Honored in the breach” More large projects and more late

or failed projects

1995 Standish Group Study

50% software projects challenged 2x budget 2x completion time 2/3 planned function

30% impaired Scrapped

20% success http://web.mit.edu/Saltzer/www/publications/

Saltzerthumbnails.pdf

Agile Methodologies

Keep only those rules and processes that help Antidote to bureaucracy License to hack

Key characteristics Adaptive People-oriented

Extreme Programming: History

Kent Beck considered the inventor Ideas developed in the early 90’s First project at Daimler Chrysler in

1996

XP Bills of Rights Developer has a right to

Clear requirements and priorities Determine how long a requirement will take to implement Revise estimates Always produce quality code

Customer has a right to An overall plan See progress in a running system Change requirements and priorities Be informed of changes to schedule and have input as to

how to adapt Cancel in the middle and still have something to show for

the investment

XP Bills of Rights Developer has a right to

Clear requirements and priorities Determine how long a requirement will take to implement Revise estimates Always produce quality code

Customer has a right to An overall plan See progress in a running system Change requirements and priorities Be informed of changes to schedule and have input as to

how to adapt Cancel in the middle and still have something to show for

the investment

XP Value System

Communication Focus on people, not documentation

Simplicity Of process and code

Feedback Mechanism to make useful progress

Courage To trust in people

People Most important factor in the quality of

software is the quality of the programmers If your life depended on a particular piece

of software, what would you want to know about it?

Bollinger (2001): that the person who wrote it was “both highly intelligent and possessed by an extremely rigorous, almost fanatical desire to make their program work the way it should.”

Knowledge Workers … prefer closed offices but communicate better

in open ones congregate in particular geographical areas move around in the course of their work collaborate concentrate work in the office communicate with people who are close by don't care about facilities gewgaws

Davenport, Why Office Design Matters 2005

Extreme Programming Project

http://www.extremeprogramming.org/