The growing availability of Internet access has led to significant increase in the use of World Wide Web. If we are to design dependable Web-based systems that deal effectively with the increasing number of clients and highly variable workload, it is important to be able to describe the Web workload and errors accurately. In this paper we focus on the detailed empirical analysis of the sessionbased workload and reliability based on the data extracted from actual Web logs of eleven Web servers. First, we introduce and rigourously analyze several intrasession and inter-session metrics that collectively describe Web workload in terms of user sessions. Then, we analyze Web error characteristics and estimate the requestbased and session-based reliability of Web servers. Finally, we identify the invariants of the Web workload and reliability that apply through all data sets considered. The results presented in this paper show that session-based workload and reliability are better indicators of the users perception of the Web quality than the requestbased metrics.

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