In order to manage the success and popularity of a web site, it needs to be designed to cope with a growing number of users. A site designed to support 50 concurrent users can't serve thousands of simultaneous visitors without collapsing. Thus, the very success of a web site could also be the cause of its failure, if it is not able to sustain the sudden and exponential growth in number of users or requests.

A recent study by Computing & Double Take revealed that 83% of UK organisations admit downtime of several hours or more. Even if you manage to avoid a complete collapse, users will not stick around on a slow-loading site. The ability to grow (and shrink!) depending on need or availability thus becomes critical, directly affecting your revenue stream. A system that's able to cope with this changing demand is called scalable.