The Art of Agile Development, 2nd Edition

Slack

AUDIENCE

Developers

We deliver on our iteration commitments.

Imagine that the power cable for your workstation is just barely long enough to reach the wall receptacle. You can plug it in if you stretch it taught, but the slightest vibration will cause the plug to pop out of the wall and the power to go off. You’ll lose everything you were working on.

I can’t afford to have my computer losing power at the slightest provocation. My work’s too important for that. In this situation, I would move the computer closer to the outlet so that it could handle some minor bumps. (Then I would tape the cord to the floor so people couldn’t trip over it, install an uninterruptable power supply, and invest in a continuous backup solution.)

Your iteration plans are also too important to be disrupted by the slightest provocation. Like the power cord, they need slack.

How Much Slack?

The amount of slack you need doesn’t depend on the number of problems you face. It depends on the randomness of problems. If you always experience exactly 20 hours of problems in each iteration, your capacity will automatically compensate. However, if you experience between 20 and 30 hours of problems, your capacity will bounce up and down. You need 10 hours of slack to stabilize your capacity and to ensure that you’ll meet your commitments.

NOTE

Remember that the team decides for itself what to commit to and whether those commitments are shared outside the team. See “Making and Meeting Iteration Commitments” ...