The Scope of the Challenge

In the easy, laid-back days of client/server systems, a system’s user base would be measured in the tens or hundreds, with a few dozen concurrent users at most. Today we routinely see active user counts larger than the population of entire continents. And I’m not just talking about Antarctica and Australia here! We’ve seen our first billion-user social network, and it won’t be the last.

Uptime demands have increased too. Whereas the famous “five nines” (99.999 percent) uptime was once the province of the mainframe and its caretakers, even garden-variety commerce sites are now expected to be available 24 by 7 by 365. (That phrase has always bothered me. As an engineer, I expect it to either be “24 by 365” or be “24 by 7 by 52.”) Clearly, we’ve made tremendous strides even to consider the scale of software built today; but with the increased reach and scale of our systems come new ways to break, more hostile environments, and less tolerance for defects.

The increasing scope of this challenge—to build software fast that’s cheap to build, good for users, and cheap to operate—demands continually improving architecture and design techniques. Designs appropriate for small WordPress websites fail outrageously when applied to large scale, transactional, distributed systems, and we’ll look at some of those outrageous failures.