The vast majority of software services and systems should aim for almost-perfect reliability rather than perfect reliability—that is, 99.999 or 99.99 percent rather than 100 percent—because users cannot tell the difference between a service being 100 percent available and less than "perfectly" available. There are many other systems in the path between user and service (laptop, home WiFi, ISP, the power grid, ...), and those systems collectively are far less than 100 percent available. Thus, the marginal difference between 99.99 percent and 100 percent gets lost in the noise of other unavailability, and the user receives no benefit from the enormous effort required to add that last fractional percent of availability

Un papier détaillé sur la haute dispo et les dépendances d'un service.