Extreme Programming

Writing software is a big fat mess and is usually tangled up with bureaucracy and Human collaboration issues. As development progresses, software typically becomes more and more fragile and more and more buggy. As we've discussed, it's rare that a system is delivered that actual does what the customer wants (let alone on time).

To fix the problem, people tried to apply engineering principles to the problem. That "analyze, design, implement, test" sequence is what I was taught as an undergraduate in the early 80's. That waterfall method produces software that is inflexible, late, and most importantly doesn't do what the user wants. Apparently it's not the solution.

The problem is that users don't know what they want until you start showing them something. Quoting Tom Burns:

"You know the least about a problem at the start of a project--that is why it doesn't make any sense to do your 'final' design at the beginning. You need to familiarize yourself with the problem space by making something useful--by writing code. Letting people use it and give feedback. Then you should upgrade your design (refactor) as you discover what is truly important."

The single biggest lesson I learned, and a point emphasized by Extreme Programming (XP), the subject of this lecture, was that requirements never stay the same for even a few days. You had better get used to it, or better yet, take advantage of it lest you drown. Interestingly, there is a close relationship between what XP espouses and between what I ended up following at jGuru, most of which I learned on the job or by listening to Tom Burns, our CEO.

This lecture summarizes the principles of XP, but does not delve into details of suggested solutions by Kent Beck and crew. I summarize "Extreme Programming Explained" here and pepper it with experience I gained from building jGuru.com.

Agile Development

XP is a form of Agile development that focuses on short development cycles and close interaction with customers. Refactoring and incremental feature alteration/addition are the name of the game. From the Agile manifesto, Agile development values...

The Agile principles are also useful.

Martin Fowler (of refactoring fame) says quote:

Agile programming has fallen short, conference told:

"McConnell noted what appears to be a contradiction in agile programming thus far. While intended to focus on individuals and interactions, agile seems to be mostly about processes and tools now, he said."

Ian MacFarland's XP slides. Ian gave a great talk to a previous CS601 class and had a great idea about determining cost and schedule for clients.

XP Principles

Erich Gamma's summary:

Does not imply that just start "daredevil" hacking. You must be disciplined.

Kent Beck's Summary:

"Extreme" implies "what is good, do to the extreme."

Designed for 2-10 programmers

What would you do if you had lots of time to build a project? You'd build lots of tests, you'd restructure a lot, and you'd talk with the customer and other programmers a lot. The normal mentality is that you never have enough time. XP says that if you operate in this counterintuitive manner, you'll get done faster and with better software.

What's the difference from other methodologies?

Overcoming Software Problems

Important XP Philosophy

XP's Four Values

  1. Communication. Problems can often be traced back to poor communication (TJP; cite class project that split,diverged because members didn't communicate). Programmers might miss telling others of important design change or not talk to customers. Manager might not ask programmer the right question. TJP: example of somebody at jGuru delivering bad news about wrong version being sent to customer.
  2. Simplicity. "What is the simplest thing that could possibly work?" It's very hard to implement something stupid and simple especially when you are always looking ahead to the future--you desire software that won't have to be changed later. But worrying about the future implies you are listening to the "changes later cost more than changes now" philosophy. The speed you gain from simplicity today can help fix speed problems later if you turn out to be horribly wrong 2% of the time. TJP: You have no idea will survive even a few days. Elegant software mostly comes from refactoring after the code has lived a few "generations." Use an evolutionary, iterative refined process. Use OO techniques and data driven software as much as possible to isolate things that might change. Example: URL->pageclass map, config files, java->rdms mapping, managers, etc...
  3. Feedback. Programmers use tests to get feedback about state of the system. Customer asks for a feature, get immediate feedback about difficulty (TJP: I remember Tom Burns asking me all the time "How hard would it be to implement ..."). Someone tracking project gives feedback to programmers and customers. Feedback also works on scale of weeks and months as customers write functional tests for features. Code and put into production most important features first so you can learn from it. In the old days, "production" meant you were done. In XP, it's always under production.
  4. Courage. When you find a serious design flaw, have the courage to go fix it; your project will surely die w/o it. Throw code away even if you've worked hard on it. Sometimes rebuilding results in much better code. Sometimes you have lots of choices for a design, have the courage to just try them out to see how they feel; toss losers and start over with promising design.

    It takes courage to fix a fundamental design flaw or make a huge simplification in an existing system. Try it out! TJP: Tom/me in France building Karel language debugger. Listen to others and try out huge simplifications. I had to throw out my design and I wasted all that meeting time.

Some Terence Thoughts


It's easy to say build lots of unit tests. It's another thing to know what a good test is. One of the things I noticed in student projects is that people test their code with that same code! Ack!

Consider testing forum message inserts.

public void testForumInsert() {
  boolean worked = db.insertForum(...);

The problem is that I can implement db.insertForum() as return true; and get a "green signal."

Another less egregious problem is the following:

public void testForumInsert() {
  Message in = ...;
  int ID = db.insertForum(in);
  Message out = db.getForum(ID);

This is a poor test because I could implement insertForum to store things in RAM (not even storing in a db) and getForum would yield the proper result.

You should test your code with "exterior code" and the rawest code you can find. For example, I would test insertForum by writing SQL via the shell or a Java program that looked at the physical db tables.


You must have an integration box that is identical to your development box that is identical to your deployment box. All using the same OS and Java version. Otherwise you will not know if a threading problem or other weird system "feature" will bite you. You must know that tests run on your integration box implies it will run on your deployment box. Conversely, if the deployment box has a problem, you need to be able to find it on a non-live box.

Load testing tools and such are crucial to reproducing conditions found on the live site.

Paranoia and "caring"

Many programmers I have worked with just didn't seem to care as much as I did about the system. My attitude follows Yoda: "there is no try, only do!" Therefore, I tested and tested and never left "an enemy at my back." When I released something at jGuru I had great confidence in it. Our first system was so bad (getting phone calls at 3 AM to reboot a frozen server sucks! I should have made the employees get up to fix it--would have resulted in better software) that I swore the second system would be well done.

If you thing something might go wrong, it will of course. The only time that the second version of jGuru has crashed due to software error was right before I went on vacation just after launch naturally. In fact, when it crashed, I knew just where the problem was immediately--the one bit of code I just threw together and didn't test for boundary conditions! We got an infinite loop. The system has crashed about 10 times: 2 power failures (induced by the moron ISP), 1 software crash, 1 disk overflow (oops), a handful of crashes due to insufficient resources (memory or file descriptors needed by lucene search engine). Naturally, there have been bugs in functionality ;)

Communicating with yourself

You need to record all conversations with the customer so you can remember later what to build.

For each task, I make a list of things to do.

I also have an overall list of tasks to build and in what order (prioritized).

As I build something, I keep a lab notebook describing what I try and also asking/answering questions as if I'm talking to myself. Stream of consciousness style. I have often been diverted from a task for a long time and the notes help a lot when I come back to it months later to restart.

I also keep a bug list on a piece of paper. For some reason this works better for me than a web-based system. I check them off as I finish them etc...