Software Architecture in Practice, Third Edition

5. Availability

With James Scott

Ninety percent of life is just showing up.

—Woody Allen

Availability refers to a property of software that it is there and ready to carry out its task when you need it to be. This is a broad perspective and encompasses what is normally called reliability (although it may encompass additional considerations such as downtime due to periodic maintenance). In fact, availability builds upon the concept of reliability by adding the notion of recovery—that is, when the system breaks, it repairs itself. Repair may be accomplished by various means, which we’ll see in this chapter. More precisely, Avižienis and his colleagues have defined dependability:

Dependability is the ability to avoid failures that are more frequent ...