Abstract

aradoxically, the larger a system is, the more critical- but less likely-it is to be highly available. We can build small ultra-available modules, but building large systems involving thousands of modules and millions of lines of code is a poorly understood art, even though such large systems are a core technology of modern society. Three decades ago, hardware components were the major source of faults and outages. Today, hardware faults are a relatively minor cause of system outages when compared with operations, environment, and software faults. Techniques and designs that tolerate these broader classes of faults are still in their infancy. This article sketches the techniques used to build highly available computer systems. Historical perspective Today's highly available systems deliver four years of

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