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Pick a movie: Understanding software testers' oracle problem

In college, I knew two guys who tested DVD players. Their job was to watch movies. Even better, they got to pick the movies! What a sweet deal right? Who wouldn’t want that testing gig?

Well, there’s a catch. There’s always a catch. While they could pick any movie, they had to watch the movie hundreds of times. Repeatedly. Again and again. I suspect that the two of them could recite every line of Gladiator in reverse order. Apparently, after you’ve seen a movie, any movie, a couple hundred times, you don’t like it any longer.

Why, might you ask, would they need to watch the same movie over and over? Well, they were testing different aspects of the audio and video drivers in the DVD player, along with the consumer-facing software that ran the device. The only way to do that, is to actually watch a movie. And the only way for you to be able to detect a small and subtle issue with the video rendering would be if you knew the movie by heart and could recognize even the smallest detail being out of place.

Talking with these fellows opened my eyes to what testers refer to as the oracle problem. An oracle is mechanism by which a tester recognizes a problem with the software. When you have an oracle — an expected result, a requirements document, a previous version of the product, etc. — you can determine when things are working and when they aren’t. For most of us, that’s text or a picture in a document that either does or doesn’t match what we see on the screen while we’re testing. For these guys, it was their memory of how the movie should sound, look, and “feel.”

The oracle problem is that all oracles are fallible. For example, requirements specifications are incomplete, contain conflicting information, or are ambiguous. Expected results in a test case only detail out a small portion of what’s expected, the tiny little portion of the application and functionality the test is designed to expose. Oracles are hard to find, require work to effectively use and always leave a tester wanting for more. That’s the problem.

For the DVD testers, the test case was the movie, and the oracle was their memory of it. Make a configuration change, watch the movie. Get a new version of a driver, watch the movie. Use a feature of the consumer-facing software, keep watching the movie. They were the oracle. They identified the problem. This context helps highlight the role we as testers play in interpreting and exercising the various oracles that we apply.

Take a second to think about the test oracles you use on a daily basis. Where do they come from? What rules do you use to interpret them? When there are ambiguities, or gaps in information, where do you go for disambiguation? What role do you play in interpreting how the oracles you use are applied, or in determining to what degree a test passes or fails?

And, if you’re not up for reflective questions about the work you do every day, instead just think about what movie you’d choose to watch over and over again — day in and day out. It’s a difficult question, because whatever movie you choose, you’ll never want to see it outside of work again.

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