Cloudonomics: The Business Value of Cloud Computing, + Website

Availability and Probability

In the game of Russian roulette, one bullet is loaded into a six-chamber revolver. Then the chamber is spun and the revolver fired repeatedly. The gun may not fire on the first round, or the second, or even the 143rd, but it will, sooner or later.

The probability that the gun fires on the first round is 1/6, and the probability that it doesn’t is 5/6, or roughly 83%. These are decent odds, or so it would seem. However, the probability that it doesn’t happen either in the first round or in the second round is 5/6 times 5/6, which is 25/36, or 69%. The probability that it doesn’t happen in the first round or in the second round or in the third round is 5/6 times 5/6 times 5/6, or , which is 125/216, or 58%. By the fourth round, the probability is , or 625/1296, or only 48%. In other words, the probability has now shifted in favor of a catastrophic event if you’re on the wrong side of the Russian roulette “wheel.” If, instead of a revolver selecting a chamber, we are rolling a die, the same math applies.

This logic is inescapable no matter how many chambers the revolver has or sides the die has. The eventual outcome is never in doubt, only the timing: Bad things are guaranteed to happen sooner or later. The likelihood that an event such as this happens either ...