Proportional Delay Differentiation Service in Web Application Servers: A Feedback Control Approach

As more and more business applications become Web-enabled, service differentiation becomes an important issue for database-driven e-commerce Websites to meet different requirements of clients. This paper presents a feedback control approach to provide proportional delay differentiation (PDD) service in Web application servers handling dynamic HTTP requests. To achieve this goal, an approximate linear time-invariant model of the database connection pool (DBCP) in Web application servers is identified experimentally and used to design a proportional-integral (PI) controller. At each sampling time, the controller can dynamically calculate and adjust the probabilities for different classes of dynamic requests to get idle database connections in the next sampling period, according to the error between the measured delay ratio and the reference value. Three kinds of workloads, which follow deterministic, uniform and heavy-tailed distributions respectively, are designed to evaluate the performance of the closed-loop system. Experiment results indicate the controller is effective in handling varying workloads, and PDD can be achieved in the DBCP even if the number of concurrent dynamic requests changes abruptly under different kinds of workloads.