Phaal: Session Based Admission Control: a Mechanism for Improving the Performance of an Overloaded Web Server

In this paper, we introduce a new, session-based workload for measuring a web server performance. We define a session as a sequence of client’s individual requests. We then measure server throughput as a number of successfully completed ses-sions. Using a simulation model, we show that an overloaded web server can experience a severe loss of throughput mea-sured as a number of completed sessions comparing against the server throughput measured in requests per second. More-over, statistical analysis of completed sessions reveals that the overloaded web server discriminates against longer sessions. We introduce a session based admission control (SBAC) to prevent a web server from becoming overloaded and to ensure that longer sessions can be completed. We show that a web server augmented with the admission control mechanism is able to provide a fair guarantee of com-pletion, for any accepted session, independent of a session length. This provides a predictable and controllable platform for web applications, and is a critical requirement for any e-business.