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February 1, 2001

Continuous Integration

Authors: Matt Di Iorio, Andrew Fuqua, Charlie Hubbard

Have you ever been on a project that practiced big-bang integration – wait until all the features are done, then throw it all together and see what happens? It usually turns out just like the name implies – a totally chaotic, primordial soup of software. Just getting the code to compile is an amazing feat. Then come the bugs. How painful it is to harness the power of nature to integrate your code!

Or say you have a daily build. But for some reason your daily build is broken daily. So you spend half the day tracking down the slacker who checked in the bad code.

"Ah, but my daily build never breaks", you say. Great! But have you ever spent more than a day tracking down one bug? I thought so.

Our team has overcome these problems using simple techniques: We check-in frequently. And we build and test continuously.

"We measure success one build at a time"

Our automated build and test process runs continuously – not daily. Well, it actually runs every half-hour, but that's close enough to continuous for me.

By building and testing continuously, we find it trivial to figure out what broke the build. Given the short time span between builds, the system is usually built with only one developer's changes at a time. If it broke, we readily know why.

By checking in frequently, we avoid file contention. This means never having to merge two sets of changes.

By doing both of these, we never need more than a few minutes to integrate a piece of code into the system. The code in MKS is always buildable – it always works – every minute of every day. I can't stress this enough: it is never in a broken state. That certainly makes integration easy. And since we check-in frequently, we never have much to integrate.

XP's practices build on each other. That's what makes the process so strong. Like XP, continuous integration's practices are multiplicative. Continuous integration draws its strength from good unit tests and frequent check-ins: The tests are more valuable because they are run more often. The frequent builds are more valuable because by running the tests they do more work each time.

Our House

You will get some benefit just by increasing check-in and build/test frequency. We discovered that some additional things we do make continuous integration even better. You may have different needs and different practices. But here is what we do.

Farewell Make

We broke our love affair with Make. She was never a very accommodating partner – always wanting things to be perfect, never compromising. I wanted spaces; She wanted tabs. We just couldn't go on.

We started using a tool called Ant. It's an extensible, cross-platform build tool written in Java. When we discovered that it uses XML for the build file, we just had to use it. Ant pulls from MKS, compiles the code, builds and signs the jar files, runs the tests, and lets us know if anything failed. A Windows Scheduled Task kicks it off every 30 minutes.

One cool Ant feature is the ability to write "build listeners". We wrote a listener to yell at us when the build fails. To notify the developers we use a "net send" command. We wanted to make it intrusive as possible so the developers couldn't ignore failures.

Developers need to build and test on their workstations before checking in their changes. Our IDE allows us to build and test. But we also wanted each developer be able to build with the same process as on the "official" build machine. This required us to modify our architecture but it helps keep the build running smoothly.

"Just the facts, ma'am"

Once you start running an automated build, you've got to see the results. Our results are on a web page. Prominently shown at the top is the Success / Failure message. Then come the details. If the build was not successful, you can see which files didn't compile or which tests failed.

When the build does fail, it only takes us a couple minutes to find and fix the problem. (Remember all the benefits I mentioned above?) In keeping with the spirit of "continuous", we don't wait 20 minutes for the next regularly scheduled build. We can kick off an unscheduled build at any time from our build page.

Our build page even has a cool plot of the number of unit tests run each day. That helps us focus on writing more tests.

Keep it up

It's always good to be on the lookout for things that you can do to make your life easier (or safer). For example, Mike Slifcak suggested we build the database from scratch before running all the tests. This ensures our production code and our install code don't get out of sync. Now we have continuous integration of the install and production code. See how much you can automate.

Our main goal with the build process was to get the most value by doing the least amount of work. There were lots things we could have added to our build process, like automatically logging all the changes between builds. But that wasn't necessary and would only give us more to maintain. Stay light!

Links

http://www.martinfowler.com/articles/continuousIntegration.html
Martin Fowler's article on continuous integration that got us started.

http://jakarta.apache.org/ant/
Here's the great great Ant.

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