Continuous availability as in 100%, 24x7? I don't believe that there is any such thing possible with today's technology.
High availability (HA) is all about the pursuit of continuous availability (CA). HA is generally achieved by making different system components redundant. The more components you make redundant, the higher the level of availability you can achieve. However, as long as it takes time to switch over applications from one system or one site to another, there will always be some downtime.
I would argue that instead of pursuing CA, you should work to make things as highly available as you can. (The difference is subtle. Did you know that eBay has 2-3 hours of scheduled downtime every week? They found that by introducing this scheduled downtime, they can reduce the amount of unscheduled downtime the rest of the week. I believe that if they can do it, just about anyone can.
Hope this helps.
Evan L. Marcus
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This was first published in September 2002