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User sessions, usually consisting of sequences of consecutive requests from customers, comprise most of an e-commerce site's workload. These requests execute e-business functions such as browse, search, register, login, add to shopping cart, and pay. Once we properly understand and characterize a workload, we must assess its effect on the site's quality of service (QoS), which is defined in terms of response time, throughput, the probability that requests will be rejected, and availability. We can assess an e-commerce site's QoS in many different ways. One approach is by measuring the site's performance, which we can determine from a production site using a real workload or from a test site using a synthetic workload (as in load testing). Another approach consists of using performance models. I look at the approach my colleagues at George Mason and I took that uses performance models in the design and implementation of automatic QoS controller for e-commerce sites.