Dealing with Web service QoS factors using constraint hierarchy

Functionality and non-functional properties are two critical factors in web service technology, but non-functional properties (quality factors) are often ignored. Usually, these are articulated as statements of objectives, as opposed to prepositional assertions. A key challenge in dealing with objectives is that there is no obvious means to decide when they are satisfied. In effect, these objectives are never fu lly satisfied, but satisficed to varying degrees. Alternative design decisions need to trade-off varying degrees of satisfaction of potentially mutually contradictory non-functional requirements. In some circumstances, non-Junctional properties are crucial; they do affect the design decision. Upon a request, there are a range o f web services that might provide the required functionality, so the web service selection can only be done based on their Quality of Service (QoS). Therefore, a quality-based web service model is in high demand. The key contribution of this paper is the use of the hierarchical constraint logic programming framework [9, 10] in dealing with quality factors. We show how quality factors can be formulated as soft constraints and how the machinery associated with constraint hierarchies can be used to evaluate the web services.