There’s an old joke about a couple of folks on a plane who hear the captain successively announce that they’ve lost one, two, then three engines. Each time, he reassures the passengers that they’re OK, but will be progressively later to land. After the losing the third engine, one passenger tells the other, “If the last one goes, we’ll be up here all night!”

It’s a remarkable aircraft that can fly on just one out of four engines. Most four engine jets need at least two to cruise. (I’ve been told that they can make a controlled descent on one engine, but can’t maintain altitude.)

Likewise, your web app probably needs more than just one functioning server to handle demand. The usual approach to computing availability is to compute the odds that at least one server survives:


If all the servers are identical, meaning that we expect them to have the same failure rate, then this reduces to the more familiar form: