Class 14: Extreme Programming (1)

Held: Monday, 27 September 2004

Summary: Today we begin to explore Extreme Programming (XP), a lightweight software design methodology.

Related Pages:

• EBoard.
• ExtremeProgramming.org.

Notes:

• Even though it's Monday, there is no quiz.
• Are there questions on the homework?
• Are there questions on the reading?

Overview:

• Context: Kinds of Software.
• Context: Heavyweight Development.
• Basics of XP.

Context: Kinds of Programs

• Because we focus so much on general principles in our curriculum, we rarely, if ever, talk about what real programmers do.
• It may help to think of three kinds of software:
  • General-purpose programs are designed for a range of environments and applications. Such programs might include macro recorders, word processors, and spreadsheets.
  • Special-purpose programs are designed for specific applications, but are intended to be distributed to a variety of customers. Such programs might include medical records software, and certain kinds of accounting programs.
  • Custom programs are designed for particular customers in particular situations.
• We can also think of programs from the point of view of a customer trying to solve a problem.
  • The customer can use off the shelf software (often special-purpose) and adapt his or her business processes to the software.
  • The customer can use more general-purpose programs and build something somewhat more custom on top of the general-purpose programs.
  • The customer can begin with open-source software that solves a similar need and customize it.
  • The customer can build a custom solution from scratch.
• Note that the last three all require programmers.
• Which, in your mind, is the focus of the XP book?

Context: Software Development

• When building a software package, how do you get from start to finish? Various software methodologies are intended to guide you.
• The traditional model is something like the following:
  • Build a clear and complete set of specifications. These specifications should indicate what the software does (and does not do) in every situation.
  • Build a comprehensive architecture and design for a program to meet those specifications.
  • Implement the design. If the design is complete enough, this should be a relatively straightforward task.
  • Test.
  • Release
  • Maintain.
• Because these steps are all large, such models are often called heavyweight.
• The traditional model has some significant strengths.
  • Because the system is viewed "in the large", potentially conflicts between different parts are identified early.
  • Because the specifications are not to change, programmers work toward a fixed target.
  • The coding is relatively straightforward.
• However, the traditional model also has some weaknesses.
  • It does not handle change well. In practice, customers change their requirements.
  • It may not distinguish between central and optional features.
  • Some conflicts aren't obvious until the system is implemented.
• Various lightweight development strategies have been developed in response to these problems.

Extreme Programming Basics

• Extreme Programming (XP) is one of the most famous (or at least most advertised) lightweight programming strategies.
• It involves a number of programming techniques and habits working together.
• Kent Beck talks about various raw materials for designing XP.
• The first raw material: The story about learning to drive.
• Next: The four values:
  • Communication
  • Simplicity
  • Feedback
  • Courage