Improving the availability of web services

In order to maintain the popularity and reputation of a web site, the quality of service perceived by users, especially the service availability, is a success factor. A service that is frequently unavailable may have negative e#ects on the reputation of the service provider, or result in loss of business opportunities. From the user's perspective, a service that exhibits poor quality is virtually equivalent to an unavailable service. In this work, we present the overall architecture and the evaluation of a middleware infrastructure which provides quality-of-service di#erentiation among classes of communication-bound processes. By communication-bound processes we mean processes whose activity is typically dominated by network communication, e.g. a video server. The proposed architecture supports di#erent classes of service, each with di#erent quality attributes concerning the network data delivery performance. In particular, the architecture is able to provide a class of service, namely guaranteed service class, which is suitable for increasing the service availability for a group of premium users, especially in overloaded servers (in absence of external faults).