Regression Testing, part I

More traffic from the Agile Testing mailing list; Grig Gheorghiu is a programmer in Los Angeles who has some thoughtful observations and questions.

I’m well aware of the flame wars that are going on between the ‘automate everything’ camp and the ‘rapid testing’ camp. I was hoping you can give some practical, concrete, non-sweeping-generalization-based examples of how your testing strategy looks like for a medium to large project that needs to ensure that every release goes out with no regressions. I agree that not all tests can be automated. For those tests that are not automated, my experience is that you need a lot of manual testers to make sure things don’t slip through the cracks.

That sounds like a sweeping generalization too. 🙂

I can’t provide you with a strategy that ensures that every release goes out with no regressions. Neither can anyone else.

Manual tests are typically slower to run than automated tests. However, they take almost no development time, and they have diversity and high cognitive value, thus they tend to have a higher chance of revealing new bugs. Manual tests can also reveal regression bugs, especially when they’re targeted for that purpose. Human testers are able to change their tests and their strategies based on observations and choices, and they can do it at an instant.

Automated tests can’t do that. At the end-user, system, and integration level, they tend to have a high development cost. At the unit level, the development cost is typically much lower. Unit tests (and even higher-level tests) are typically super-fast to run and ensure that all the tests that used to pass still pass.

When the discussion about manual tests vs. automated tests gets pathological, it’s because some people seem to miss the point that testing is about making choices. We have an infinite number of tests that we could run. Whatever subset of those tests we choose, and no matter how many we think we’re running, we’re still running a divided-by-infinity fraction of all the possibilities. That means that we’re rejecting huge numbers of tests with every test cycle. One of the points of the rapid testing philosophy is that by making intelligent choices about risks and coverage, we change our approach from compulsively rejecting huge numbers of tests to consciously rejecting huge numbers of tests.

So the overall strategy for regression testing is to make choices that address the regression risk. All that any strategy can do–automated, manual, or some combination of the two–is to provide you with some confidence that there has been minimal regression (where minimal might be equal to zero, but no form of testing can prove that).

Other factors can contribute to that confidence. Those factors could include small changes to the code; well-understood changes, based on excellent investigation of the original bug; smart, thoughtful developers; strolls through the code with the debugger; unit testing at the developer level; automated unit testing; paired developers; community ownership of the code; developers experienced with the code base; readable, maintainable, modular code; technical review; manual and automated developer testing of the fix at some higher level than the unit level; good configuration management. People called “testers” may or may not be involved in any of these activities, and I’m probably missing a few. All of these are filters against regression problems, and many of them are testing activities to some degree. Automated tests of some description might be part of the picture too.

When it comes to system-level regression testing, here are the specific parts of one kind of strategy. As soon as some code, any code, is available, rapid testers will test the change immediately after the repair is done. In addition, they’ll do significant manual testing around the fix. This need not require a whole bunch of testers, but investigative skills are important. Generally speaking, a fix for a well-investigated and well-reproduced bug can be tested pretty quickly, and feedback about success or failure of the fix can also be provided rapidly. We could automate tests at this higher level, but in my experience, at this point it is typically far more valuable to take advantage of the human tester’s cognitive skills to make sure that nothing else is broken. (Naturally, if a tool extends our capabilities in some form, we’ll use it if the cost is low and the value high.)

In an ideal scenario, it’s the unit tests that help to ensure that this particular problem doesn’t happen again. But even in a non-ideal scenario, things tend to be okay. Other filters heuristically capture the regression problems–and when regressions make it as far as human testers, thoughtful people will usually recognize that it’s a serious problem. If that happens, developers and development management tend to make changes farther up the line.

It was ever thus. In 1999, at the first Los Altos Workshop on Software Testing, a non-scientific survey of a roundtable of very experienced testers indicated that regression tests represented about 15% of the total bugs found over the course of the project, and that the majority of these were found in development or on the first run of the automated tests. Cem Kaner, in a 1996 class that I was attending that an empirical survey had noted a regression rate of about 6%. Both he and I are highly skeptical about empirical studies of software engineering, but those resultscould be translated with suitable margin of error into “doesn’t happen very much”.

With agile methods–including lots of tests at the level of functional units–that turns into “hardly happens at all”, especially since the protocol is to write a new test, at the lowest level possible, for every previous failure determined by other means. This extra check is both necessary and welcome in agile models, because the explicit intention is to move at high velocity. Agile models mitigate the risk of high velocity, I perceive, by putting in low-level automated unit tests, where it’s cheap to automate–low cost and high value. Automated regression at the system level is relatively high cost and low value–that is, it’s expensive, if all we’re addressing is the risk of developers making silly mistakes.

I’ll have more to say about some specific circumstances in a future post. Meanwhile, I hope this helps.

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