Brain Surgery Guides

I was working with a client a few weeks ago. He had asked me to help him get started developing a feature for one of his applications. He wanted to learn the flow of acceptance testing, unit testing, and coding.

You can see this flow by reading the Craftsman articles, starting at #25 and going forward.

We used FitNesse to write a simple acceptance test. Then we started executing the acceptance test. Since the Fixtures weren't written, we had to implement each of them one step at a time. At first the fixtures were skeletal, having just enough code in them to allow FitNesse to proceed without exceptions. We implemented each Fixture in the simplest possible terms. The Fixtures would compile, but did very little else. Within half an hour or so we got to the point where the acceptance test was executing to completion, and failing.

Next we had to tie the Fixtures to the application. Since there was no application at this point, this was an act of design. We had to determine how the fixtures would interact with the application, and build the skeletal structured within the application that would support that. When we were done with that, the acceptance test still ran to completion, and still failed.

Next, one step at a time, we made the acceptance test pass. This involved adding code the the application to process the information in a manner consistent with what the acceptance test expected. We wrote unit tests for all the code in the application that was more than trivial.

In the end, my client something to this effect:
I thought this was an interesting observation. It reminded me of a NOVA I saw once. It was about a person who had uncontrollable tremors. He went to a neurosurgeon who determined that the tremors were being caused by a small cluster of neurons that were misfiring deep in the patient's brain. He decided that he could stop the tremors by destroying that cluster of neurons.

Destroying a cluster of neurons is simply a matter of heating them up until they die. You can heat them with a very tiny needle with a heating element on the end. You insert the needle into the brain, positioning it right in the middle of the errant cluster of neurons, and then pulse the heat for a fraction of a second.

In order to do as little collateral damage as possible, the needle must be very fine, and it must be positioned perfectly. To do this, the surgeon imobilizes the patient's head, removes the necessary portion of the skull, and then using PET scans, locates the errant cluster of Neurons in 3 dimensions to within fractions of a millimeter. Then he builds a set of guides, anchored to the patient's head. The needle will be threaded through the guides. The guides control the azimuth and altitude angles of the needle relative to the patient's head, and also the depth of the needle.

Setting up these guides takes a lot of time and effort. Setting up the guides is the operation. The actual insertion and heating requires barely more than a minute.

Acceptance tests, Unit tests, and the practices of Agile Development are like those guides. They set everything up. They constrain and restrict what the production code will eventually do. They make sure that the production code does what is needed, and only what is needed.

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