Introduction

What's all the buzz about high availability? Why is everyone so intent on achieving the Utopia of high server availability: Five Nines? It really all comes down to one thing: economics. The economics of today's Internet-centric world demand that critical services and servers be available 100% of the time. In the absence of perfection , which no one has delivered yet, the bar for high availability solutions has been set at five nines: 99.999% uptime. What exactly does that equate into, though?

Five nines availability allows you to have critical services offline for 5.25 minutes per year! That's an unbelievably low number no matter how you look at it. But that's the goal of high availability solutions. As you may know, 5 minutes per year is barely enough time to apply a hot fix, much less a service pack. The answer to this problem: high availability server solutions.

When discussing high availability solutions, you can look at the problem in two distinctly different ways: hardware and software. Windows Server 2003 provides you with two types of software-based high availability: clustering and network load balancing, each of which is discussed in detail in the following sections.