How to write a bug
In order to break my slump I thought I’d give a quick description of a project I recently worked on. I was helping set up a demo of an application that was moved onto the fabric. The app was written in C and we didn’t change any of its code. We ended up running it by shelling out from Java to make it easy for the developers who didn’t know C. The person asking for the demo wanted to see our reliability feature but didn’t want to see the usual hardware demo we give which involved pulling a power cord or network cable. So I had to invent a software bug that would illustrate how an application can be made reliable by running it on top of Appistry’s software.
So to make a short story long, here’s what my first try looked like:
Random generator = new Random(System.currentTimeMillis());
if( (generator.nextInt() % 4 ) > 0 )
{
doSomething();
}
The basic idea being that by generating a random integer in the code running on the fabric I could then use the modulo operator to inject a bug about 25% of the time. Because it’s random, it would be unpredictable (the best kind of bug). The modulo operator would give me values from 0-3 and therefore if I got a 0 I would run the bug:
else
{
throw new Exception();
}
The error case just throws an Exception because the fabric will attempt to retry the job if an unchecked exception is thrown. We tested out our new bug and it didn’t work. It was running the error case way too much. So basically I had a bug in my bug. After reading the javadoc on the Random.nextInt() method I figured out that it will generate negative numbers! So I added the following fix:
if( Math.abs(generator.nextInt() % 4 ) > 0 )
Now I get my 25% failure rate like we wanted. We show the client running and submitting a handfull of requests to the fabric. Then, on the log on, the fabric, we show in realtime that the failure randomly occurs. But when it does, the job is automatically resubmitted on the fabric and the answer is sent back to the client. The client doesn’t have to be concerned with retry logic and only gets its correct answer.
I thought a cool web demo would be to use Ajax and setup a button for people to cause a software bug to happen while the results are coming back to the browser in realtime. Maybe I’ll write that in my "spare" time…
-jasen
