This model isn’t based on release planning at Guinness. It’s based on drinking Guinness.
It’s the end of a work day. You and your mates are going out for a drink. Initially, you’re thinking you’d like to have three pints of Guinness. How do you place your order?
If you’re a traditionally-minded software development project manager, you’ll order all three pints at once, in the same glass. When the bartender tells you three pints of Guinness won’t fit in a one-pint glass, you’ll pound your fist on the bar and shout until he finds a way to make them fit. After all, the reason he doesn’t want to pour three pints into the glass is that he’s either lazy or incompetent, or both. Once you let him know who’s boss in no uncertain terms, he’ll get those three pints into the glass, one way or another.
If you’re in the UK, you have to pay when you order, and not after you’ve received your drinks. So, you’re out the price of three pints, even though two of them are spilling off the bar and onto the floor. You’ve blown your project budget already, and there’s no way you’re getting three pints, unless you want to lap it up off the floor with your tongue. You can’t place a second order without going back to your spouse…er, that is, the budget committee…to request additional funding.
As a traditionally-minded manager, of course, your first thought will be to rent several of those inexpensive three-pint glasses from India. That way, you can fit three pints into one glass every time you place an order. But then you start to run into quality problems. By the time you’ve drunk the first pint, the rest of it has started to go flat and warm (too warm, even, for the UK). Either that, or you’ve changed your mind. After the first pint, maybe you fancy a Smithwick’s, or maybe you’re satisfied and don’t want a second.
You’ve committed to three pints of Guinness, you’ve paid for three, and you’ve got no flexibility to change. And the rent keeps coming due on those three-pint glasses, so you naturally feel as if you’ve got to keep them filled, or else you aren’t getting your money’s worth.
What’s the alternative? Well, you could arrive at the pub thinking you’ll be having three pints of Guinness, but you order just one pint. Once you finish it, you’ve still got 2/3 of your budget remaining, and you haven’t committed to anything more. Nothing has spilled onto the floor, and your tongue is still clean. If you feel like a Smithwick’s, or if you just want to make it an early night, you’ve got the flexibility to decide at the last moment. At the end of the evening, you’ve got none of those cheap three-pint glasses to pay for.
It’s quite possible all the functionality everyone ever dreamed of won’t fit into your next release cycle. The question is, what will you do about it?