Testing Cartwheel, and How Testing Improves Code

Over the last few days, I put a bunch of time into my ongoing refit of Cartwheel.

Cartwheel is a sizeable toolkit for doing bioinformatic sequence analysis; it's got a Web interface, a batching and queuing system built on PostgreSQL, and a client library for manipulating things over XML-RPC. So, it's got a lot of moving parts, which means that testing it is important, but it's also hard.

(I wrote earlier about how automated testing is hard, even when you wrote all of the software involved, and I also wrote a bit about how to enable in-process testing of XML-RPC.)

Cartwheel is actually one of the main motivating factors behind twill, my framework for functional Web testing. I noticed that over the years, Cartwheel had gotten more and more brittle: as I'd add features, old ones would break or give me trouble. PBP kept popping up on my radar, and finally I took the time to go try it out & eventually rewrote it into twill.

So anyway, I've been adding to the functional tests for the Cartwheel Web and XML-RPC interfaces (what would that be, the HTTP-level stuff? yeah, that), and I ran into a whole slew of problems connected to releasing the database handles. It turns out that even though I had some finalization code for the database interface, it wasn't enough, and even the stuff that was there didn't work. Since the finalization code was never actually run in deployment -- Linux was actually doing the cleanup when the process died, of course -- I hadn't understood just how badly I'd written things. Database connections weren't getting closed, and this caused various operations to hang because I was trying to drop tables while connections were open.

Several hours of head-scratching, code examination, and 'print' statements later, I now have a large set of tests for both HTTP-level interfaces, and I'm much more confident that my cleanup routines are working. Moreover, I understand the consequences of my original architecture decisions much better.

This brings me to the meta-lesson: before this experience, I knew that good finalization routines were important because having them improved the usability of the code and also having them was simply the Right Way to program. Since I never actually had to re-use this code in other contexts, this was largely a theoretical consideration. However, once I had to do setup and teardown for my functional tests, which effectively meant I was reusing the code, I ran smack into the wall of reality: yes, it is important, and if I'd been smart enough to write things properly Back When my last two days would have been much more pleasant.

Next up: CMake, KWWidgets, and the Kitware experience.

--titus

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