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The next topic under spares and redundancy is clustering and web farms. In the previous section we covered RAID and how it provides redundancy for disk drives, but what about doing the same for servers? This is addressed with redundant servers or clustering: multiple servers acting as a single logical unit. An example of this in action is Amazon. It may appear that you're only connected to a single server when you're shopping from the comfort of home, but behind the scenes, there is a multitude of servers handling your requests! Clustering is primarily for fault tolerance, but another implementation also provides load balancing for performance optimization. We point out that clustering and load balancing are not the same and that various implementation are available to support redundancy, load balancing, or both.