Specifying and measuring quality of service in distributed object systems

Distributed applications are difficult to build and maintain and are even more difficult when the applications are distributed over wide-area networks. Distributed Object Computing middleware has emerged to simplify the building of distributed applications by hiding implementation details behind functional interfaces. However critical applications have non-functional requirements, such as real-rime performance, dependability, or security, that are as important as the functional requirements, but are also hidden by the middleware. Because current distributed object middleware doesn't support these aspects of critical applications, application developers often find themselves bypassing the distributed object systems, effectively gaining little or no advantage from the middleware. We have developed Quality Objects (QuO), a framework for including Quality of Service (QoS) in distributed object applications. QuO supports the specification of QoS contracts between clients and service providers, runtime monitoring of contracts, and adaptation to changing system conditions. A crucial aspect of QuO is a suite of Quality Description Languages for describing states of QoS, system elements that need to be monitored to measure the current QoS, and notification and adaptation to trigger when the state of QoS in the system changes. This paper gives a brief overview of QuO and describes the syntax and semantics of CDL, the component of QDL for describing QoS contracts.