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-time 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 middle-ware. 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. 1: Introduction Distributed applications are becoming more prevalent in both local-area networks (LANs) and wide-area networks (WANs) despite the added complexity in developing and maintaining them. Distributed object middleware, such as CORBA [9], has contributed to the prevalence of distributed applications by simplifying their development and maintenance in the following ways: • Object implementations and their interfaces are developed separately, facilitating abstract design of an application's component objects and their interactions without worrying about the low-level specifics of how each object accomplishes its function • Implementation languages, operating systems, and computer architectures are hidden from the application developer, who is building the application at the object interface level • Standard interface definitions and code generators enable heterogeneous objects to interoperate without regard to their implementation details, platform or system dependencies, or distribution across the network. However, there are increasingly more distributed applications that have quality of service (QoS) requirements along with functional requirements. Applications such as multimedia, video-on-demand, national security, military, health care, medical, and financial systems often have critical requirements, such as real-time performance, synchronization of data, security, dependability, and fault tolerance. Distributed object middleware falls short in providing support …