QoS Guarantee in Tomcat Web Server: A Feedback Control Approach

In this paper, classical feedback control theory is used to contribute two types of QoS guarantees, relative delay guarantee ( RDG) and absolute delay guarantee (ADG). RDG aims to maintain the average queuing delay ratio between different classes of requests according to pre-specified parameters, and ADG aims to ensure the average queuing delay for requests with high priority is no more than the threshold configured. All components of the closed-loop of QoS guarantee is implemented for HTTP dynamic requests in the database connection pool (DBCP) in Tomcat Web Server. Through off-line system identification, the approximate linear time-invariant model of the DBCP was established; RDG and ADG controllers were designed to use the Root Locus method. The controller was invoked periodically to calculate and adjust the probabilities for different classes of requests to use a limited number of database connections, according to the error between the measured queuing delay and the reference value. The experiments were conducted to evaluate the performance of the closed-loop systems. Experiment results demonstrated that, the controllers designed were capable of handling fluctuating workloads, and achieved RDG and ADG effectively in Tomcat web server.