Tuesday 20 November 2012

From Test Harness To Support Tool

Before I discovered the wonders of unit testing I used to follow the suggestion from Steve Maguire in his book “Writing Solid Code” about stepping through your code in the debugger to test it. In particular he was suggesting that you look at the common pitfalls, such as off-by-one errors when traversing arrays and writing loops. This had a profound effect on the way that I partitioned my code because to perform this practice efficiently I needed to get my code executing in the debugger as quickly as possible. That in turn meant that code was generally packaged into small (static) libraries and then linked into both the main product and, more importantly for this discussion, a test harness.

The test harnesses may have started out as trivial applications, but the library often has some potentially reusable[1] value outside the main product. Consequently I often ended up putting more than a cursory amount of effort into them. Yes, sometimes I even contrived a feature just to allow a particular code path to be executed and therefore tested. In hindsight it was not an overly productive use of my time compared to, say, unit testing, but it did have the benefit of creating both a system/integration-test tool, and sometimes as a by-product a support tool. From what I’ve read about some of the support tools Microsoft ships, they also started out as internal utilities that others eventually found useful and the word spread.

Take for example my own, simple DDE and ODBC libraries. These weren’t written with any particular application in mind, but I created a simple GUI based test harness for each of them - DDEQuery came from the former and PQT from the latter. Although only intended as personal test tools I have used them both on many occasions to aid in the development and support of my real work[2]. OK, these are my tools and therefore I have more than a vested interest in using and maintaining them. But the same idea has paid dividends many times over on the other systems I’ve been paid to work on.

I began to realise how useful the test tools could be outside the team when I started working on a multi-language project for the first time. The front-end was written in PowerBuilder and the back-end in C. I was part of a team tasked with making substantial performance improvements, but the first problem I could see was that I had to waste more than 4 minutes loading the GUI every time before I even hit any of my team’s code. What I felt we needed was our own cut-down tool, not necessarily UI based, that would allow us to invoke the entry points more rapidly for testing and more importantly provide an alternate means through which we could invoke the profiler.

The watershed moment was when the tool got bundled with the main GUI during testing. Although it presented a very Matrix-esque view of the data it was enough to allow them to workaround some UI bugs, such as when an item was missing from the dashboard so you couldn’t even select it, let alone manipulate it. Nowadays even though I’d expect automated unit and collaboration tests I’d still want some sort of harness just to enable profiling and code checking, such as with a tool like BoundsChecker. Unit testing does not make any other sort of testing redundant, but you might get away with doing less of it.

I ended up in a similar kind of position many years later, but this time it was a VB front-end and C++ back-end with a good dose of COM thrown in the middle. This time they already had a command line tool for driving the calculation engine but still nothing to act as a front-end. Once again I was somewhat amazed that the first port of call when debugging was to invoke the VB front-end which an age to start up. This also implied that the COM interface was never tested in isolation either - it must have been painful working on the front/back-end interface.

I was particularly pleased with one simple little tool that made my life much easier. It allowed me to extract portions of a large data set so that I could turnaround my local testing much quicker. What I never expected, when broadcasting the existence of this tool to the rest of the team, was that I would be chastised for using company time to write a tool that helped me do my job more efficiently! I also built a simple GUI to act as a mock front-end, just like before, but this time I was far more careful about who I mentioned it to.

There were other simple test harnesses in the VCS repository from days gone by, but given the attitude of those up above it’s not surprising that they were left to rot. When I came to try them out I found none of them even built. Only one was of any vague use and so I nurtured that and made sure it became part of the standard build along with any other new tools I wrote. I then packaged it along with the others into a separate Utils package that could be deployed into an environment on demand to aid in any manual testing or support.

I eventually felt vindicated when the tool I was chastised over started to be used by both the BAs and BAU team to generate custom data sets for testing specific scenarios. In one of the essays that Fred Brooks published as part of The Mythical Man Month he suggests there might be:-

“It is not unreasonable for there to be half as much code in scaffolding as there is in product”

Nowadays with unit testing I would expect that figure to be even higher. Of course there is a cost to all this “scaffolding” as it needs to be maintained just like the production code. I suspect that the more naive managers believe that if they skimp on test code they will have less waste to maintain and therefore more time can be spent on building new features instead. In my experience those projects I worked on that had a plethora of tests and tools delivered new releases an order of magnitude more frequently than those without.


[1] I, along with many others, genuinely believed that we would be writing lots of reusable, pluggable components that would save ourselves time as we approached each new project. For a software house this may well make sense, and it certainly helped in some cases when I worked at one. But for in-house development trying to share code across projects and teams is just too hard it seems.

[2] The DDE tool rarely sees the light of day now, but PQT is still used by me virtually every day for testing and support.

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