| In heterogeneous network and operating system environments with a fair amount of performance variations, multiple applications compete and share a limited amount of system resources, and suffer from variations in resource availability. These complex applications, such as OmniTrack, a client-server based omni-directional visual tracking application, are thus desired to adapt themselves and to adjust their resource demands dynamically. We believe that appropriate application-level QoS adaptation decisions can be achieved with the assistance of a middleware architecture, where both application and system level dynamics can be observed and analyzed to decide when, how and to what extent adaptation has to occur.; In this work, we present Agilos, a novel Middleware Control Architecture to enforce the best possible adaptation decisions for distributed multimedia applications, via dynamic controls and reconfigurations of their internal parameters and functionalities. Several major contributions are presented in this dissertation. First, for modeling adaptors, we developed a Task Control Model based on control theory, in order to enact graceful degradation or upgrade paths within a required QoS range. Second, we deployed a Fuzzy Control Model in the design of middleware configurators, in order to model the process of choosing among application-specific parameter-tuning and reconfiguration choices.; Third, in order to optimally design the reconfiguration rules in configurators so that the best possible application QoS is achieved, we designed mechanisms and protocols for the Quality of Service probing and profiling services, named QualProbes, for the discovery of relationships among QoS parameters and reconfiguration actions. Fourth, estimation errors and significant end-to-end delay may obstruct desired accurate adaptation measures. We devised an optimal state prediction approach to estimate current states based on available state observations. Finally, we present a gateway-centric approach that facilitates on-the-fly reconfiguration of client-server mappings. We have built a prototype of the Agilos architecture, and validated it with OmniTrack, a client-server based omni-directional visual tracking system. In this work, we show that the Agilos architecture effectively achieves the best possible performance with respect to the most critical QoS parameter, the tracking precision, while the adaptive actions are stable, flexible and configurable according to the needs of individual applications. |
