Thursday, 13 March 2008

Big Hairy Tables

I like Keith Braithwaite's "software gauges" metaphor. As he explains, a gauge is a shortcut for deciding whether something passes or fails some criteria. For example, if your bag fits inside the metal cage at an airport then it can be taken onboard as hand luggage. You don't need to use a tape measure. What's interesting about gauges is that you don't necessarily need to be able to articulate "the rules"; if you need to know the rules you can infer them from the gauge.

However, I'm not convinced with what Keith says about trader spreadsheets making good gauges. Obviously he's had success with them, so they definitely can be made to work, but would they have worked even better another way? Basically what it boils down to is that I like tables; I just don't like big hairy tables!

If the hand-luggage cage can be taken as an example of good practice then some of the properties of a good gauge appear to be: sturdy and reliable; correct enough for all practical purposes; obvious; unambiguous; quick to use; and simple to understand by the gauge user.

The spreadsheets Keith showed me contained a large number of sparsely populated columns of denormalised data with lots of magic numbers and magic strings. This doesn't seem to stack up well against the list of desirable traits for a gauge. They're not designed with the gauge user in mind. If your domain model doesn't fit the gauge, is it because the model is wrong? Or the gauge is wrong? Or your interpretation of the gauge is wrong? With so many moving parts, it's got to be difficult to work out, even with help from the traders.

Isn't it better to treat the spreadsheets as a starting point rather than an ending point? And then have someone skilled at analysis and abstraction work with the traders and extract smaller, more practical gauges. There's nothing to stop you holding these gauges in spreadsheets too. I have a lot of respect for the customer, but I don't see why, just because the traders have written a spreadsheet, you can't help them improve it. They've got their skills you've got yours.

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